Escherichia) on a lab report as the cause of a patient’s
urinary tract infection. Therefore, it is important to
learn both genus and species names of important
pathogens.
NORMAL FLORA
Each of us has a natural population of microorganisms
living on and within us. This is our normal flora.
These microbes may be further categorized as resi-
dents or transients. Resident floraare those species
that live on or in nearly everyone almost all the time.
These residents live in specific sites, and we provide a
very favorable environment for them. Some, such as
Staphylococcus epidermidis, live on the skin. Others, such
as E. coli, live in the colon and small intestine. When
in their natural sites, resident flora do not cause harm
to healthy tissue, and some are even beneficial to us.
However, residents may become pathogenic if they
are introduced into abnormal sites. If E. coli, for exam-
ple, gains access to the urinary bladder, it causes an
infection called cystitis. In this situation, E. coliis con-
sidered an opportunist, which is a normally harmless
species that has become a pathogen in special circum-
stances.
Transient floraare those species that are found
periodically on or in the body; they are not as well
adapted to us as are the resident flora. Streptococcus
pneumoniae, for example, is a transient in the upper res-
piratory tract, where it usually does not cause harm in
healthy people. However, transients may become path-
ogenic when the host’s resistance is lowered. In an eld-
erly person with influenza, S. pneumoniaemay invade
the lower respiratory tract and cause a serious or even
fatal pneumonia.
The distribution of our normal flora is summarized
in Table 22–1. You can see that an important function
of normal flora is to inhibit the growth of pathogens
in the oral cavity, intestines, and in women, the vagina.
The resident bacteria are believed to do this by pro-
ducing antibacterial chemicals but also by simply
being there and providing competition that makes it
difficult for pathogens to establish themselves. An
example may be helpful here. Let us use botulism.
Typical food-borne botulism is acquired by ingesting
the bacterial toxin that has been produced in food.
Infants, however, may acquire botulism by ingesting
the spores (dormant forms) of the botulism bacteria
on foods such as honey or raw vegetables. The infant’s
colon flora is not yet abundant, and the botulism
spores may be able to germinate into active cells that
produce toxin in the baby’s own intestine. For older
children or adults, botulism spores are harmless if
ingested, because the normal colon flora prevents the
growth of these bacteria.
Recent research suggests that some specific bacte-
ria in our colon flora are even more important than we
thought. One species, Bacteroides fragilis, produces a
polysaccharide, Psac, that may be necessary for the
normal development of certain aspects of the human
immune system: production of normal T cells by the
thymus gland and the proper functioning of the
spleen. We have much more to learn about our rela-
tionship with our resident flora.
Resident flora may be diminished by the use of
antibiotics to treat bacterial infections. An antibiotic
does not distinguish between the pathogen and the
resident bacteria. In such circumstances, without the
usual competition, yeasts or pathogenic bacteria may
be able to overgrow and create new infections. This is
most likely to occur on mucous membranes such as
those of the oral cavity and vagina.INFECTIOUS DISEASE
An infectious disease is one that is caused by microor-
ganisms or by the products (toxins) of microorganisms.
To cause an infection, a microorganism must enter and
establish itself in a host and begin reproducing.
Several factors determine whether a person will
develop an infection when exposed to a pathogen.
These include the virulence of the pathogen and the
resistance of the host. Virulenceis the ability of the
pathogen to cause disease. Host resistanceis the total
of the body’s defenses against pathogens. Our defenses
include aspects of innate immunity such as intact skin
and mucous membranes, and the sweeping of cilia to
clear the respiratory tract, as well as adequate nutri-
tion, and the adaptive immune responses of our lym-
phocytes and macrophages (see Chapter 14).
To illustrate these concepts, let us compare the
measles virus and rhinoviruses (common cold). The
measles virus has at least a 90% infectivity rate, mean-
ing that for every 100 non-immune people exposed,
at least 90 will develop clinical measles. Thus, the
measles virus is considered highly virulent, even
for healthy people. However, people who have recov-
ered from measles or who have received the measles
vaccine have developed an active immunity that
increases their resistance to measles. Even if exposed500 An Introduction to Microbiology and Human Disease