The Normal Bacterial Flora of Humans (page 4)
(This chapter has 5 pages)
© Kenneth Todar, PhD
Beneficial
Effects of the Normal Flora
The effects of the normal flora are inferred by microbiologists from
experimental comparisons between "germ-free"
animals (which are not
colonized by any microbes) and conventional animals (which are
colonized with a typical normal flora). Briefly, some of the
characteristics of
a germ-free animals that are thought to be due to lack of exposure to a
normal flora are:
1. vitamin deficiencies, especially vitamin K and vitamin B12
2. increased susceptibility to infectious disease
3. poorly developed immune system, especially in the gastrointestinal
tract
4. lack of "natural antibody" or natural immunity to bacterial infection
Because these conditions in germ-free mice and hamsters do not occur
in conventional animals, or are alleviated
by introduction of a bacterial flora (at the appropriate time of
development), it is tempting to conclude that the
human normal flora make similar contributions to human nutrition,
health and
development. The overall beneficial effects of microbes are summarized
below.
1. The normal flora synthesize and
excrete
vitamins in excess of their own needs, which can be absorbed
as nutrients by their host. For example, in humans, enteric bacteria
secrete Vitamin
K and Vitamin B12, and lactic acid bacteria produce certain B-vitamins.
Germ-free animals may be deficient in Vitamin K to the extent that it
is
necessary to supplement their diets.
2. The normal flora prevent
colonization
by
pathogens by competing for
attachment
sites or for essential nutrients. This is thought to be their
most
important beneficial effect, which has been demonstrated in the oral
cavity,
the intestine, the skin, and the vaginal epithelium. In some
experiments,
germ-free animals can be infected by 10 Salmonella bacteria,
while
the infectious dose for conventional animals is near 106
cells.
3. The
normal flora may antagonize
other
bacteria
through the production of substances which inhibit or kill
nonindigenous
species. The intestinal bacteria produce a variety of substances
ranging
from relatively nonspecific fatty acids and peroxides to highly
specific
bacteriocins, which inhibit or kill other bacteria.
4. The normal flora
stimulate the
development
of certain tissues, i.e., the caecum and certain lymphatic
tissues
(Peyer's patches) in the GI tract. The caecum of germ-free animals is
enlarged,
thin-walled, and fluid-filled, compared to that organ in
conventional
animals. Also, based on the ability to undergo immunological
stimulation,
the intestinal lymphatic tissues of germ-free animals are
poorly-developed
compared to conventional animals.
5. The
normal flora stimulate the
production
of natural antibodies.
Since the normal flora behave
as antigens in an animal, they induce an immunological response, in
particular,
an antibody-mediated immune (AMI) response. Low levels of
antibodies
produced against components of the normal flora are known to cross
react
with certain related pathogens, and thereby prevent infection or
invasion.
Antibodies produced against antigenic components of the normal flora
are
sometimes referred to as "natural" antibodies, and such antibodies are
lacking in germ-free animals.
Harmful
Effects of the Normal Flora
Harmful effects of the normal flora, some of which are observed
in
studies with germ-free animals, can be put in the following categories.
All but the last two are fairly insignificant.
1. Bacterial
synergism between a member of the normal flora and a potential
pathogen. This means that one organism is helping another to grow or
survive. There are examples of a member of the normal flora supplying a
vitamin or some other growth factor that a pathogen needs in order to
grow. This is called cross-feeding
between microbes. Another example of
synergism occurs during treatment of "staph-protected
infections" when a penicillin-resistant staphylococcus that is a
component
of the normal flora shares its drug resistance with pathogens that are
otherwise susceptible to the drug.
2. Competition
for nutrients Bacteria in the gastrointestinal tract must absorb
some of the host's nutrients for their own needs. However, in general,
they transform them into other metabolisable compounds, but some
nutrient(s) may be lost to the host. Germ-free animals are known to
grow more rapidly and efficiently than conventional
animals. One explanation for incorporating
antibiotics into the food of swine, cows and poultry is that the animal
grows
faster and can therefore be marketed earlier. Unfortunately, this
practice contributes to the development and spread of bacterial
antibiotic resistance within the farm animals, as well as humans.
3. Induction of a low grade
toxemia Minute amounts of bacterial toxins (e.g.
endotoxin)
may be
found in the circulation. Of course, it is these small amounts of
bacterial antigen that stimulate the formation of
natural antibodies.
4. The normal
flora may be agents of disease. Members of the normal flora may
cause endogenous disease if
they reach a site or tissue where they cannot be restricted or
tolerated by
the host defenses. Many of the normal flora are potential
pathogens, and if
they gain access to a compromised tissue from which they
can invade, disease may result.
5. Transfer to susceptible hosts
Some pathogens of humans that are members of the normal flora may also
rely on their host for transfer to other individuals where they can
produce disease. This includes the pathogens that colonize the upper
respiratory tract such as Neisseria
meningitidis, Streptococcus
pneumoniae, Haemophilus influenzae and Staphylococcus aureus, and
potential pathogens such as E. coli,
Salmonella or Clostridium
in the gastrointestinal tract.
chapter continued
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