Bacterial Pathogens of Humans (page 2)
(This chapter has 6 pages)
© Kenneth Todar, PhD
The Major Groups of Bacterial Pathogens
This article deals with the major
groups of bacterial pathogens. Specific chapters in the textbook
address many individual groups or members of a group. All groups are
defined by at least one bacteriological criterion such as Gram stain,
metabolism, morphology, spore formation, etc. However, there is often
some genetic or phylogenetic relationship between members of a group.
Although we organize bacterial pathogens into natural groups for
discussion based on
criteria, rather than on the basis of affected organ, mode
or type of disease, two summary tables are provided at the end of this
reading that identify bacterial pathogens of humans on the basis of
specific bacterium, type of disease, and usual mode of transmission.
When one searches for clusters of pathogens in the Bacterial Domain of
the Tree of Life, they are found primarily among the Gram-positive
bacteria and the Gram-negative proteobacteria. Most of the bacterial
pathogens of humans are classified as Gram-positive
or Gram-negative, but some notable exceptions include the
chlamydiae, spirochetes and the mycobacteria.
The spirochetes are a phylogenetically distinct group of bacteria
have a unique cell morphology and mode of motility. Spirochetes are
thin, flexible, spiral-shaped bacteria that move by means of
called axial filaments or endoflagella. The flagellar filaments are
within a sheath between the cell wall peptidoglycan and an outer
The filaments flex or rotate within their sheath, which causes the
to bend, flex and rotate during movement. Most spirochetes are free
(in muds and sediments), or live in associations with animals (e.g. in
the oral cavity or GI tract). A few are pathogens of animals,
occasionally transmitted to humans (e.g. leptospirosis). The two major
pathogens of humans are Treponema
pallidum, the agent of syphilis,
a sexually transmitted disease, and Borrelia burgdorferi,
cause of Lyme
Disease, transmitted by the bite of the deer tick.
Figure 4. Spirochetes: A.
section of a spirochete showing the location of endoflagella between
inner membrane and outer sheath; B.
Borrelia burgdorferi, the agent
of Lyme disease; C. Treponema pallidum, the spirochete that
Spirilla and other curved bacteria
Spirilla are Gram-negative bacteria with a helical or spiral shape.
Their metabolism is respiratory and never fermentative. Unlike
they have a rigid cell wall and are motile by means of ordinary polar
Two important pathogens of humans are found among the spiral forms.
jejuni is the cause of bacterial diarrhea, especially in
children. The bacterium is transmitted via contaminated food, usually
poultry or shellfish, or untreated drinking water.
pylori is able to colonize the gastric mucosal cells of
i.e., the lining of the stomach, and it has been well established as
cause of peptic
ulcers and there is strong evidence for its involvement in
Figure 5. Helicobacter
The term vibrio refers to a Gram-negative bacterium which
the cell shape of a curved rod or a comma. Members of the genus Vibrio
consists of common bacteria in aquatic environments, especially marine
They have structural and metabolic properties that overlap with both
enterics and the pseudomonads. Vibrios are facultative anaerobes (grow
or absence of O2), like enterics, but they have polar
are oxidase-positive, and degrade sugars in the same manner as the
In aquatic habitats, they overlap with the pseudomonads in their
although pseudomonads favor fresh water and vibrios prefer salt water.
Some marine vibrios are bioluminescent (they emit light) and some are
of fish, squid and other marine life. Vibrio
cholerae causes epidemic or Asiatic cholera which,
untreated, is one of the most rapidly fatal infectious diseases known.
The pathology is related to diarrheal diseases caused by the enteric
except it is relentless, and a patient can die rapidly from
The cholera toxin, which is the classic model of a bacterial
is also produced by some strains of E. coli.
Figure 6. Vibrio cholerae,
the agent of Asiatic or epidemic cholera.