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CHAPTER 28: MICROBIOLOGY
LECTURE OUTLINE
28.1 The Microbial World
Leeuwenhoek was among the first to view microscopic life forms in a drop of water. Louis
Pasteur devised the experiments that disproved the theory of spontaneous generation.
Microbiology is the study of microbes, a term that includes the viruses, bacteria, archaea, protists,
and some fungi. Many microbes have beneficial, and sometimes essential, roles to play in human
health and the biosphere.
28.2 Bacteria and Archaea
Both bacteria and archaea are prokaryotes. Prokaryotes do not have nuclei or membrane-bound
organelles.
Biology of Bacteria
Bacteria have three basic shapes: rod, spherical, and spiral. All bacterial have a plasma
membrane and a cell wall. Bacteria are differentiated by a Gram stain. Most bacteria
have a single circular chromosome located in the nucleoid region of the cytoplasm.
Bacterial Reproduction
Bacteria reproduce asexually by binary fission.
Bacterial Metabolism
Bacteria demonstrate a remarkable range of metabolic abilities. Some are
heterotrophs, some are chemoautotrophs, and some photosynthesize.
Bacterial Diseases in Humans
Most bacteria do not cause disease, but a significant number do. Pathogenic microbes
often carry genes that code for specific virulence factors that determine the type and
extent of illness they are capable of causing.
Streptococcus Infections
More different types of human disease are caused by bacteria from the genus
Streptococcus than by any other type of bacterium. Streptococcus bacteria cause
pneumonia, meningitis, middle ear infections, tooth decay, strep throat, impetigo,
rheumatic fever, and necrotizing fasciitis.
Tuberculosis
Tuberculosis is one of the leading worldwide causes of death due to infectious
disease. It is caused by Mycobacterium tuberculosis, a very slow growing
organism.
Food Poisoning
Various bacteria cause food poisoning, including Salmonella, Staphylococcus,
and Clostridium botulinum.
Chlamydia Infections
Chlamydia bacteria are responsible for trachoma and one of the most common
sexually transmitted diseases in the United States.
Drug Control of Bacterial Diseases
Antibiotics kill or inhibit bacteria by interfering with their unique metabolic
pathways. Growing resistance of bacteria to antibiotics has become a significant
problem.
Biology of Archaea
Archaea and bacteria are not close relatives. Most of the archaea studied to date live in
extreme environments.
Archaeal Structure
The plasma membrane of archaea differs markedly from those of bacteria and
eukaryotes. Archaea reproduce asexually by binary fission.
Archaeal Metabolism
Some archaea are heterotrophs, while others are autotrophs.
28.3 Protists
Protists are eukaryotes. For the most part, protists are unicellular and microscopic. Protists are
structurally diverse.
Biology and Diversity of Algae
Algae can be unicellular or form colonies or filaments. Some types are multicellular
seaweeds. Algae contain chloroplasts and perform photosynthesis. In aqueous
environments they are termed phytoplankton.
Green Algae
Green algae are believed to be closely related to the first plants. Green algae are
symbiotic with other organisms, particularly fungi and lichens.
Diatoms
Diatoms are the most numerous unicellular algae in the oceans. Diatoms have a
wide variety of elaborate shells made of silica.
Dinoflagellates
Dinoflagellates are best known for the red tide they cause when they greatly
increase in number. One species in red tides produces a very potent toxin.
Red Algae
Red algae are mainly multicellular and produce a number of useful gelling
agents.
Brown Algae
The brown algae are the conspicuous multicellular seaweeds.
Euglenoids
Euglenoids are freshwater unicellular organisms that typify the problem of
classifying protists. Many have chloroplasts, but some do not.
Biology and Diversity of Protozoans
A protozoan is a usually motile, eukaryotic, unicellular protist. This text groups the
protozoans as all heterotrophic by ingestion. In aquatic environments, they are part of the
zooplankton.
Ciliates
Ciliates are the largest group of protozoans. All of them have cilia. Paramecium
is the most widely known ciliate.
Amoeboids
Amoeboids move by pseudopods, processes that form when cytoplasm streams
forward in a particular direction. Parasitic amoeboids in the genus Entamoeba
cause amoebic dysentery.
Zooflagellates
Zooflagellates are heterotrophic protozoans that propel themselves using one or
more flagella. Most are symbiotic, and many are parasitic, causing African
sleeping sickness, Chagas disease, and epidemics of diarrhea.
Sporozoans
Sporozoans produce spores and are either intercellular or extracellular parasites.
Malaria
Malaria is the most widespread and dangerous sporozoan disease. The
malaria parasite, one of several Plasmodium species, has a complex life
cycle that involves transmission by a mosquito vector.
Other Sporozoan Diseases
Other sporozoan diseases include Toxoplasma infection and
Crytosporidium infection.
Molds as Protists
Water molds and slime molds are classified as protists.
Water Molds
Most water molds are saprotrophic. They usually live in water.
Slime Molds
Slime molds feed on dead plant material in forests and woodlands.
Cellular Slime Molds
Cellular slime molds exist as individual amoeboid cells.
28.4 Fungi
Fungi are a structurally diverse group of eukaryotes that are strict heterotrophs. Most fungi are
saprotrophs.
Biology of Fungi
The body of a fungus is composed of a mass of individual filaments called hyphae;
collectively, the mass of filaments is called a mycelium. The cell walls of fungal cells
contain chitin, not cellulose. The energy reserve of fungi is not starch, but glycogen.
Fungi produce windblown spores during both asexual and sexual reproduction.
Diversity of Fungi
Fungi are differentiated on the basis of their mode of sexual reproduction.
Zygospore Fungi
Stalks bear sporangia, capsules that produce zygospores. The common bread
mold belongs to this group.
Sac Fungi
The sac fungi are named for their characteristic cuplike sexual reproductive
structure called an ascocarp. Important sac fungi include morels and truffles, and
the original source of penicillin.
Yeasts
The term yeasts is generally applied to unicellular fungi, and many of
these organisms are sac fungi. Brewer’s yeast is a representative of
budding yeasts.
Club Fungi
Club fungi are named for their characteristic sexual reproductive structure called
a basidium. Mushrooms and shelf fungi are basidiocarps.
Environmental Aspects of Fungi
Fungi often have symbiotic relationships with other organisms.
Fungi and Photosynthesizers
Lichens are associations between fungi and cyanobacteria or green algae.
Lichens are efficient at acquiring nutrients and moisture, and therefore can
survive in poor soils as well as on rocks. Mycorrhizal fungi form mutualistic
relationships with the roots of most plants, helping them grow more successfully
in dry or poor soils.
Fungal Diseases of Plants
Smuts and rusts are club fungi that parasitize cereal crops.
Fungal Diseases of Humans
Mycoses, fungal diseases of humans, vary in levels of seriousness. Moldlike fungi cause
infections of the skin called tineas. Histoplasma capsulatum can cause a mild “fungal
flu” or serious disease. Candida albicans causes the widest variety of fungal infections.
Control of Fungi
The strong similarities between fungal cells and human cells make it difficult to design
antimicrobials against fungi.
25.5 Viruses, Viroids, and Prions
Viruses are not composed of cells, and are not alive. Viroids are strands of RNA that can
reproduce inside a cell, and prions are protein molecules that cause other proteins to become
prions.
Biology of Viruses
There is considerable variety in the structure and reproduction of viruses.
Viral Structure
Most viruses are much smaller than bacteria. They come in a variety of shapes.
A virus always has at least two parts—a capsid composed of protein subunits and
an inner core of either DNA or RNA. In some viruses, the capsid is surrounded
by a membrane called an envelope.
Viral Reproduction
Viruses are specific to a particular host. The life cycle of a typical enveloped
animal RNA virus has six steps: attachment, entry, replication, biosynthesis,
assembly, and budding.
Latency
Some animal viruses can become latent (hidden) inside the host cell.
Retroviruses have a genome of RNA, but are able to convert their
genome into DNA because they contain an enzyme called reverse
transcriptase.
Viral Diseases in Humans
Viruses cause many important human diseases. The best protection against most viral
diseases is immunization utilizing a vaccine.
The Common Cold and Influenza
Colds are most commonly caused by rhinoviruses, and the symptoms usually
include a runny nose, mild fever, and fatigue. The flu is caused by the influenza
virus and is characterized by more severe symptoms. The influenza virus can
change rapidly via antigenic drift and antigenic shift.
Measles
Measles is one of the most contagious human diseases. It is a major killer
worldwide. A vaccine exists for measles.
Herpesviruses
Herpesviruses cause chronic infections that remain latent for much of the time.
These include cold sores and fever blisters, chickenpox, and infectious
mononucleosis.
Antiviral Drugs
Because viruses use the machinery of host cells for viral replication, it is difficult to
develop drugs that affect viral replication without harming host cells. Antibiotics are not
effective against viral infections.
Viroids and Prions
Viroids and prions are also acellular pathogens. Viroid replication causes diseases in
plants, the only known hosts. Prions are proteinaceous infectious particles that cause
degenerative diseases of the nervous system in humans and other animals. These disease
include scrapie, Creutzfeldt-Jakob disease, and mad cow disease.