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Mechanisms of Bacterial Pathogenicity (page 1)
(This chapter has 8 pages)
© Kenneth Todar, PhD
Introduction
A pathogen is a microorganism that is able to cause disease
in
a plant, animal or insect. Pathogenicity is the ability to
produce
disease in a host organism. Microbes express their pathogenicity by
means
of their virulence, a term which refers to the degree of
pathogenicity
of the microbe. Hence, the determinants of virulence of a
pathogen
are any of its genetic or biochemical or structural
features that enable it to produce disease in a host.
The relationship between a host and a pathogen is dynamic, since
each
modifies the activities and functions of the other. The outcome of such
a relationship
depends on the virulence of the pathogen and the relative degree
of resistance or susceptibility of the host, due mainly to the
effectiveness
of the host defense mechanisms.
Staphylococcus aureus, arguably
the most prevalent pathogen of humans, may cause up to one third of all
bacterial diseases ranging from boils and pimples to food poisoning, to
septicemia and toxic shock. Electron
micrograph from
Visuals Unlimited,
with permission.
The Underlying Mechanisms of Bacterial
Pathogenicity
Two broad qualities of pathogenic bacteria underlie the means by which
they cause disease:
1. Invasiveness is the ability to invade tissues. It encompasses
mechanisms for colonization (adherence and
initial
multiplication), production of
extracellular substances which facilitate
invasion (invasins) and ability to bypass or overcome host defense
mechanisms.
2. Toxigenesis
is the ability to produce toxins. Bacteria may produce two types of
toxins called exotoxins and endotoxins.
Exotoxins
are released from bacterial cells and may act at tissue sites removed
from
the site of bacterial growth. Endotoxins are cell-associated
substance. (In a classic sense, the term endotoxin refers to the
lipopolysaccharide component of the outer membrane of Gram-negative
bacteria).
However, endotoxins may be released from growing bacterial cells and
cells that are lysed as a result of effective host defense (e.g.
lysozyme)
or the activities of certain antibiotics (e.g. penicillins and
cephalosporins).
Hence, bacterial toxins, both soluble and cell-associated, may be
transported
by blood and lymph and cause cytotoxic effects at tissue sites remote
from
the original point of invasion or growth. Some bacterial toxins may
also
act at the site of colonization and play a role in invasion.
Acid-fast stain of Mycobacterium tuberculosis, the
agent of tuberculosis (TB). The bacteria are the small pink-staining
rods. More than one-third of the world population is infected.
The organism has caused more human deaths than any other bacterium in
the history of mankind. Although its ability to produce disease is
multifactorial, it is not completely understood. American Society of
Microbiology, with permission.
chapter continued
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