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
to
members of the genus Pseudomonas. Their metabolism is
respiratory
and never fermentative. Important human pathogens include Pseudomonas
aeruginosa, Neisseria gonorrhoeae, Neisseria meningitidis, Bordetella
pertussis,
Haemophilus influenzae, Legionella pneumophila, Brucella, Francisella,
and a few others. Many bacteria in this physiological group are
free-living
in soil and water, and they play an important role in decomposition,
biodegradation,
and the C and N cycles. Also, many bacteria that are pathogens of
plants
are found in this group, including Pseudomonas, Xanthomonas
and Agrobacterium.
Figure 7. Three looks at Pseudomonas,
the
head of the Gram-negative aerobic rods. A. Electron micrograph,
negative
stain. B. Scanning electron micrograph. C. Gram stain.
Pseudomonas
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
become
infected by Pseudomonas if the host defenses are weakened, and
it
is difficult to treat due to inherent and acquired resistance to
antimicrobial
agents.
It is usually involved in soft tissue infections, urinary tract
infections
and pneumonia.
Whooping cough (or pertussis) is caused by Bordetella
pertussis. The disease is particularly serious in infants
and
young children and has a high mortality rate. Whooping cough is
controlled
by vaccination with the acellular pertussis vaccine, which
is
usually given in association with diphtheria, tetanus and sometimes H.
influenzae type b (Hib), as part of the childhood immunization
program
in the U.S.
Legionnaires' pneumonia is caused by Legionella
pneumophila.
This pneumonia, and the bacterium, were not discovered until 1976, when
there was an outbreak of disease at a Legionnaire's meeting in
Philadelphia.
It took several months to find, culture and grow the bacterium. The
incident
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
nothing
about.
Neisseria
gonorrhoeae causes the sexually-transmitted disease gonorrhea,
and Neisseria
meningitidis is the agent of meningococcal meningitis.
The neisseriae are discussed below with the pyogenic cocci.
Haemophilus
influenzae is also a cause of meningitis, but the
incidence
of the disease has declined rapidly with the use of the Hib vaccine
which
began in 1994. Haemophilus is sometimes involved in infections
of
the upper respiratory tract, particularly the sinuses.
Brucellosis is a chronic debilitating infection in humans
associated
with reproductive failure in domestic animals. Person-to-person
transmission
of brucellae is extremely rare. Brucella abortus is the
species
usually involved in human disease. The primary reservoir of the
organism
is in cattle, although bison are sometimes wrongfully accused.
Enterics
Enteric
bacteria are Gram-negative rods with facultative anaerobic
metabolism
that live in the intestinal tracts of animals in health and disease.
This
group consists of Escherichia coli and its relatives,
the
members of the family Enterobacteriaceae. Enteric
bacteria
are related phenotypically to several other genera of bacteria such as
Pseudomonas
and Vibrios. Generally, a distinction can be made on the ability
to ferment glucose; enteric bacteria all ferment glucose to acid end
products
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
extremely
large number of enteric bacteria have been isolated and characterized.
Escherichia coli is, of course, the type species of
the
enterics.
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
because
of its natural occurrence and the ease and speed of growing the
bacterium in
the laboratory. It has been used in hundreds of thousands of
experiments
in cell biology, physiology, and genetics, and was among the first
cells
for which the entire chromosomal DNA base sequence (genome) was
determined.
In spite of the knowledge gained about the molecular biology, genetics
and
physiology
of E. coli, surprisingly little is known about its ecology, for
example, why it consistently associates with humans, how it helps its
host,
how it harms its host, etc. A few strains of E. coli are
pathogenic
(one is now notorious, strain 0157:H7, that has been found to
contaminate raw
hamburger, vegetables, unpasteurized milk and drinking water) .
Escherichia
coli causes intestinal tract infections
(usually
acute and uncomplicated, except in the very young) or uncomplicated
urinary tract infections and neonatal meningitis.
Figure 8. E. coli
O157.H7.
© David E. Graham. Virginia Polytechnic Institute and State
University,
Blacksburg, Virginia. Image by William Ghiorse, Department of
Microbiology,
Cornell University, Ithaca, New York. Licensed for use by ASM Microbe
Library
http://www.microbelibrary.org.This
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
humans: Salmonella
and Shigella.
Shigella dysenteriae causes bacillary
dysentery: Salmonella enterica, causes food
poisoning and gastroenteritis.
Salmonella
typhi, which infects via the intestinal route, causes typhoid
fever.
Some bacteria that don't have an intestinal habitat resemble
E.
coli in enough ways to warrant inclusion in the enteric group. This
includes Proteus, a common saprophyte of decaying
organic
matter and Yersinia pestis, which causes
bubonic plague.
Also classified as an enteric is Erwinia, a
pathogen
of plants that causes fireblight in pear and apple trees and soft rot
of
carrots and potatoes.
chapter continued
Previous Page