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General Characteristics of Different Types of Acellular Microorganisms
Lesson: General Characteristics of Different Types of
Acellular Microorganisms
Lesson Developer: Dr. Vandana GuptaDr. Indira
P.Sarethy, Dr. Sanjay Gupta
College / Department: RLA, JIIT
Lesson Reviewer: Dr. Pooja Gulati
College/ Department: Department of Microbiology,
Maharishi Dayanand University, Rohtak, Haryana
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General Characteristics of Different Types of Acellular Microorganisms
Institute of Lifelong Learning, University of Delhi
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General Characteristics of Different Types of Acellular Microorganisms
Table of Contents
Lesson: General Characteristics of Different Types of Acellular
Microorganisms

Introduction

Virus

Size

Symptoms of Some of the Viral Diseases

Viral structure and Morphology
o
Nucleic Acid
o
Capsid
o

Classification

Some Common Viruses

Icosahedral viruses

Helical viruses

Complex viruses
Viral Envelope
Cultivation of Viruses




TMV

Bacteriophages

Polio Virus
Replication of Viruses

Lysogeny

Lytic Cycle

One Step Growth Curve of Bacteriophage

Structure

Plant Diseases Caused by Viroids
Viroids

Prions

Summary

Exercises

Glossary

References
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General Characteristics of Different Types of Acellular Microorganisms
Introduction
Various life forms, especially microbes occur in nature in amazing diversity of almost
all sizes,shapes, textures and are found in deviatingsettings such as at
astonishingpressures,temperatures, salinity and are almost ubiquitous.Because of the
astonishing diversity of microorganisms, it becomesabsolutelyinevitable to classify
microbes in different groups based on similar characteristics.
All the microorganisms are divided into two groups based on their structural and
functional organization namely, acellular and cellular forms. Acellular microbes
include viruses, viroids and prions as theyneither have a cellular organization nor do
they replicate by cell division.Acellular life forms not only lack the complex structural
features such as organelles (mitochondria, chloroplast, Golgi complex, endoplasmic
reticulum etc.), but they also cannot perform important life functions such as DNA
replication, transcription, protein synthesis, ATP synthesis when present outside the
host cell. These organisms when present outside the living host cell are considered
non-living since they behave as inert particles. However, once inside the host cell they
replicate and behave like living organisms. They use host machinery for replication
and synthesisof all their structural components and assemble into hundreds of new
particles. All the acellular forms are obligate intracellular parasites. Unlike cellular
microorganisms, which divide by cell division, acellularmicroorganisms are formed by
self-assembly of their structural components. The mechanism of self-assembly is
similar to the folding of a polypeptide in a proper conformation and its association with
other polypeptides and proteins to form a functional entity (which is governed by the
principle of achievement of minimum energy state by the molecules).
Viruses are acellularmicroorganisms, which contain nucleic acid (either DNA or RNA)
that can either be single stranded or double stranded, circular or linear, segmented or
non-segmented. The nucleic acid is encapsidated in a capsid that is made of proteins.
It is either a polyhedral structure with 20 triangular faces and 12 vertices (corners), or
a rod like structure with protein subunits arranged helically around the genome of the
virus and in still other viruses the structure is complex. In some of the viruses, an
extra covering composed of lipids, proteins and carbohydrates is present, and such
viruses are called enveloped viruses. Viruses can undergo two types of life cycles- lytic
(they completely destroy the host cells and are released in large numbers from these
cells) or lysogenic (integration of viral genome into the host chromosome and its
replication with the host chromosome). They cause great economic losses as they give
rise to several diseases in humans, animals and most of the cash crops.
Prions are proteinaceous infectious particles, which lack nucleic acid and cause slow,
progressive neurological disorders in humans and animals. On the other hand, viroids
are single stranded, circular RNA molecules capable of causing diseases in plants.
They are completely devoid of protein coat.
This chapter is aimed to provide an insight to the undergraduate students in the
general characteristics of different types of acellularmicroorganisms.
Viruses
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General Characteristics of Different Types of Acellular Microorganisms
Viruses (= venom or poisonous fluid) are very small (1/1000 of the host cell) obligate
intracellular parasites (cannot multiply outside living host cell), and behave as inert
entitiesoutside a living host. They areextraordinarily simple microorganisms in
comparison to all others. They are so simple in structure that they can even be
crystallized just like protein molecules.
By definition, a complete virus particle or virion is a sub microscopic entity that:
1. Containsonly one type of nucleic acid (either RNA orDNA),
2. Multiplies inside a living cell and uses"macromolecule synthesizing machinery"
of the host cell, and
3. Contains a protein coat (sometimes enclosed in an envelope made up of lipids,
proteins and carbohydrates) surrounding the nucleic acid. Sometimes, it may
also have additional layers.
Viruses can infect organisms across all the domains of life including bacteria, fungi,
protozoa,algae, vertebrates,invertebrates and plants. Viruses are of great medical,
social and economical concern as they are obligate pathogens. Viruses infecting
bacteria are called bacteriophages or phages. Viruses provided scientists with an
easy system to look into the intricacies of molecular biology and helped in solving a
number of most basic biological questions. Hershey and Chaseused bacteriophageas
amodel organism andprovided evidence that the DNA, not protein, was the genetic
material.Now a days, viruses find anextensive range of applications in biotechnology
and research.
When viruses invade susceptible host cells, they display some properties of living
organisms, and hence are said to be on the borderline between living and non-living.
Some of their living features are:
1. Their genetic material replicates inside a suitable host cell, by programming its
machinery. Viral progeny is released either by rupturing the cell or by budding
(similar to exocytosis) from the cell.
2. They are sensitive to heat, certain chemicals and radiation.
3. They possess antigenic property.
4. They exhibit host specificity.
Their non-living features include:
1.
2.
3.
4.
They can‟t grow and divide on their own.
They can be crystallized.
They are inert outside their specific host cells.
They are devoid of cell membrane, cell wall and all other cellular ultra
structural details.
Size
Viruses are much smaller than other (cellular) microorganisms; they fall in the size
range of 20 – 300 nm. Some of the examples are as follows:
Poliovirus
30𝜂m
Adeno virus
60 – 90 𝜂m
Influenza virus
80 -100𝜂m
Herpes virus
120–220𝜂m
Tobacco Mosaic virus
15 x 300𝜂m
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General Characteristics of Different Types of Acellular Microorganisms
Poxvirus
200-300 𝜂m
Symptoms of Some of the Viral Diseases
As already mentioned, viruses cause diseases in plants as well as animals. In plants,
different viruses induce almost similar symptoms in varied plants. The most commonly
observed symptoms are:
Leaf Mosaic Disease: Large, mottled, light green-yellow patches develop on upper
leaf surface. Growth of plant is stunted.Fruits develop mosaic patches.
Bunchy Top: Leaves fail to emerge from the stem and the plant appears stunted with
lot of small leaves at the top.
Leaf curl: Young leaves curl and mottle, and veins are cleared.
Leaf enations: Irregularly thick lesions appear on the leaves.
Stunting: Plant growth is stunted.
However, in humans, viruses infect different organ systems and cause various
symptoms. These range from mild respiratory, intestinal, febrile (viral fever) disorders
(which are self limiting) to life threatening conditions such as hepatitis, encephalitis,
AIDS and hemorrhagic fever.
Viral Structure and Morphology
Viruses are simple in structureand consist of a nucleic acid (DNA or RNA) enclosed or
encapsidated in a proteinaceous coat (capsid). Their components are described in
Figure 1. Some viruses are enclosed in an extra bilayer of lipids and proteins called
envelope (which is similar in structure to the unit membrane but contains virus
encoded proteins). Such viruses are called enveloped virusesin contrast to the
viruses lacking lipid envelope called naked or non-envelopedviruses (Figure 2). In
enveloped viruses, the envelope confers antigenic, biological and chemical properties
on the virus.
Figure 1: Various Components of a Virion.
Source: Author & ILLL
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General Characteristics of Different Types of Acellular Microorganisms
Figure 2: Naked and Enveloped Viruses.
Source: Author & ILLL
Earlier, when structure of all the viruses were not known, they were divided into two
groups, namely ether sensitive and ether insensitive, based on their sensitivity to
organic solvents. The viruses with envelope are sensitive to organic solvents such as
ether, alcohol, acetone etc. because of the disruption of lipid bilayer in the presence of
organic solvents and lose of infectivity.
Nucleic Acid
The genetic material in virusesis eitherDNA or RNA and it can be single stranded (ss)
or double stranded (ds).Itmay have alinear or a circular shape depending on the
virus(Table 1). In some viruses, the genetic information is present in several
segments calledsegmented genome, for example,Influenza virus, in which 7-8
segments of linear RNA are present and each segment carries unique genetic
information. The length of nucleic acidsalsovariesin viruses. It ranges from a few
thousand nucleotides (nt)to a few hundred thousand nucleotides. In Poliovirus,the ss
RNA genomeconsists ofonly 7000 nt whilethe ds DNA genome of pox virus is250-350
kblong.
Table 1: Types of Genome in Various Viruses
Source: Author
Parameter
Types
Nucleic acid
 DNA or
 RNA
Shape
 Linear or
 Circular or linear
 Segmented
Strandedness
 Single-stranded or
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General Characteristics of Different Types of Acellular Microorganisms
 Double-stranded
Sense
(applicable
for
single
stranded
RNA viruses)
 Positive sense (+) (can directly function as mRNA)
 Negative sense (−) (has to be transcribed first to be
able to function as mRNA)
 Ambisense (+/−) (some part of genome acts as
positive sense, whereas the other acts as negative
sense genome)
Capsid
The overall shape of virus particle varies in different groups. Commonly seen shapes
of viruses include rod shaped (tobacco mosaic virus), filamentous (Ebola virus),
icosahedral (polio virus), brick shaped (pox virus), bullet shaped (rabies virus),
pleomorphic and irregular (influenza virus), complex (bacteriophage T4),etc. These
shapes are mostly determined by the capsid structure (Figure 3).
Figure 3: Different Sizes and Shapes of Viruses.
Source:http://arthropodsbio11cabe.wikispaces.com/file/view/virus(cc)
A capsid is a protein coat that encloses nucleic acid of the virus and protects it from
nucleases present in extracellular as well as cellular fluids. It also helps in attachment
of virus to the host cell. Capsid is made up of protein subunits called capsomeres. The
arrangement of capsomeres is typical of a particular type of virus. Viruses are divided
into (i) helical viruses, (ii) icosahedral viruses and (iii) complex viruses, based on
capsid architect.
Helical viruses: Theyappear like rods, which are either rigid or flexible and are
hollow inside to accommodatenucleic acid. In fact,the capsomeres are arranged
around the nucleic acid in a helical manner (e.g., Tobacco Mosaic Virus, TMV) as
shown in the Figure 4. TMV is a plant virus and the very first virus to be discovered.
Later, it wasfound to have helical symmetry. Other examples of important viruses
having helical symmetryinclude influenza, corona,mumps, measles and Ebola
hemorrhagic fever viruses. These are also enveloped viruses.
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General Characteristics of Different Types of Acellular Microorganisms
Figure 4: Structure of a helical tobacco mosaic virus showingRNA coiled in a helix of
repeating protein subunits or capsomeres.
Source: http://upload.wikimedia.org/wikipedia/commons/3/3f/TMV_Structure.png (cc)
Icosahedral viruses: Theseappear
as
circular or hexagonalempty ball
like.Icosahedral capsid is composed of capsomeres arranged in a regular array of 20
identical equilateral triangular faces or planes (sides) and 12 corners or vertices as
shown in Figure 5A. A regular icosahedron is a closed shell formed from identical sub-
A
B
Figure 5:A.Computer simulated model of adenovirus showing
symmetry.B. Transverse section of an icosahedron showing nucleic acid.
Sources: A. Modified from http://en.wikipedia.org/wiki/capsid
B. ILLL
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icosahedral
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General Characteristics of Different Types of Acellular Microorganisms
units. The minimum number of identical capsomers required is twelve, each composed
of five identical sub-units. Capsomers at the vertices are surrounded by five other
capsomers and are called pentons. Capsomers on the triangular faces are surrounded
by six others and are called hexons. Hexons are essentially flat and pentons (at the
12 vertices) are curved (Figure. 5A).The same protein may act as the subunit of both
the pentamers and hexamers or they may be composed of different proteins.Nucleic
acid is enclosed inside the icosahedral capsid (Figure 5B). Viruses causing polio,
Dengue fever, herpes, common cold, and HIV are examples of icosahedral viruses
(Figure 6).
Figure 6. Electron micrograph of icosahedralviruses.
Source: http://commons.wikimedia.org/wiki/File:Icosahedral_Adenoviruses.jpg (cc)
Complex Viruses:Viruses that are neither helical nor icosahedralare called complex
viruses. For example, bacteriophages of T-even series (T2, T4 and so on) contain
icosahedral capsid (head that encloses the nucleic acid). However, they also
haveadditional structureslike a tail showing helical symmetry, tail sheath and tail
fibers attached to it (Figure. 7). A symmetry created by the presence of icosahedral
and helical shapes together, is also called as binal symmetry. Another example of
complex virus is poxvirus, which lacks clearly identifiable capsid symmetry.
A
B
Figure7: A.Structural details of a complex virus (bacteriophage T4) and B. Electron
micrograph of T4.
Sources:A.ILLL &B. http://www.abovetopsecret.com/forum/thread470839/pg1 (cc)
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General Characteristics of Different Types of Acellular Microorganisms
Viral Envelope
In enveloped viruses, an extra layer is present surrounding the capsid. Itis composed
of lipids, proteins and carbohydrates.The envelope is derived from either host cell
membrane, or endoplasmic reticulum/Golgi complex membranes, ornuclear membrane
depending on the site of replication and release of virus from the host cell. Envelopes
are embedded with viral encoded glycoproteins and are involved in the receptormediated attachment of the virus to the host cell. Figure 8 shows schematic diagram
of the envelopes of viruses with different symmetries. For example, influenza virus
has hemagglutinin spikes, which impart the ability to agglutinate red blood cells to the
virus and also causeattachment of the virus to the host cell. Similarly,SARS
coronavirus andrabies virus have S glycoprotein andG glycoprotein spikes on their
surfaces, respectively.
A
B
Figure 8: Schematic diagram of enveloped viruses with(A)icosahedraland (B)helical
symmetries.
Sources: A. http://commons.wikimedia.org/wiki/File:Enveloped_icosahedral_virus.svg
(cc) &B. http://en.wikipedia.org/wiki/Coronaviridae (cc)
Reality Check
Popular belief: Taking antibiotics can cure viral diseases.
Reality: Antibiotics have no effect on viruses. It is because viruses do
not carry out their own biochemical reactions; instead they use host cell
machinery to do so. Hence, antibiotics do not affect them.
Most antibiotics interfere with the reproduction of bacteria,
hindering their creation of new genetic instructions or new cell walls.
Source: science.howstuffworks.com/life/cellular-microscopic/virius-human2.htm
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General Characteristics of Different Types of Acellular Microorganisms
Cultivation of Viruses
As viruses are obligate intracellular parasites, theycan only be grown in live cells. This
presented a major hurdle in the discovery and initial developments in the field of
Virology. Initially, viruses were cultivated in live animals and plants. Later
on,embryonated eggs were used for the cultivation of animal viruses (chick embryo
technique). 7-10 days old fertilized eggs are inoculated at various sites (allantoic
cavity, yolk sac, chorioallantoic membrane and amniotic cavity) for growing different
viruses. More recently, viruses are grown in cell cultures on monolayers of animal
cells. When the viruses infect cells, they cause changes in the morphology of the cell,
called cytopathic effects. These effects are characteristic for each virus and can be
used for diagnosis of viral infections. Plant viruses are grown in plants, protoplast
cultures, plant tissue cultures,etc., and bacteriophages are grown on the bacterial
lawns.
Classification of Viruses
With the discovery of more and more viruses, a need to classify them and name them
was felt for proper communication among the scientific community. Initially, viruses
were classified based on the symptoms of diseases they caused, for example, all the
viruses causing similar symptoms of liver inflammation and jaundice were grouped as
hepatitis viruses.But with increase in knowledgeabout structure of the virus, type of
nucleic acid present (whether DNA or RNA, single or double stranded,etc.) and
themode of replication of the genome, an International Committee on Taxonomy of
Viruses (ICTV) was constituted in 1966.
Viruses arenow grouped into families based on(a) type of nucleic acid (RNA or DNA, double stranded or single stranded, positive or
negative sense)
(b) mode of replication, and
(c) morphology.
The suffix –virus, -viridae and –ales are used for genus, family and order names,
respectively. Viral species is a group of viruses sharing the same genetic information
and host range and are designated by descriptive names such as human herpes virus
(HHV), with subspecies designated by a number (HHV-1). Figure 9 shows
variousfamilies of viruses infecting animals along with comparisons of their shapes,
sizes and symmetry.
Plant viruses, bacteriophages and viruses of fungi and algae are classified in a similar
manner. Most of the plant viruses are single stranded RNA viruses and they are
mostly non-enveloped with few exceptions. Most of the bacteriophages contain DNA
genome and like plant viruses they are mostly non-enveloped.
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General Characteristics of Different Types of Acellular Microorganisms
Figure9: Comparisons of shapes, sizes and symmetry of different families of viruses
infecting animals.
Source: http://www.vetmed.ucdavis.edu/viruses/02-01_450.jpg (cc)
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General Characteristics of Different Types of Acellular Microorganisms
Some Common Viruses
Tobacco Mosaic Virus (TMV)
It was the first virus to be discovered or we can say that the concept of viruses was
developed with this virus. Adolf Mayer worked on TMV infected plants and showed that
the disease was transmissible from infected plant to a healthy plant. Dmitri Ivanofsky
further followed his work and showed that the sap from infected plants retained the
infectivity even after filtration through bacteriological Chamberland filter. Since he
could neither grow nor visualize the viral particles,he believed that a toxin produced
by
bacteria
might
have
caused
infection. In
1898,
it
was
aDutch
microbiologist, MartinusBeijerinck,who became convinced that the filtered solution
contained a new form of infectious agent. He also inferred from his studies that the
agent multiplied only in living cells. Based on his findings, he gave the concept of
ContagiumVivumFluidum (soluble living germ). This was followed bypurification
andcrystallizationof TMV from infected tobacco plants byW. M.Stanleyin 1935.Later in
the year 1939, after the invention of electron microscope,TMV particles were observed
as rods of approximately 300 x 15 𝜂m in size (Figure 10A). This virus is nonenveloped helical virus with positive sense RNA molecule (about 6,000 nucleotide).
A
B
Figure10: A. Electron Micrograph of Tobacco Mosaic Virus,&B. Leaves of plant
infected with TMV (note the variegation of yellow, light and dark green coloured
patches).
Sources:A.http://en.wikipedia.org/wiki/Tobacco_mosaic_virus (cc)
B. http://en.wikipedia.org/wiki/Tobacco_mosaic_virus (cc)
TMV infects members of family Solanaceae, especially tobacco plant.
The infection causes characteristic patterns on leaves such as mottling
(mosaic-like) and variegation of different coloured patches (Figure
10B).
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General Characteristics of Different Types of Acellular Microorganisms
Bacteriophages
Bacteriophages are viruses that infect and parasitize bacteria.Infact, they are so host
specific, that „typing‟ of bacteria can be done based upon phage infection.
Twort (1915) described an infectious agent that distorted the appearance of
Staphylococcal colonies. Felix d‟Herelle(1917) observed that the filtrates of fecal
cultures of dysentery patients induced transmissible lysis of broth culture of a
dysentery bacillus and hypothesized the presence of an invisible antagonist in fecal
filtrates. He is the only one to be credited with the discovery of phages.
Bacteriophages have variousshapes and sizes ranging from filamentous, circular,
irregular, lemon shaped, icosahedral and complex. E.coliT even phages (T2, T4)
andlambda phages have been extensively studied. Their genomes have been
sequenced completely. A number of molecular mechanisms including the discovery
that DNA is the genetic material (Harshey and Chase 1952) and regulation of gene
expression have been elucidated using bacteriophages. Several genetic engineering
techniques use bacteriophage based vectors for cloning, expression of different
proteins and also study of the function of various genes. Bacteriophage based vectors
are also used for creating whole genome libraries of various organisms.
T-even phages are tadpole shaped and possess a head and a tail.The head is
hexagonal in shape and consists of a tightly packed core of nucleic acid (ds DNA),
enclosed within a protein coat called a capsid. The size of the head varies in different
phages from 28 – 100 𝜂m. Phage T4 head is 100 𝜂m long with 65 nm head diameter.
Its tail is composed of a hollow core surrounded by a contractile sheath and a terminal
base plate, which has got „prongs‟ attached, or tail fibers (usually 6 in number), or
both. Tail of phage T4 is 100 𝜂m in length and 25 𝜂m in diameter (Figure
11A).Bacteriophage lambdatoo has a complex symmetry but it lacks base plate and
tail fibers.Rather, it has a tail pin at the end of its helical tail and its tail is flexible
(Figure 11B).
A
B
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General Characteristics of Different Types of Acellular Microorganisms
Figure 11: Bacteriophages-(A) T4 (With kind permission: Dr. Robert Duda,
Department of Biological Sciences, University of Pittsburgh), and (B) Lambda.
Sources: A. http://www.asm.org/division/m/foto/T4Mic.html
B. https://www.biochem.wisc.edu/faculty/inman/empics/virus.htm
Poliovirus
Poliovirus is an animal virus that has affinity for nervous tissue. Poliovirus causes
poliomyelitis (Figure 12A), a major health concern in developing countries. It is a nonenveloped positive sense RNA virus, 27-30 𝜂m in diameter, with a capsid composed of
60 capsomeres arranged in icosahedral symmetry (Figure 12B). Each capsomere is
made up of one molecule each of 4 virionproteins: VP1, VP2, VP3 and VP4.Poliovirus
belongs to the family Picornaviridae which includes many of the other important
human viruses such as Hepatitis A virus, Enteroviruses (cause intestinal infections),
Rhinoviruses (cause common cold infections),etc. Poliovirus is on the priority list of
WHO for its eradication from earth like the small poxvirus by mass vaccination of
children up to the age of 5.
A
B
Figure 12: A.A child showing symptoms of polio,&B.Electron micrograph of poliovirus
Sources: A. http://commons.wikimedia.org:wiki:File/Polio_EM_PHIL_1875_lores.PNG
(cc)B.http://creationwiki.org/pool/images/thumb/0/01/Polio_virus.jpg/110pxPolio_virus.jpg (cc)
Viruses are one of the most abundant groups of biological entities and exist in
different shapes and sizes. It is not possible to study each one of them here. Thus, a
few other important virusesare described in Table 2 along with their morphology and
diseases caused by them.
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General Characteristics of Different Types of Acellular Microorganisms
Table2:Morphology of some important viruses and their examples
Sources: As described.
Shape
Examples
Pox virus(causes small pox and cow pox diseases)
Source:http://en.wikipedia.org/wiki/File:Smallpox_vir
us_virions_TEM_PHIL_1849.JPG (cc)
Complex
Orthomyxovirus(causes influenza)
Source:https://upload.wikimedia.org/wikipedia/comm
ons/3/3a/Influenza_virus_particle_color.jpg (cc)
Pleomorphic, usually spherical
Ebola virus (causes hemorrhagic fever)
Source:http://upload.wikimedia.org/wikipedia/commo
ns/a/a7/Ebola_Virus_TEM_PHIL_1832_lores.jpg(cc)
Filamentous
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General Characteristics of Different Types of Acellular Microorganisms
Rhabdovirus (causes rabies)
Source:
en.wikipedia.org/wiki/Vesicular_stomatitis_virus (cc)
Bullet shaped
Retrovirus (Human immunodeficiency virus, causes
AIDs)
Source:http://commons.wikimedia.org/wiki/File:ElecM
icro_of_HIV_Retrovirus_serum_isolate_SampHM47.jpg (cc)
Variable (condensed round
to cone shaped core)
Replication of viruses
Replication of viruses in host cells results in two alternative outcomes, either a lytic or
a lysogenic cycle.These two cycles have been extensively studied using
bacteriophages (Figure 13).
Lytic cycle

Adsorption to the host cell: Virus attaches to specific macromolecules, present
on the surface of acell, called receptors.

Injection of the viral genome: Either genome of the virus enters the cell either
through injection (as in most of the bacteriophages) or complete virus enters
inside the cell by a process similar to endocytosis and then viral genome gets
uncoated in the cytoplasm (as in most of the animal viruses).

Synthesis of viral proteins and nucleic acid: After its entry, genome of the virus
takes over the host translational machinery and starts synthesizing its own
proteins and nucleic acid.
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General Characteristics of Different Types of Acellular Microorganisms

Assembly of the viral particles: The newly synthesized viral structural proteins
come together and self assemble into a virus, to which a copy of the genome is
added.

Release of viral particles from the cell: Host cell undergo lysis and the viral
particles are released.
For
an
animation
of
the
lytic
cycle
see
the
following
http://sites.fas.harvard.edu/~biotext/animations/lyticcycle.html
web
link:
Interesting Facts: The sequence of events that occurs when we come down with the
flu or a cold is a good demonstration of how a virus works:
1. An infected person sneezes near us.
2. We inhale the virus particle, and it attaches to cells lining the sinuses in our
nose.
3. The virus attacks the cells lining the sinuses and rapidly reproduces to generate
its progeny in large numbers.
4. The host cells break, and new viruses spread into our blood and also into our
respiratory system. Since the cells lining our sinuses get lysed, fluid flows into
our nasal passages and we get a running nose.
5. Viruses in the fluid that drips down our throat start attacking the cells lining the
throat thereby causing a sore throat.
6. Viruses in our bloodstream can also attack muscle cells and cause muscle
aches.
Source: science.howstuffworks.com/life/cellular-microscopic/virius-human2.htm
Lysogenic cycle
A French biologist, Andre Lwoff, first explained the process of lysogenic cycle in early
1950s. In this cycle,the genetic material of the virus gets integratedinto the host cell‟s
genome and is called aprovirus. In such state, virus does not cause damage to the
host cell and the cell continues to live and replicate normally. This state also allows
the virus to remain dormant or latent within the host cell until it is exposed to certain
stimuli like UV rays. An animation of the process can be seen on the following web
link:http://sites.fas.harvard.edu/~biotext/animations/lysogeny.html.
Following this, the genome of the virus is excised from the host genome and begins to
encodeconstituents of the virus as in lytic cycle, which then assemble into viral
particles. These are then released from the host cell by its lysis.
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General Characteristics of Different Types of Acellular Microorganisms
Interesting Facts: Even though a virus is merely a set of genomic sequence
surrounded by a protein coat, and can‟t carry out any biochemical reactions of its own,
yet it can live for years or longer outside a host cell. Some of the viruses can "sleep"
inside the host cells for years before reproducing.They do this by integrating their
genome into the host cell genome. For example, a person infected with HIV can live
without showing symptoms of AIDS for years, but he or she can still spread the virus
to others.
Source: science.howstuffworks.com/life/cellular-microscopic/virius-human2.htm
Figure 13: Multiplication of bacteriophages by lytic and lysogenic cycles.
Source: http://cnx.org/content/m44597/latest/?collection=col11516/latest (cc)
One Step Growth Curve of Bacteriophage
To study a single replication cycle of viruses, one-step or single-step
growth/multiplication curve is developed.It was first given by Max Delbruck and Ellis in
the year 1939 using an E. coli-T4 bacterial system. The experiment for one step
growth curve starts with the mixing of phages with the bacterial host cell suspension.
Samples of the medium are withdrawn at regular intervals and the number of
extracellular as well as intracellular phages is determined. A curve is plotted withtime
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General Characteristics of Different Types of Acellular Microorganisms
interval on the x- axis and phage titer on the y-axis (Figure 14).Growth of phages can
be divided into several phases:
1. Adsorption (initial phase): During this phase, the numberof extracellular
phages added at beginning of theexperiment is reduced as the phages adsorb
onto the bacterial cells.
2. Eclipse phase: It is defined as the time period during which phage genome
enters the cell and undergoes transcription, translation and replication.
Synthesis of its structural proteins occurs and after a threshold concentration
of various constituents is achieved, assembly of virus particles starts.
Therefore, no infectious virus particles can be detected during this period, even
inside the cell (checked after cell lysis).
3. Productive phase: This phase includes maturation and release of progeny
viruses. They become detectable in the medium. Their time of appearance may
vary from one to several hours or even days depending on the virus.
The time elapsed after phageadsorption into the cell till the first progeny
phage is released in the medium is called latent period.During this period,
extracellular phages cannot be detected. After this phase is over,there is a
steady increase in the extracellular titer of phages, after which a plateau is
achieved. Plateau indicates that all the host cells are lysed and progeny phages
are released.
Burst size:The burst sizeof a virus is the “number of infectious virus particles
produced per infected cell”. It can be calculated from the results of a one-step
growth experiment by dividing the final number of phages obtained by the total no
of bacterial cells used in the experiment. Burst size for viruses generally
variesfrom 10 to 10,000. Viruses vary significantly in terms of the kinetics of their
replication and hence length of the eclipse and latent period. Generally speaking,
bacteriophages multiply faster and have shorter growth cycle and animal viruses
have longer growth cycle.
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General Characteristics of Different Types of Acellular Microorganisms
Figure 14: One step growth curve of the lytic bacteriophages.
Source: Author & ILLL
Viroids
These are single stranded circular RNA molecules that lack a protein coat.Viroids infect
plants and were discovered by Theodor O‟ Diener while he was attempting to isolate
and characterize the agent of Potato Spindle Tuber Disease (earlier assumed to be
caused by a virus). Later on, many plant diseases, e.g., Citrus exocortis,
Chrysanthemum stunt, coconut cadang-cadangetc. were attributed to the infections
caused by viriods.
Structure
A viroid consists of a very short strand of RNA without any protective protein coat
(Figure 15). Electron microscopic studies of purified Potato Spindle Tuber Viroid
(PSTV) revealed that it had a single stranded RNA molecule containing 250 – 350
nucleotides. This RNA does not have any open reading frame meaning that it cannot
code for a protein. Instead, it takes over the transcriptional machinery of the host and
synthesizes its own RNA genome copies. This causesphysiological disturbances in the
infected plant cells ultimately leading to development of a disease.
Figure 15: Viroid structure.
Source: http://emu.arsusda.gov/typesof/images/viroid.jpg (cc)
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General Characteristics of Different Types of Acellular Microorganisms
Plant Diseases caused by Viroids
Viroid diseases are symptom-wise not much different from viral diseases.Stunting,
vein discoloration or clearing, leaf distortion, localized chlorotic or necrotic spots and
death of the whole plant results from viroidal infections (Figure 16).Viroids are
responsible for crop failures (as they cause infection in some important crop plants
such as potato, coconut, cucumber, tomato etc.) and the loss of millions of dollars in
agricultural revenue each year.
.
A
B
C
Figure 16: A.Oblong tubers obtained from plants infected with Potato spindle tuber
viroid (PSTVd), B. Stiff and upright growth of potato plants infected withPSTVd.(With
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General Characteristics of Different Types of Acellular Microorganisms
kind permission: Prof. Thomas A. Zitter, Plant Pathology & Plant-Microbe Biology
Section, Cornell University),&C.Mild, intermediate, or severe (RG1) symptoms of
PSTVd infection in foliage of tomato plants.
Source: A & B.http://vegetablemdonline.ppath.cornell.edu/
C. apsnet.org/edcenter/intropp/lessons/viruses/pages/PotatoSpindleTuber.aspx (cc)
Prions
In the 1920s, H G Creutzfeldt and A M Jacob observed lots of cases of a slow but
progressively degenerative disease independently. The disease, now called
Creutzfeldt-Jakob Disease (CJD), is characterized by mental degeneration and loss of
motor function that ultimately leads to death. Since then, many other similar
neurological diseases have been described. One of them is Kuru disease, which
involves loss of voluntary motor control. Animal diseases caused by prions include
scrapie of sheep and goats, bovine spongiform encephalopathy (mad cow disease),
that causes slow loss of neuron function and eventually death. Consuming infected
cattle meat has led to development of new human disease termed New Variant CJD.
It was not until 1982, that Stanley Prusiner discovered the causative agent of above
diseases. It was found to be an infectiousproteinaceousparticle, described for the first
time and named asPrions. Thisdiscoverywas made, while working ontheneurological
disease called scrapie in sheeps. He observed that the infecting ability of scrapie
infected brain tissue reduced by treatment with proteases but not by treatment with
radiation. This suggestedthat the infectious agent is purely proteinaceous in nature.It
was difficult to believe as in the absence of DNA or RNA, replication of prions could not
be explained. But later with lot of further research it was established that prions are
the miss-folded form of normal cellular protein (Figure 17) and that this miss-folding
is catalyzed by the presence of similarly miss-folded proteins in the body.
Therefore,infectionsby prions progress slowly. No treatment or vaccineis available for
these infections. Hence, they are also known as “slow progressive encephalopathies”.
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General Characteristics of Different Types of Acellular Microorganisms
Figure 17: Conformational change from the normal prion protein (left) to the
abnormal prion protein (right)(With kind permission: Dr. Akio Nomoto, Institute of
Microbial Chemistry, Tokyo).
Source:www.bikaken.or.jp/english/project/prionase.html
These diseases are sometimes inherited indicating a possible genetic cause (mutation
in normal cellular prion protein causing it to fold improperly),but they are also
infectious, as mad cow disease originated due to feeding cattle with themeat
ofscrapie-infected sheep. CJD has been transmitted with transplanted nerve
tissuesandKuru transmitted through tradition of cannibalism in tribal people.
Some of the characteristic features of prions are:
1. They are resistant to inactivation by heating to 90 °C (which inactivates
viruses).
2. Prion infection is not sensitive to radiation treatment that is enough to damage
viral genomes.
3. Nucleases (enzymes that digest DNA or RNA)can not destroy prions.
4. Prions are sensitive to protein denaturing agents, such as phenol and urea.
They are also sensitive to proteases (as they are made up entirely of protein).
After studying the general characteristics of acellular organisms like viruses,
viroidsand prions, let us compare their features. These are listed in Table 3.
Table 3: Comparison between Viruses, Viroidsand Prions
Source: Author
Feature
Virus
Viroid
Prion
Nucleic Acid
ss or ds DNA or
RNA)
ss RNA
-
Presence of Capsid
or Envelope
+
-
-
Presence of Protein
+
-
+
Need for Helper
Virus
+ /- (Smaller virus
e.g., parvovirus
needs)
Electron Microscopy
No
No
+
Nucleotide
sequence
identification
-
Host cell damage
and Electron
Microscopy
+
+
+
-
Viewed by
Affected by heat &
protein denaturing
agents
Affected by
Radiation /
Nucleases
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General Characteristics of Different Types of Acellular Microorganisms
Host
Bacteria, animals
or plants
Plants
Mammals
Summary
Acellular microbes lack a cellular organization and they do not replicate by cell
division. Viruses, viroids and prions are included in acellular microorganisms. These
organisms when present outside the living host cell are considered non-living, as they
behave as inert particles, but inside the host cell they can replicate and behave like
living organisms. They use host machinery to synthesis all their components as well as
genome replicationand assemble into hundreds of new particles.
Viruses contain nucleic acid, a protein coat and sometimes an extra covering called
envelope. The protein coat called capsid is either a polyhedral structure with 20
triangular faces and 12 vertices (icosahedral), or a rod like structure with protein
subunits arranged helically around the genome of the virus (helical) or complex.
Viruses are host specific and they also exhibit tissue or cell tropism (infect a particular
type of tissue or cell). Viruses can be cultivated in whole organisms (an animal or a
plant for growing animal or plant viruses, respectively) or in cell or tissue cultures.
Viruses are classified into different classes and further into orders and families based
on nucleic acid, structure of the viral particle, strandedness (single or double
stranded), sense of the genome (positive or negative sense) and the mode of
replication of their genome).
Viruses can undergo lytic (they completely destroy the host cells and are released in
large numbers from these cells) or lysogenic (multiply slowly and are released slowly
from the host cell) life cycle.
Viroids are single stranded, circular RNA molecules capable of causing diseases in
plants. They are completely devoid of protein coat.
Prions are proteinaceous infectious particles, which lack nucleic acid and cause slow,
progressive neurological disorders called slow progressive encephalopathies (SPEs) in
humans and animals.
Exercises
1. Write short notes on the followinga.
b.
c.
d.
Acellular microorganisms
Viral structure
Viral genome
Viral capsid symmetries
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General Characteristics of Different Types of Acellular Microorganisms
e.
f.
g.
h.
i.
Viroid
Prions
Lytic replicative cycle of viruses
Lysogenic replicative cycle of viruses
Cultivation of viruses
2. Define the followinga.
b.
c.
d.
e.
f.
g.
h.
i.
j.
k.
l.
m.
n.
Capsomeres
Capsid
Ambisense genome
Negative sense genome
Positive sense genome
Leaf mosaic disease
Leaf curl
Enveloped viruses
Provirus or prophage
Complex virus
Icosahedral virus
Helical virus
Binal symmetry
Cytopathic effect
3. Differentiate among the followinga.
b.
c.
d.
Enveloped and non-enveloped virus
Viroid and prion
Lytic and lysogenic cycle in viruses
Segmented and non-segmented genome
4. State true or falsea.
b.
c.
d.
e.
f.
g.
h.
i.
j.
k.
l.
m.
n.
o.
p.
q.
r.
Viral envelope is composed of lipids, proteins and carbohydrates.
T4 bacteriophage is an example of helical virus.
Pox virus is the largest known animal virus.
Sizes of the viruses vary from 0.2-2 micrometers.
Viroids are single stranded linear RNA molecules.
Prions infect plants.
Cell cultures cannot be used for growing viruses.
Whole plants, callus or protoplast cultures are used to grow viruses.
All the bacteriophages have head and tail morphology.
Plant viruses enter the host plant cells through receptor-mediated
entry.
T. O. Deiner gave the concept of viruses ContagiumVivumfluidum.
Prions cause slow progressive neurological disorders.
Tobacco mosaic virus was the first virus to be discovered.
Prions can be grown in chick embryo culture.
Viroids are sensitive to proteases.
Ether sensitive viruses are non–enveloped.
Kuru is an example of viral disease.
Viruses contain only one type of nucleic acid, either DNA or RNA.
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General Characteristics of Different Types of Acellular Microorganisms
Glossary
Acellular microorganism: An organism that lacks the
organization and multiply by assembly rather than cell division.
detailed
cellular
Bacteriophage: A virus infecting bacterial cells.
Binal symmetry: The capsid symmetry in some of the viruses is a combination of
icosahedral and helical symmetry. Such viruses are called complex virus.
Burst size: The number of phage particles produced by a host cell during lytic life
cycle.
Capsid: A protein coat surrounding the nucleic acid of a virus.
Capsomere: Protein subunit of the capsid.
Complex virus: see binal symmetry.
Cytopathic effects: The changes in the cell structure resulting from viral
infections, which can be visualized using light microscopes, such as accumulation
of inclusion bodies in the cytoplasm or nucleus, ballooning of the cells or formation
of multinucleated giant cells.
Envelope: A membranous layer composed of phospholipids, carbohydrates and
proteins that covers the nucleocapsid in some viruses. These viruses are called
enveloped viruses.
Enveloped viruses: see envelope.
Helical virus: Virus with a helical capsid surrounding its genome.
Icosahedral: A type of viral symmetry composed of 20 equilateral triangular faces
and twelve corners.
Lysogeny: A state in which viral genome is integrated into the host genome and
the virus replicates along with the host cell.
Lytic cycle: A life cycle pattern in virus that results in lysis of the host cell.
Microorganism: An organism that cannot be viewed properly through naked eye.
Mycophage: A virus infecting fungi.
Phycophage: A virus-infecting algae.
Plaque: A clear area in a bacterial lawn or animal cell monolayer that is produced
due to destruction of cells by viruses.
Prion: A proteinaceous infectious particle that lacks genome and causes slow
progressive neurological diseases.
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General Characteristics of Different Types of Acellular Microorganisms
Proviral DNA: Viral DNA integrated into the host genome.
Retroviruses: Viruses having ssRNA genome that replicates through a DNA
intermediate by using RNA dependent DNA polymerase or reverse transcriptase.
Segmented genome: A viral genome that is composed of more than one
fragment or segment, where each fragment carries different genetic information
e.g. influenza virus genome.
Virion: The extracellular phase of the life cycle of a virus.
Viroid: An infectious agent composed of only ssRNA which is circular and does not
code for any protein, infects plants.
References
Books
1.
Prescott, Harley & Klein (2011). Microbiology. 8 th edition, The McGraw-Hil
Companies. Inc.
2.
Pelczar MJ, Chan ECS &Krieg NR (2003). Microbiology: Concepts and
Applications. 5th edition, The McGraw-Hil Companies. Inc.
3.
Madigan MT, Martinko JM, Dunlap PV & Clark DP (2009).Brock Biology of
Microorganisms. 12th edition, Pearson Prentice Hall.
Web Links
Science.howstuffworks.com/life/cellular-microscopic/virius-human2.htm
https://discovery.wisc.edu/.../deacea6d-6172-4e93-97a7-404e19f5809c
http://www.els.net/WileyCDA/ElsArticle/refId-a0000434.html
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