Bacterial Pathogens of Humans (page 5)
(This chapter has 6 pages)
© Kenneth Todar, PhD
Endospore-forming bacteria produce a unique resting cell called an endospore.
They are Gram-positive and usually rod-shaped, but there are
The two medically-important genera are Bacillus,
the members of which are aerobic sporeformers in the soils, and Clostridium,
species are anaerobic sporeformers of soils, sediments and the
tracts of animals.
Figure 10. Endospore-forming
bacilli (phase contrast illumination). Endospores are dehydrated,
cells appearing as points of bright light under phase microscopy.
bacteria are characterized by the location (position) of the endospore
in the mother cell (sporangium) before its release. The spore may be
terminal or subterminal, and the sporangium may or may not be swollen
accommodate the spore.
Some sporeformers are pathogens of animals, usually due to the
of powerful toxins. Bacillus
anthracis causes anthrax,
a disease of domestic animals (cattle, sheep, etc.), which may be
transmitted to humans. Bacillus
cereus causes food poisoning. Clostridium
botulimum causes botulism, a form of food
tetani is the agent of tetanus. Clostridium
causes food poisoning, anaerobic wound infections and gas
gangrene, and Clostridium difficile causes a severe
of colitis called pseudomembranous colitis. Whenever the
act as pathogens, it is not uncommon or surprising that their spores
somehow involved in transmission or survival of the organism between
Figure 11. Robert Koch's
photomicrographs of Bacillus anthracis. In 1876, Koch
by careful microscopy that the bacterium was always present in the
of animals that died of anthrax. He took a small amount of blood from
an animal and injected it into a healthy mouse, which subsequently
diseased and died. He took blood from that mouse and injected it into a
another healthy mouse. After repeating this several times he was able
recover the original anthrax organism from the dead mouse,
for the first time that a specific bacterium is the cause of a specific
disease. In so doing, he established Koch's Postulates, which still
supply the microbiological standard to demonstrate that a specific
is the cause of a specific disease.
Listeria monocytogenes is a Gram-positive rod-shaped
related to Bacillus and Clostridium, but it does not form
monocytogenes is the agent of listeriosis, a
infection caused by eating food contaminated with the bacteria.
has recently been recognized as an important public health problem in
United States. The disease affects primarily pregnant women, newborns,
and adults with weakened immune systems.
Figure 12. Listeria
Actinomycetes and related bacteria
The actinomycetes are not thought of as pathogenic bacteria,
but two of their relatives are among the most important pathogens of
these being the agents of tuberculosis and diphtheria.
Actinomycetes are a large group of Gram-positive bacteria that usually
grow by filament formation, or at least show a tendency towards
and filament formation. Many of the organisms can form resting
called spores, but they are not the same as endospores. Branched forms
superficially resemble molds and are a striking example of convergent
of a procaryote and a eucaryote together in the soil habitat.
such as Streptomyces have a world-wide distribution in
They are important in aerobic decomposition of organic compounds and
an important role in biodegradation and the carbon cycle. Actinomycetes
are the main producers of antibiotics in industrial settings, being the
source of most tetracyclines, macrolides (e.g. erythromycin), and
(e.g. streptomycin, gentamicin, etc.).
Two genera of bacteria that are related to the actinomycetes,
and Mycobacterium, contain important pathogens of
many nonpathogenic mycobacteria and corynebacteria live in normal
tuberculosis is the etiologic agent of tuberculosis
(TB) in humans. Tuberculosis is the leading cause of death in
world from a single infectious disease. Mycobacterium
infects 1.7 billion people/year which is equal to 33% of the entire
population. The bacterium is responsible for over 3 million
After a century of decline in the United States, cases of tuberculosis
have increased slightly, and multiple drug-resistant strains have
in cases is attributable to changes in the social structure in cities,
the HIV epidemic, and patient failure to comply with treatment
A related organism, Mycobacterium leprae, causes leprosy.
Figure 13. Mycobacterium
tuberculosis Acid-fast stain. 1000X magnification. ©
J. Delisle and Lewis Tomalty, Queens University, Kingston, Ontario,
Licensed for use by ASM Microbe Library
bacteria were observed in a sputum sample from a patient with active
The genus Corynebacterium consists of a diverse group
of bacteria including animal and plant pathogens, as well as
Some corynebacteria are part of the normal flora of humans, finding a
niche in virtually every anatomic site. The best known and most widely
studied species is Corynebacterium
diphtheriae, the causal agent of diphtheria.
The study of Corynebacterium diphtheriae traces closely
development of medical microbiology, immunology and molecular biology.
Many contributions to these fields, as well as to our understanding of
host-bacterial interactions, have been made studying diphtheria and the
Rickettsias and chlamydiae are
unrelated groups of bacteria that are obligate intracellular
eucaryotic cells. Rickettsias cannot grow outside of a host
because they have leaky membranes and are unable to obtain nutrients in
an extracellular habitat. Chlamydiae are unable to produce ATP
amounts required to sustain metabolism outside of a host cell and are,
in a sense, energy-parasites.
Rickettsias occur in nature in the gut lining of arthropods
fleas, lice, etc.). They are transmitted to vertebrates by an arthropod
bite and produce diseases such as typhus
Spotted Fever, Q fever and ehrlichiosis. See Rickettsial
Diseases, including Rocky Mountain Spotted Fever.
Chlamydiae are tiny bacteria that infect birds and mammals.
may colonize and infect tissues of the eye and urogenital tract in
trachomatis causes several important diseases in humans: chlamydia,
the most prevalent sexually transmitted disease in the U.S., trachoma,
a leading cause of blindness worldwide, and lymphogranuloma venereum.
Chlamydia pneumoniae is a cause of pneumonia and has
been recently linked to atherosclerosis.
Figure 14. Ehrlichia
© Vsevolod Popov, J. Steven Dumler, and David H. Walker.
of Texas Medical Branch at Galveston. Licensed for use by ASM Microbe
Ehrlichia are obligate intracellular parasites related to the
rickettsiae that are tick-borne pathogens of dogs and humans. In
they cause human granulocytic ehrlichiosis (HGE) and human monocytic
(HME). In this electron micrograph, dense-core cells of E.
seen exiting the host cell following rupture of the cytoplasmic
The ehrlichiae will now go on to infect additional host cells or they
be ingested by a feeding tick, and spread to another animal.
Mycoplasmas are a group of
that lack a cell wall. The cells are bounded by a single triple-layered
membrane. They may be free-living in soil and sewage, parasitic
of the mouth and urinary tract of humans, or pathogens in animals and
In humans, Mycoplasma pneumoniae causes primary
pneumonia, also called walking pneumonia.