Bacterial Pathogens of Humans (page 3)
(This chapter has 6 pages)
© Kenneth Todar, PhD
The Gram-negative aerobic rods and cocci
This group consists of Gram-negative bacteria phenotypically related
members of the genus Pseudomonas. Their metabolism is
and never fermentative. Important human pathogens include Pseudomonas
aeruginosa, Neisseria gonorrhoeae, Neisseria meningitidis, Bordetella
Haemophilus influenzae, Legionella pneumophila, Brucella, Francisella,
and a few others. Many bacteria in this physiological group are
in soil and water, and they play an important role in decomposition,
and the C and N cycles. Also, many bacteria that are pathogens of
are found in this group, including Pseudomonas, Xanthomonas
Figure 7. Three looks at Pseudomonas,
head of the Gram-negative aerobic rods. A. Electron micrograph,
stain. B. Scanning electron micrograph. C. Gram stain.
aeruginosa is the quintessential opportunistic pathogen
of humans. It is a leading cause of hospital-acquired infections (nosocomial
infections), and it is difficult to eradicate due to its resistance to
most antimicrobial agents. There is probably no tissue that cannot
infected by Pseudomonas if the host defenses are weakened, and
is difficult to treat due to inherent and acquired resistance to
It is usually involved in soft tissue infections, urinary tract
Whooping cough (or pertussis) is caused by Bordetella
pertussis. The disease is particularly serious in infants
young children and has a high mortality rate. Whooping cough is
by vaccination with the acellular pertussis vaccine, which
usually given in association with diphtheria, tetanus and sometimes H.
influenzae type b (Hib), as part of the childhood immunization
in the U.S.
Legionnaires' pneumonia is caused by Legionella
This pneumonia, and the bacterium, were not discovered until 1976, when
there was an outbreak of disease at a Legionnaire's meeting in
It took several months to find, culture and grow the bacterium. The
was a wake-up call to public health officials that there were probably
a lot of disease-producing bacteria in the environment that they know
gonorrhoeae causes the sexually-transmitted disease gonorrhea,
meningitidis is the agent of meningococcal meningitis.
The neisseriae are discussed below with the pyogenic cocci.
influenzae is also a cause of meningitis, but the
of the disease has declined rapidly with the use of the Hib vaccine
began in 1994. Haemophilus is sometimes involved in infections
the upper respiratory tract, particularly the sinuses.
Brucellosis is a chronic debilitating infection in humans
with reproductive failure in domestic animals. Person-to-person
of brucellae is extremely rare. Brucella abortus is the
usually involved in human disease. The primary reservoir of the
is in cattle, although bison are sometimes wrongfully accused.
bacteria are Gram-negative rods with facultative anaerobic
that live in the intestinal tracts of animals in health and disease.
group consists of Escherichia coli and its relatives,
members of the family Enterobacteriaceae. Enteric
are related phenotypically to several other genera of bacteria such as
and Vibrios. Generally, a distinction can be made on the ability
to ferment glucose; enteric bacteria all ferment glucose to acid end
while similar Gram-negative bacteria (e.g. pseudomonads) cannot ferment
glucose. Because they are consistent members of the normal
flora humans, and because of their medical importance, an
large number of enteric bacteria have been isolated and characterized.
Escherichia coli is, of course, the type species of
E. coli is such a regular inhabitant of the intestine
of humans that it is used by public health authorities as an indicator
of fecal pollution of drinking water supplies, swimming beaches, foods,
etc. E. coli is the most studied of all organisms in biology
of its natural occurrence and the ease and speed of growing the
the laboratory. It has been used in hundreds of thousands of
in cell biology, physiology, and genetics, and was among the first
for which the entire chromosomal DNA base sequence (genome) was
In spite of the knowledge gained about the molecular biology, genetics
of E. coli, surprisingly little is known about its ecology, for
example, why it consistently associates with humans, how it helps its
how it harms its host, etc. A few strains of E. coli are
(one is now notorious, strain 0157:H7, that has been found to
hamburger, vegetables, unpasteurized milk and drinking water) .
coli causes intestinal tract infections
acute and uncomplicated, except in the very young) or uncomplicated
urinary tract infections and neonatal meningitis.
Figure 8. E. coli
© David E. Graham. Virginia Polytechnic Institute and State
Blacksburg, Virginia. Image by William Ghiorse, Department of
Cornell University, Ithaca, New York. Licensed for use by ASM Microbe
is a phase contrast image of cells immobilized on an agar-coated slide.
The enteric group includes two other important some other
intestinal pathogens of
Shigella dysenteriae causes bacillary
dysentery: Salmonella enterica, causes food
poisoning and gastroenteritis.
typhi, which infects via the intestinal route, causes typhoid
Some bacteria that don't have an intestinal habitat resemble
coli in enough ways to warrant inclusion in the enteric group. This
includes Proteus, a common saprophyte of decaying
matter and Yersinia pestis, which causes
Also classified as an enteric is Erwinia, a
of plants that causes fireblight in pear and apple trees and soft rot
carrots and potatoes.