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Life’s Gray
Zone: Viruses
and Prions
5
It almost seems that not a day goes by without news about some new disease that threatens humanity. Ebola and Zika virus are just two of the more
recent entries into the rogues’ gallery of dangerous maladies. And it is not
just humans that are vulnerable to such new threats. Elephant endotheliotropic herpesvirus (EEHV) is killing baby Asian elephants. First documented in 1995, there have been over 50 cases in zoos across North America
and Europe, only nine of which were successfully treated. Wild Asian elephants are also at risk. Although there have also been serious cases reported
in adults, most infected older elephants do not develop life-threatening illness. In affected elephants, extensive damage to the cells lining the capillaries causes blood loss and hemorrhaging, which ultimately leads to
shock. The disease can be treated with the rapid application of appropriate
drugs, but this has only been successful in around a third of cases. Young
elephants apparently contract the disease through physical contact with
infected adults.
Meanwhile, deer and elk across North America are succumbing to chronic
wasting disease (CWD). First recognized in 1967 in Colorado, this invariably fatal disease has spread to 23 US states and two Canadian provinces—most recently to Maryland in 2011. Michigan, with a single case on
a captive breeding farm in 2008, reported its first case in a wild deer in
2015. The disease is characterized by weight loss ending in death, and
behavioral symptoms such as repetitive walking in a set pattern. It is
thought to spread from animal to animal through saliva. There is no
treatment.
None of the diseases mentioned above are caused by prokaryotic or eukaryotic microbes. EEHV, for instance, is a virus, related to those herpesviruses
that plague humans. CWD is a prion disease. Neither viruses nor prions are
composed of cells, nor do they display many of those characteristics of living
things discussed in Chapter 1. Rather, they occupy a nebulous position, not
really alive in the conventional sense, but certainly not the same as nonliving matter either. Here in Chapter 5, we will explore exactly what they are,
what characteristics they possess, and how they affect our wellbeing in so
many ways.
All case studies have a few questions
at the end, the answers to which will
become apparent as you read the
sections following the case.
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Viruses are acellular parasites, requiring a host cell
in which to replicate
CASE: “DEATH” IN THE RUE MORGUE
Edgar Allan Poe died October 7, 1849, at the age of 39.
His death has always been attributed to the effects and
complications of alcoholism and drug abuse. Michael
Benitez at the University of Maryland, on the other hand,
believes Poe died of rabies. Benitez came to this conclusion
by studying records of Poe’s symptoms at the time of his
death. He was first delirious with tremors and hallucinations
and then slipped into a coma. He emerged from his coma
and was calm and lucid before he lapsed back into delirium.
After 4 days, he died. These symptoms, say Benitez, are telltale for rabies but are not typical for alcoholism. Additionally,
records show that Poe had abstained from drinking for
6 months prior to his death. We will never know for certain
if rabies killed one of the greatest American authors (Figure
5.1), but can we continue to definitively blame Poe’s death
on the bottle? “Nevermore.”
1. What type of infection is rabies? How is the agent that
causes rabies different from the other prokaryotic or
eukaryotic organisms we have already discussed?
2.Why are the symptoms associated with a rabies
infection primarily neurological?
https://scholar.vt.edu/access/content/group/97b91a99-7258-44a2-8002-9
b7c83a84bd5/WebDev/Website/Gallery/
EnglishGallery/ePGalleryPatrickM/index
_files/Edgar_Allan_Poe_2.jpg
Figure 5.1 Edgar Allan Poe.
Rabies, caused by rabies virus, has been recognized as a serious disease
since ancient times. As far back as 4000 years ago, the Mesopotamians had
written laws detailing the responsibilities of dog owners, designed to reduce
the risk of this dreaded illness. But the doctors who treated Edgar Allan Poe
can be forgiven for not understanding what was affecting the author. At the
time of Poe’s death, it would still be over 40 years before anyone even suggested that something called a virus existed.
What exactly are viruses and how do they differ from other infectious agents?
Nobel Prize-winning immunologist Sir Peter Medawar described them as
“bad news wrapped in protein,” which is a fairly good definition.
First of all, most viruses are small (Figure 5.2a). A typical bacterium might
have a length of approximately 5 micrometers (μm; 1,000,000 μm = 1 meter).
Viruses range in size from about 20 to several thousand nanometers (nm) in
length (1000 nm = 1 μm, so 1,000,000,000 nm = 1 meter). The largest human
virus, ironically the smallpox virus, is approximately 300 nm long and is
barely visible with the strongest light microscope. The rabies virus is 170 nm
long and about 70 nm wide. Not all viruses are quite so tiny, however. In the
past 15 years or so, several giant viruses, all of which infect protozoa, have
been discovered (Figure 5.2b).
Second, unlike prokaryotes and eukaryotes, viruses are acellular—they are
not composed of cells. At its simplest, a viral particle consists of only nucleic
acid, enclosed in a protein coat, much as Medawar described them. As we
will see shortly, many viruses are somewhat more complex than that, but
viruses are far simpler than the cell-based microorganisms we have discussed so far.
Third, we have previously noted that all living things use DNA to code for the
proteins they need to survive. Some viruses also encode genetic information
Most viruses have one of a few basic structures
(a)
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(b)
Mammalian red blood cell
10,000 nm
Typical bacillus
length = 3000 nm
Poliovirus
30 nm
HIV
50 nm
Influenza
virus
85 nm
Rabies
virus
~ 70 ¥ 170 nm
Smallpox
virus
~300 ¥ 200
¥ 100 nm
1 μm
Figure 5.2 Sizes of viruses. (a) Most viruses are significantly smaller than typical bacterial and animal cells shown for
comparison. (b) In recent years, several very large viruses have been discovered. Pithovirus is the largest known virus, 1.5
micrometers in length and 0.5 micrometers in width. It is as large as some small bacteria, and it is larger than the smallest eukaryotic
cells, found in a type of alga. Discovered in Siberia in 2014, pithovirus infects certain amebas.
in DNA, but many rely on RNA as genetic material. Consequently, viruses
are often broadly classified as either DNA or RNA viruses.
Fourth, all viruses are obligate intracellular parasites; they can reproduce
only if they invade a host cell. A viral particle has only a small number of
genes. This genetic material codes primarily for the structural proteins of the
coat and other required proteins, including some enzymes needed for replication. Everything else is supplied by the host cell. Outside of a host cell,
viruses cannot replicate and they show few if any of the properties commonly attributed to living things. However, once inside an appropriate cell,
viruses essentially commandeer the host cell’s machinery and use it to replicate themselves. In this manner, an infected cell becomes a viral factory. The
virus uses host enzymes and other resources, including amino acids, as well
as host structures like ribosomes, to make copies of itself. Newly assembled
viral particles are released from the host cell, which frequently kills the cell
or seriously compromises normal functioning.
Because they are acellular, and because they lack many fundamental
characteristics of living things, are viruses even alive? The answer to this
question, however, really depends on how we define “life.” Viruses certainly replicate and evolve as other living things do, and the way we treat
viral disease is the same whether or not we consider them to be alive.
Perhaps the most accurate way to view viruses is to think of them, as
implied in this chapter’s title, as straddling the border between the living
and nonliving world.
Most viruses have one of a few basic structures
An individual, complete viral particle is called a virion. As previously stated,
all viral particles are composed of genetic material surrounded by a protein
coat. Depending on the type of virus, the genetic material may be DNA or
RNA. The protein coat is called the capsid, and the individual subunits that
make up the capsid are called capsomeres. Each capsomere is composed of
one or more proteins. Collectively, the nucleic acid and the capsid are called
the nucleocapsid.
Most capsids have one of two basic shapes. Many capsids are composed of
20 capsomeres, all in the shape of equilateral triangles. Such viruses have the
appearance of geodesic spheres and are known as icosahedral (20-sided)
viruses (Figure 5.3a). Helical viruses (Figure 5.3b) have capsomeres that fit
together to form a spiral around the enclosed nucleic acid. Apart from these
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Chapter 5: Life’s Gray Zone: Viruses and Prions
(a)
(b)
(c)
(d)
Nucleocapsid
Envelope
Capsomere
Nucleic acid
Capsid
Glycoprotein
Nonenveloped
icosahedral nucleocapsid
Nonenveloped
helical nucleocapsid
Enveloped
icosahedral nucleocapsid
Enveloped
helical nucleocapsid
Figure 5.3 Viral structure. The two main shapes for viral capsids are (a) icosahedral and (b) helical. Both of these shapes may
be enveloped, (c) and (d), surrounded by a phospholipid bilayer that is embedded with glycoproteins.
two most common shapes, some viruses have a complex structure.
Smallpox virus, for example, has a capsid that is composed of multiple layers
of protein, neither icosahedral nor helical in shape.
Many viruses are surrounded by an envelope that is composed of a phospholipid bilayer in which viral proteins called glycoproteins are embedded
(Figures 5.3c and 5.3d). Other viruses are nonenveloped and lack this
structure. As we will see, the presence or absence of an envelope and its
associated glycoproteins tells us a great deal about how a particular virus
enters and leaves a host cell. Many viral nucleocapsids contain a small number of enzymes. These are enzymes lacking in the host cell, which are
required for successful viral replication.
Specific viruses usually infect only certain hosts
and certain cells within those hosts
An important characteristic of a virus is its host range—the spectrum of
hosts that it is able to infect. Some viruses infect only animals, while others
infect only plants, fungi, and so forth. Even within a group such as animal
viruses, however, a given virus cannot infect all animal species. For example,
cats are susceptible to feline leukemia virus (FLV). While dangerous to cats,
this virus is incapable of infecting humans. On the other hand, while humans
are susceptible to Epstein–Barr virus, the virus responsible for mononucleosis (mono), cats are not.
Even within an appropriate host, only certain types of cells can be infected.
For example, the rabies virus that may have killed Edgar Allan Poe can also
kill dozens of other mammalian species. However, rabies virus is able to
infect only certain cell types within these hosts. The specificity of the virus
describes the cell types a particular virus can successfully infect. What
determines specificity? Rabies virus is an enveloped virus. The glycoproteins that are embedded in the viral envelope can bind to proteins found in
the plasma membrane of nerve cells (neurons). The protein in the plasma
membrane of the neuron is similar to a lock, while the glycoprotein is like a
key. Only cells that have the proper membrane protein can be infected by
this virus (Figure 5.4). This explains why the symptoms accompanying
rabies are largely neurological. Hepatitis B virus, on the other hand, lacks
Viruses must first attach to and enter an appropriate host cell
(a)
Viral glycoprotein
Capsid
Envelope
Nucleic acid
Host cell
membrane protein
Host cell
plasma membrane
(b)
the necessary glycoproteins to bind to neurons and therefore cannot infect
this cell type. Hepatitis B virus has a different envelope glycoprotein, one
that allows it to bind to proteins on the surface of liver cells. While some
viruses are very specific, others can infect many cell types; the host molecule to which they attach is widespread and found on a variety of different
cells. Nonenveloped viruses attach to target host cells via components of
their capsid. In poliovirus, for instance, the point where three capsomeres
come together forms a region called a canyon. Cells lining the intestine have
proteins in their membranes that can fit snugly into these canyons, permitting infection by the virus.
Replication of animal viruses proceeds through
a series of defined steps
The infection of a susceptible animal cell by a virus can conveniently be
divided into several steps, which we will consider in sequence. Many of the
differences that occur in the replicative cycle of different viruses depend on
whether or not the virus has an envelope, and whether its genetic material is
DNA or RNA.
Viruses must first attach to and enter an appropriate
host cell
The first step in the infection process, called attachment, is the contact
between a virion and its target host cell. A virion is not motile. It is merely
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Figure 5.4 Viral host-cell specificity.
The ability of a virus to infect specific host
cells is due in large part to the ability of the
virus to bind to proteins found in the host-cell
membrane. (a) The virus has glycoproteins that
match the three-dimensional structure and
chemical properties of the host-cell membrane
protein. Chemical attraction between the virus
and the host proteins binds the virus to the host
cell. The virus can then enter the cell via one of
several mechanisms. (b) The same virus cannot
bind a different cell type, because proteins found
on this cell are not complementary to the viral
glycoproteins. Consequently, this cell type is
resistant to infection by this specific virus.
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transported passively until it happens to interact with a cell bearing the
proper receptor. For enveloped viruses, the glycoproteins in the envelope
bind the proper receptor in the host-cell plasma membrane. In nonenveloped viruses, proteins forming part of the capsid often function as attachment sites. The receptor molecule to which a particular virus attaches is
typically a membrane protein of some sort. Only cells bearing the correct
receptor are vulnerable to a specific virus. For instance, human immunodeficiency virus (HIV) can infect only cells that bear proteins called CD4 on
their surface.
Following attachment, the virion must enter the host cell in a step called
penetration (Figure 5.5). For nonenveloped viruses, entry occurs via endocytosis (Figure 5.5a) (see Chapter 3, pp. 63–64, for a discussion of endocytosis). The binding of the virion to the receptor causes the cell to transport the
virus across the membrane and into the cytoplasm inside a membranebound vesicle. Enveloped viruses may also enter a cell via endocytosis, but
many employ a different strategy called fusion (Figure 5.5b). During fusion,
viral attachment brings the plasma membrane and the viral envelope into
close proximity. The two lipid bilayers actually fuse together, releasing the
nucleocapsid into the cell cytoplasm. Many of the details of fusion remain
unclear, but it appears that when viral glycoproteins bind to their receptors,
(a)
Envelope
Viral glycoprotein
Capsid Nucleic acid
Host-cell
membrane protein
Host-cell
plasma membrane
(b)
Figure 5.5 Entry of an animal virus into a host cell. (a) Entry via endocytosis. Following viral attachment to host-cell
membrane receptors, the host cell transports the virus into its cytoplasm. Once endocytosis is complete, the viral particle is released
into the cytoplasm. Both enveloped and nonenveloped viruses may enter a cell via endocytosis. (b) Entry via fusion. Following viral
attachment, the host-cell plasma membrane and the viral envelope are brought into close proximity. Following fusion of the two
lipid bilayers, the viral nucleocapsid is released into the host-cell cytoplasm. Only enveloped viruses enter host cells by fusion.
The final two stages are the assembly of new virions and their release from the host cell
the glycoproteins change their shape due to the interaction. This disrupts
the envelope and plasma membrane, allowing them to fuse.
Once they have entered the cell, viruses release their
protein coat prior to replication
Before it replicates, the virus must shed its capsid (Figure 5.6). This process,
called uncoating, may occur in endocytotic vesicles, where the pH is low. In
other cases, host enzymes called proteases digest the viral protein coat.
Once a virus has uncoated, we say that it has entered the eclipse phase. An
intact viral particle no longer exists; the eclipse phase ends only when newly
replicated virions are assembled at the end of the replicative cycle.
Following uncoating, two important tasks must be completed before new
viral particles are assembled. First, the nucleic acid must be replicated, and
second, new structural proteins must be produced (see Figure 5.6). These
two important processes are collectively called the synthesis stage. Different
viruses achieve these two goals in various ways that primarily reflect the type
of nucleic acid they carry. We will return to this topic shortly, following our
completion of the viral replicative cycle.
The final two stages are the assembly of new virions
and their release from the host cell
Once new viral nucleic acid and structural proteins are made, new virions
are formed in a process called assembly (see Figure 5.6). In some viruses,
the capsomeres first assemble to form the capsids, and the genetic material
is then inserted. In others, the capsomeres latch onto the genetic material,
ultimately producing the new viral particle.
The final step in the viral replicative cycle is the release of newly formed
virions from the infected cell. Nonenveloped viruses are generally released
by cell lysis. The host cell literally explodes, releasing the newly assembled
virions. The released virions may now contact other host cells, to begin a
new round of viral replication. Lysis invariably kills the host cell.
Host-cell plasma membrane
Host-cell cytoplasm
Newly replicated
viral DNA
Uncoating
Virion that has
successfully
penetrated host cell
Viral RNA
Newly synthesized
viral proteins
Newly
assembled
virions
Figure 5.6 Uncoating, synthesis, and assembly of a DNA virus. Once a virion successfully penetrates a host cell,
it must release its nucleic acid. This may occur either due to the acidity of an endocytic vesicle or the activity of host enzymes.
The subsequent synthesis stage consists of two events: The viral DNA is replicated into many new DNA copies and the DNA is
transcribed into viral RNA, which in turn is used to produce new viral proteins. The new viral proteins then assemble around the
newly synthesized DNA, producing new progeny virions.
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(a)
Figure 5.7 Viral release by budding.
(a) Stages in the release of an enveloped helical
virion are shown. Enveloped viruses acquire
their envelope as they bud through the plasma
membrane of the infected host cell. The envelope
is actually a portion of the host-cell plasma
membrane. Viral glycoproteins, already produced
during the synthesis stage (see Figure 5.6),
become embedded in the plasma membrane
and are acquired along with the lipid bilayer as
newly assembled viral particles leave the cell.
(b) Electron micrograph of several vesicular
stomatitis virus (VSV) particles budding from
the surface of an infected cell. Closely related to
rabies virus, VSV affects cattle, horses, and pigs as
well as humans. It is transported by biting insects.
(b)
Viral glycoproteins embedded
in host-cell plasma membrane
Helical
nucleocapsid
Cytoplasm
New enveloped virion
Enveloped viruses, on the other hand, are typically released from infected
cells by budding (Figure 5.7), wherein the assembled nucleocapsid pushes
through the plasma membrane. In doing so, it becomes coated with plasma
membrane material that now forms the viral envelope. The virus has already
coded for envelope glycoproteins, which are already present in the plasma
membrane, and the new virus will acquire them along with its envelope.
Unlike lysis, budding does not necessarily result in cell death.
The synthesis step described above is substantially different for DNA and
RNA viruses. In the next sections we consider some of the details in the synthesis stage for these two viral groups.
DNA viruses must replicate their DNA and use
it to direct synthesis of viral proteins
Once a DNA virus releases its DNA into the cell, some of the viral genes
begin to code for protein. This process is similar to what occurs in both
eukaryotic and prokaryotic cells and involves two crucial steps. First, the
viral genes, composed of DNA, are used to produce RNA that carries the
genes’ genetic instructions. Second, the RNA associates with host ribosomes, and the genetic message is converted into the specified viral protein.
The details of this two-step process will be explored in Chapter 7. Some of
the proteins that are made first are those needed to replicate the viral DNA.
The viral DNA is then replicated, and hundreds or even thousands of new
DNA molecules are produced. Finally, another round of protein synthesis
takes place, primarily using host enzymes and resources. This time the proteins being produced are the structural proteins that will make up the completed virions. Examples of DNA viruses that replicate in this manner include
Retroviruses use their RNA to produce DNA
Viral enzymes needed
to replicate DNA
Newly replicated
viral DNA
Viral RNA
Newly
assembled
virions
Viral DNA
Host-cell cytoplasm
Newly synthesized
capsid proteins
Figure 5.8 Synthesis in a DNA virus. To produce new progeny viral particles, the infecting virion must both replicate its
DNA and synthesize necessary proteins. Initially, enzymes that the virus needs to replicate its DNA are produced. DNA replication
then follows. Additional structural proteins including capsid proteins are then produced, permitting the assembly of new virions.
the herpesviruses and the papillomaviruses that cause warts. The synthesis
stage of a DNA virus is illustrated in Figure 5.8.
RNA viruses rely on RNA rather than DNA as their
genetic material
Unlike any prokaryote or eukaryote, RNA viruses encode their genetic information in RNA. For some RNA viruses, DNA plays no role whatsoever in
their synthesis stage. Although many of the details vary for different RNA
viruses, a general overview of their synthesis stage is provided in Figure
5.9a. For these viruses, the RNA is replicated many times to provide the
genetic material for newly synthesized virions. The RNA also guides the production of viral proteins, including capsid proteins, which will assemble
around the newly replicated RNA genetic material.
RNA viruses are unique in their ability to make copies of their RNA in this
way. To do so, they require an enzyme called RNA-dependent RNA polymerase, which is unique to RNA viruses. A few familiar RNA viruses include
poliovirus, rhinoviruses (which cause colds), measles virus, and influenza
virus. The rabies virus introduced in our case is likewise an RNA virus.
Retroviruses use their RNA to produce DNA
Although they are RNA viruses, retroviruses use a very different replication
strategy in their synthesis stage. Retroviruses also carry RNA in their nucleocapsid, but instead of replicating it into new RNA molecules, they convert
their RNA to DNA in a process called reverse transcription (Figure 5.9b).
Reverse transcription requires an enzyme called reverse transcriptase. The
newly synthesized DNA is then transcribed back to many copies of the viral
RNA. This RNA serves as the genetic material for new virions and is translated into structural proteins. By far, the most important human retrovirus is
HIV, the causative agent of AIDS.
Because reverse transcriptase is not used by our cells, it makes an attractive
target for antiviral drugs. Ideally, interfering with this enzyme should have
no adverse consequences for the host cell. Many of the important currently
available anti-HIV medications act by targeting this enzyme. The topic of
antiviral drugs will be more fully explored in Chapter 13.
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Production of new
viral proteins
(a)
Assembly
Production of new RNA
molecules by RNAdependent RNA polymerase
Newly assembled virion
Viral RNA
Production
of new viral
proteins
(b)
Infecting
retrovirus virion
Production
of DNA using
reverse
transcriptase
Assembly
Production
of new RNA
DNA
RNA
New retrovirus virion
Figure 5.9 Replication of RNA viruses. (a) RNA viruses use RNA, both to copy into new RNA molecules and to guide the
production of new viral proteins. These newly produced proteins assemble around the newly synthesized RNA molecules to form
new virions. (b) In retroviruses, RNA is converted into DNA, which is then used to make many copies of RNA. The newly synthesized
RNA is used both for protein production and for incorporation into new progeny virions.
Not all viral infections cause symptoms or kill host cells
Not all viral infections affect the host in the same way. If the virus undergoes
repeated rounds of replication, the infection is termed acute; infected cells
essentially devote themselves to viral production and ultimately die as a
consequence. Some viruses, such as influenza and rabies viruses, cause
acute infections only. The viruses keep reproducing and infecting new cells
until either the host immune system eliminates the intruder or the host dies
(Figure 5.10).
Chronic infections are characterized by a slow release of viral particles that
may or may not kill the host cell; if such infections do result in individual cell
death, host-cell replication is able to keep up with cell death, and there may
be few or no signs and symptoms of disease in an infected individual. A good
Relative amount of virus in host tissues
Time highlighted in pink corresponds to when signs and symptoms of disease are apparent.
Viral reactivation
of previously latent
infection
Acute
infection
Chronic infection following
initial acute infection
Latent infection following
initial acute infection
Time (days, months, or years depending on virus)
Figure 5.10 Types of viral infections.
Symptoms of acute infections are associated with
that period of time when levels of virus are high.
Chronic infections typically start with an acute
infection; viral levels then diminish, and viral
replication continues at a lower rate. The amount
of virus produced during the course of a chronic
infection may or may not be sufficient to cause
symptoms. Latent infections also typically begin
as acute infections. Following the acute infection
phase, the virus enters a period of dormancy (the
latent period), during which it does not replicate.
This latent period lasts indefinitely; reactivation
does not necessarily occur. If reactivation does
occur, there will be a new acute episode.
Viruses damage host cells in several ways
example is a hepatitis B virus (HBV) infection. A person infected with HBV
initially suffers through an acute phase with severe illness. In many individuals, the virus is destroyed by the immune system and the patient returns to
the uninfected state. In others, the immune system can eventually suppress
but not entirely eliminate the virus. Consequently, low levels of virus continue to be produced over a prolonged period of time, sometimes lasting
many years.
Some DNA viruses and retroviruses cause latent infections. Latent infections begin as acute infections, but then the virus enters a quiescent period
during which there is no additional production of viral progeny (see Figure
5.10). During this latent phase, the viral DNA codes for few if any viral proteins. Furthermore, it does not replicate independently of the host DNA. If
the host cell divides, however, the viral DNA is duplicated along with the
host DNA prior to cell division. Consequently, if a cell infected with a latent
virus divides, the two newly produced daughter cells will both be infected
with the virus.
Viruses may persist in this latent state for many years or in some cases for the
entire life of the host. However, a sudden change in the health of the host or
certain environmental factors can cause viral reactivation. A reactivated
virus is a previously latent virus that has resumed replication. During this
time, symptoms may become apparent in an infected individual.
Herpesviruses undergo replication cycles of this type. For instance, human
herpesvirus type 1 (HHV-1) is the causative agent of cold sores. When a person is first infected, an acute infection causes cell death, resulting in the oral
lesion. An effective immune response eventually destroys replicating viruses.
However, the virus will remain latent and may be reactivated at any time. The
reappearance of the cold sore reflects such reactivation. If you are infected
with HHV-1, you will remain infected for life, even if you never experience
any symptoms after the initial infection. If you get a cold sore a second time
in the same spot, it is a consequence of the same initial infection event. This
is quite unlike an infection with influenza virus; once you recover from a
bout of the flu, the virus is gone. If you suffer through another case of the flu
in the future, you can blame it on a second infection. EEHV, the virus discussed in the introduction, is a herpesvirus specific to elephants. The baby
elephants that die due to EEHV succumb during the early, acute phase. If
they survive that period, the elephants remain infected for life, with the virus
entering a latent phase. Young elephants apparently become infected when
they interact with adults who carry the latent virus. In these adults, the virus
may reactivate in a few cells—not enough to cause symptoms in the adult,
but enough to infect the young elephants.
We are only just beginning to understand the factors that might cause a
latent virus to reactivate. Anything that suppresses the host immune system,
such as cancer chemotherapy or HIV infection, makes reactivation more
likely. In the case of HHV-1, factors such as exposure to UV light seem to
play a role.
Viruses damage host cells in several ways
Disease-causing viruses create problems for cells by interfering with their
normal functions. Any such interference is known as the virus’ cytopathic
effects. One important cytopathic effect is the ability of many viruses to
interfere with the host-cell plasma membrane. Recall from Chapter 3 that
the plasma membrane maintains cell integrity and is important in regulating transport into or out of the cell. In virally infected cells, the budding of
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enveloped viruses can significantly compromise the ability of the plasma
membrane to carry out these functions. Viruses can also damage cells by
interfering with the ability of the cell to carry out its own protein synthesis or
by simply using resources that the cell requires for its own needs. Some
viruses prevent the host cell from copying its DNA. Others can initiate a process that ultimately leads to cancer.
Let us return to Edgar Allan Poe and the rabies virus that may have killed
him. Oddly, in spite of the extreme symptoms exhibited by rabies victims,
examination of infected neurons reveals little damage. There are, however,
large, prominent clumps of viral proteins visible in the cytoplasm. These
Negri bodies, as they are called, are often the only visible sign of infection.
Presumably, Negri bodies interfere with normal neuron function, resulting
in the neurological symptoms typical of rabies.
Like animals, plants are susceptible to many
viral infections
Unlike animal cells, plant cells are surrounded by a rigid cell wall and plant
viruses are unable to reach the plasma membrane when the cell wall is intact.
Consequently, plant viruses infect cells through breaks or wounds in the cell
wall. In many cases this damage is caused by insects, bacteria, or fungi.
In other respects, plant viruses are similar to animal viruses. Like animal
viruses, they may carry DNA or RNA. Their mode of replication is essentially
the same, as is their overall morphology. In one important respect, however, viral infections in plants are quite different. Plant cells are connected
to one another via small junctions, which essentially form connections
between the cytoplasm of adjacent cells. In effect, all cells linked by these
junctions have a common cytoplasm. Progeny of plant viruses can spread
to new host cells through these junctions and through the phloem, which
functions as the plant’s circulatory system. Because of this, plant infections
are often systemic. In other words, whereas animal viruses usually affect
only specific organs or tissues, plant viruses can affect cells throughout the
entire organism.
Many important plant diseases are caused by viruses. Plant viruses take a
large economic toll in crops such as corn and wheat, but they cause even
greater problems in perennial plants like potatoes. Plants suffering from
viral infections are often stunted (Figure 5.11a). Irregular coloration often
appears on leaves and stems, and green pigments may be lost. Unlike animals, plants rarely recover from a viral infection because, as mentioned previously, the entire plant is often infected. They also lack the ability to mount
a highly specific immune response. In a few happy cases, viral infection in
plants is actually desirable. The attractive variegated color of some tulips is
due to a viral infection that is transmitted through the bulbs (Figure 5.11b).
(a)
(b)
Bacteriophages are viruses that infect bacteria
Even bacteria do not get a free ride when it comes to pathogens. They are
subject to infection with bacteria-attacking viruses called bacteriophages
or phages for short.
The most intensively studied phages are those that infect E. coli. A specific
phage known as T4 provides an illustrative example. A T4 virion has a complex structure, with an icosahedral head containing DNA and a hollow, helical tail (Figure 5.12). The tail region is associated with other characteristic
structures called the tail fibers, tail pins, and base plate.
Figure 5.11 Plant viruses. (a) Tobacco
mosaic virus. The leaf on the right is from a
tobacco plant infected with tobacco mosaic
virus. Note its stunted and discolored appearance
compared with the leaf from an uninfected plant
on the left. (b) The attractive variegated color in
some tulips is the result of a viral infection.
Bacteriophages are viruses that infect bacteria
(a)
Figure 5.12 Structure of T4, a
representative bacteriophage. (a) Note
the complex structure, including the icosohedral
head encompassing the nucleic acid, and the
helical tail, which makes initial contact with
the host cell. (b) An electron micrograph of a
bacteriophage.
(b)
Nucleic acid
Icosahedral
head
Capsid
103
Collar
Sheath or
tail
Tail pins
Tail fibers
Base plate
The initial step in the infection cycle of bacteria involves the contact and
adherence to the bacterial surface by the phage (Figure 5.13a). This process,
called adsorption, cannot occur on just any bacterial cell. To successfully
adhere, molecules on the phage tail and tail fibers must match specific molecules on the bacterial surface that serve as receptors. A bacterium lacking
these molecules is resistant to infection.
(a) Adsorption
(f) Release
(b) Penetration
(c) Replication
of nucleic acid and
production of
phage proteins
(e) Maturation
(d) Assembly
Figure 5.13 Replicative cycle of
bacteriophage T4. This bacteriophage
is able to infect E. coli cells. (a) Adsorption:
The phage makes contact and adheres to the
bacterial surface. (b) Penetration: The helical
sheath contracts, forcing the viral DNA into the
cell cytoplasm. (c) Replication involves both
replication of the viral DNA and inhibition of hostcell activity. (d) Assembly of new phage particles.
(e) Maturation of phages into newly produced
infective viral particles. (f ) Lysis of the host cell
and release of newly produced, mature phages.
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Chapter 5: Life’s Gray Zone: Viruses and Prions
Once the phage has attached, it injects its DNA into the bacterium in a step
called penetration (Figure 5.13b). This involves the contraction of the helical sheath, which forces the hollow tube into the cell cytoplasm, much like a
microscopic syringe. In the process, the viral DNA is released into the cell’s
interior. The viral capsid does not enter the cell. It remains as an empty shell,
attached to the cell exterior.
Once DNA has entered the host cell, the metabolic activity of the cell is
blocked and the cell begins producing proteins coded for by the viral DNA
(Figure 5.13c). These include proteins that block normal host-cell activity,
enzymes required for viral replication, and structural proteins needed to
construct new capsids. The viral DNA is also repeatedly copied. All energy
required for these processes is provided by the host cell.
Phage DNA enters host cell
As new viral DNA and structural proteins are produced, they spontaneously
assemble into new virions (Figure 5.13d). Up to several hundred new phages
may ultimately be produced (Figure 5.13e). Eventually, the host cell bursts,
releasing the progeny phages (Figure 5.13f). These newly released phages
may then contact and infect other susceptible cells.
The T4 replicative cycle we have just described is termed a lytic cycle,
because it results in bacterial cell lysis and death. Not all phages, however,
undergo a lytic cycle. Infection of E. coli with phage lambda, for instance,
does not necessarily cause lysis. After releasing its DNA into a susceptible
bacterium, lambda does not immediately reproduce. Rather, it enters an
inactive period called the prophage state (Figure 5.14). In the prophage
state, the viral DNA is actually incorporated into the bacterial DNA. If the
bacterium itself divides, the viral DNA will also replicate along with it, but
otherwise, the infected cell remains largely unaffected. This state of viral
inactivity inside an infected cell is called lysogeny. Under certain conditions, however, the virus may begin to replicate, returning to the lytic cycle.
Ultraviolet light or exposure to certain chemicals is known to promote a
return to the lytic cycle in phage lambda.
Phage DNA integrates
into host-cell DNA,
entering prophage stage
When host cell replicates
its DNA, phage DNA
is also replicated
Phages can influence the number of bacteria
and even the diseases that they cause
Although they do not infect animals or plants, bacteriophages affect humans
in several important direct and indirect ways. First, the number of phages in
the environment is almost inconceivable. A single milliliter of sea water may
contain close to 50 million of them, and virologists have found similar numbers in soil. According to some estimates, in the oceans alone, up to 40% of
all bacteria may be destroyed each day by phages. Consequently, they may
influence the entire world’s food supply by limiting the numbers of bacteria
available for other organisms to eat. The vast numbers of killed bacteria no
doubt impact nutrient cycling in the ocean and other environments.
Of perhaps more immediate concern to humans is the effect lysogenic
phages have on the virulence of some bacteria. For example, cholera is
caused by Vibrio cholerae, a Gram-negative species (Figure 5.15). This
often deadly disease is caused by a toxin produced by the bacteria that
results in massive diarrhea. However, the bacterium cannot make the toxin
by itself. Rather, lysogenic phage genes inside the cell instruct the host to
create the toxin. Bacteria that are uninfected with these phages do not
cause disease.
Yet other phages may actually provide us with an invaluable service—
namely, protection from potentially pathogenic bacteria. It has recently
Host cell divides;
phage DNA is present
in both daughter cells
Figure 5.14 Lysogenic cycle of
bacteriophage lambda. Once phage DNA
is released into the cytoplasm of the host cell,
it incorporates into the host DNA. During the
lysogenic cycle, no new progeny phages are
produced. If the bacterial cell replicates, the
phage DNA, called the prophage, is replicated
along with the host DNA. Under appropriate
conditions, prophages in the lysogenic cycle may
return to the lytic cycle.
Prions are infectious proteins
105
been proposed that phages in our intestine help keep intestinal bacteria in
check, and thus unable to easily cause disease. These phages typically adhere
in almost astronomical numbers to the mucus lining the intestine.
Researchers have speculated that these phages stand ready to attack potentially disease-causing bacteria that attempt to penetrate the mucus to reach
the underlying intestinal lining. The phages benefit from this relationship
because of their access to a steady stream of bacteria in which to reproduce.
In other words, this may be an odd and ancient partnership between animals and phages that is beneficial to both.
Finally, because they destroy specific bacteria, some have suggested that
bacteriophages could be utilized therapeutically to treat bacterial infections. This idea was first proposed in the early twentieth century. Spotty
results, however, combined with the development of powerful antibiotics
dampened interest in phage therapy. However, in recent years, with the
development of antibiotic resistance by many bacteria, some researchers
are taking a fresh look at this approach. We will return to this topic in
Chapter 13.
Prions are infectious proteins
CASE: LAST LAUGH FOR THE
“LAUGHING DEATH”
Up through the 1950s, 2–3% of the Fore people of New Guinea
mysteriously died of the “laughing death” each year. The
disease, also called kuru, started with increased clumsiness,
headaches, and a tendency to giggle inappropriately.
Within a few months, victims could no longer walk. Total
incapacitation, followed by death, occurred within a year.
Curiously, the disease affected women and young children
almost exclusively (Figure 5.16). Older boys and men were
spared. The reason for this was unknown until 1957, when
Carleton Gajdusek, of the US National Institutes of Health,
determined how the disease was transmitted. The Fore people
practiced ritual cannibalism as part of their burial ceremony.
When a tribal member died, it was customary for female family
members to prepare the body and eat parts of the deceased’s
brain. Small children also consumed brain tissue, but older
boys and adult men did not participate. Gajdusek postulated
that an infectious agent was transmitted through the brain
tissue, explaining why only those who had consumed such
tissue were affected. Kuru has now been eradicated, because
cannibalism has not been practiced since the 1960s.
Figure 5.15 Cholera. This 1912 painting by
the French painter Jean-Loup Charmet depicts
the horror associated with cholera epidemics.
1. What exactly causes kuru?
2. How is this infectious agent unlike any other that we
have thus far discussed?
3. What other diseases are caused by similar agents?
Figure 5.16 A young Fore child infected
with kuru. This photo was taken in the early
1950s, before measures to eradicate kuru were
initiated.
Kuru may be gone, but other diseases, such as Creutzfeldt–Jakob disease
(CJD) and bovine spongiform encephalopathy (BSE), also known as mad cow
disease, are still with us. Likewise, chronic wasting disease (CWD), discussed
in the introduction, continues to plague deer and elk in North America. These
diseases all cause neurological symptoms and they are all uniformly fatal.
They are all caused by highly unusual infectious agents called prions.
Prions are infectious proteins. Their very existence was not proposed until
1982, and it is only since the 1990s that most of the scientific community has
accepted the notion that a protein could cause infection. The concept of prions was hotly contested because their existence violates a basic tenet of biology—that only nucleic acids can replicate themselves and serve as genetic
material.
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Chapter 5: Life’s Gray Zone: Viruses and Prions
But prions do not replicate in the traditional sense. Prions seem to be
abnormally folded forms of a protein that already exists in the brain (Figure
5.17). The best-studied prion disease is scrapie, which affects sheep (see
Figure 1.6b). It is named for the behavior of infected animals, which
includes scraping themselves bare against fence posts or other solid
objects. On the surface of the sheep’s brain is a protein called PrPc.
Occasionally this protein folds up incorrectly. It is then called PrPSc. This
aberrant protein is able to interact with normal PrPc proteins, forcing them
to refold in the abnormal three-dimensional shape (Figure 5.18). In other
words, PrPSc acts like a mold or template, which forces PrPc to take on the
disease-causing shape. Eventually, the PrPSc proteins build up to such an
extent that they begin to form abnormal plaque on the surface of brain
cells. Whether or not it is these plaque deposits that are the direct cause of
the disease remains to be seen. Other prion diseases, including kuru, CJD,
and CWD are also believed to be caused by improper folding of PrPc and
the ability of the aberrant protein to cause normal proteins to adopt the
abnormal shape.
PrPc
Prion disease can be caused by mutation of the gene coding for PrPc, but it
can also be transmitted between individuals. Creutzfeldt–Jakob disease, for
instance, can be transmitted via transplants or surgical instruments. Ninety
percent of all prion disease in humans has been identified as CJD. Typical
age of onset is approximately 65, and the disease is characterized by progressive dementia. Like all known prion diseases, it is always fatal.
Bovine spongiform encephalopathy (BSE) is a prion disease of cattle. The
disease is named for the brain’s spongy appearance in affected cows (Figure
5.19). It is also popularly called mad cow disease because of the erratic
behavior of affected cattle.
It is believed that the disease was spread by the previously common practice of adding bone meal to cattle feed. Bone meal often has nerve tissue
attached to it, and such tissue may contain infectious prion particles. The
disease came to the attention of the general public when it suddenly
appeared in British cattle in 1980. There were over 180,000 cases and millions of cattle were slaughtered in an effort to contain the epidemic. The
use of animal products in cattle feed was banned. Many were concerned
that humans who consumed meat from infected cows might develop a
similar disease. There was also speculation that CJD was a human form
of mad cow disease. This is probably not the case, but disturbingly, in
1996, a new form of CJD called variant CJD (vCJD) was first reported in
Great Britain. Unlike CJD, vCJD has an average age of onset of 27.
Furthermore, the disease kills its victims in just over a year, as opposed
to about 3 years for CJD. About 230 people have died of vCJD, mostly in
Great Britain.
Could vCJD be the human equivalent of mad cow disease, and did British
victims become infected by eating contaminated meat? Certainly the sudden appearance vCJD in 1996 is consistent with such a link.
Looking back and looking forward
We have now completed our examination of our microbial roster. Viruses
and prions are far simpler than the cellular life forms reviewed in Chapter 4.
Viruses, the ultimate parasites, cannot replicate unless they infect appropriate host cells. Once inside, viruses commandeer the cell, diverting host
resources to their own replication, often with dire consequences for the host
PrPSc
Figure 5.17 Normal and abnormal
forms of the PrP protein. The PrPc protein
is normally found on brain cells. The normal PrPc
sometimes takes on the form of the aberrant
PrPSc. The normal protein in its correct threedimensional structure has four spiral-like regions
called alpha helices. In the abnormal form, two
of these alpha helices, shown in red, lose their
helical form and take on a different, elongated
structure, indicated by the colored arrows.
Looking back and looking forward
Normal PrPc
protein
DNA
RNA
Normal
cell
107
Figure 5.18 Conversion of the normal
PrP protein to the abnormal form. The
aberrant PrPSc can force normal PrPc proteins
to take on the aberrant shape. (1) PrPSc (the
red square) may be acquired by consuming
contaminated animal products or may be due
to a mutation in the gene coding for the protein.
(2) The aberrant protein is able to interact with
the normal protein (blue circles) and force it to
take on the aberrant shape. (3)–(6) As this process
continues, more and more of the normal PrPc is
converted to PrPSc. High levels of the aberrant
protein form abnormal plaque deposits on the
surface of brain cells, which may be involved
in the nervous disorders seen in individuals
suffering from prion disease.
Infective PrPSc
protein
1.
2.
3.
4.
5.
6.
Infected
cell
cell. Prions, even less complex than viruses, are essentially rogue proteins
that force other host proteins into an aberrant shape. As these abnormally
folded proteins build up, symptoms of disease appear.
So now, the stage is set. We know who the microorganisms are, and we are
starting to understand what they do. But before we continue to investigate
them, we will consider what they did. In Chapter 6, we examine microbes
with a historian’s eye and investigate some of the astounding ways in which
they have influenced human affairs throughout history. We will see that
empires have fallen and societies altered, all because of microorganisms. We
will also review the colorful history of microbiology itself, looking at some of
the important discoveries that have advanced the science of microbiology to
where it is today.
Figure 5.19 Stained cross section of
the brain from a cow that died of
BSE. The white areas scattered across the brain
tissue are places where neurons have died due
to the buildup of the prion protein. Brain tissue
preparations from individuals that have died of
other prion diseases, including CJD and CWD
appear similar.
108
Chapter 5: Life’s Gray Zone: Viruses and Prions
Garland Science Learning System
•• http://garlandscience.rocketmix.com/students/
•• Investigate viral factories and their assembly lines
•• Test your knowledge of this chapter by taking the quiz
•• Familiarize yourself with the terminology used in this chapter by using the
vocabulary review
•• Get help with the answers to the Concept questions
Concept questions
These questions are designed to help you start thinking
like a microbiologist. The answers are not always simply
found in the text. Instead, you will need to take the concepts about which you have learned and apply them to
new situations. Some of the questions may not even have
just a single correct answer. Help is provided as part of
the GSLS resources, which can be accessed through
http://garlandscience.rocketmix.com/students/.
1. What is fundamentally different about the way viruses
reproduce compared with cellular forms of life?
2. Referring back to Chapter 1, in which the properties of
living things were discussed, make an argument that
(a) viruses are not living things or (b) viruses are living
things.
3.An anti-HIV drug called Enfuvirtide binds to and
interferes with the glycoproteins found in the viral
envelope. Which stage of the viral replicative cycle is
Enfuvirtide interrupting?
4.RNA-dependent RNA polymerase is a unique viral
enzyme produced by many RNA viruses. It is not produced by any prokaryotic or eukaryotic cells. Would
this enzyme make a reasonable target for drugs active
against the RNA viruses that produce it? Why or
why not?
5. If you know that a particular virus is enveloped, what
does this information alone suggest about its replicative cycle?
6. What is meant by the “eclipse phase” in the viral replicative cycle? Referring back to the steps described in
the replicative cycle here in Chapter 5, after which
step does it begin? After which step does it end?
7. Suppose that a particular virus has a wide host range
but narrow tissue specificity. What precisely does this
mean? What about a narrow host range and broad tissue specificity?
8.Chickenpox is caused by the chickenpox virus. If a
child is unvaccinated and exposed to the virus, she
might develop clinical signs of this disease. Many
years later, this same person may develop a case of
shingles, caused by the same viral infection that
caused chickenpox as a child. Referring back to the
types of viral infections described in this chapter, state
the type of infection the individual is experiencing
when she had chickenpox. What type of infection
does she have in the time between chickenpox and
shingles? What kind of infection does she have while
she is experiencing shingles?
9.Aphids use a remarkable trick to protect themselves
from certain eukaryotic parasites. The aphids have
symbiotic bacteria that live in their gut. These bacteria
often are infected with phage. The phages produce a
protein that is toxic to the parasites. What type of
phage infection do the bacteria within the aphid’s
digestive system have?
10.Before the idea of prions was proposed, diseases
caused by what we now know to be prions were
believed to be caused by as-yet unidentified viruses.
Some of the evidence used to show that such diseases
were not caused by viruses included the use of
DNAses (enzymes that digest DNA) and RNAses
(enzymes that digest RNA). Experiments were performed in which brain tissue from infected mice was
treated with either DNAse or RNAse. Other, previously
healthy mice were then exposed to the DNAse- or
RNAse-treated tissue. In either case, these newly
exposed mice developed the same disease as the original mice from which the brain tissue had been taken.
How do these results help demonstrate that a virus
was not involved?