WORLD OF MICROBIOLOGY AND IMMUNOLOGY Microbial genetics381•
The flora of the gastrointestinal tract in animals has
been studied intensively. These studies have demonstrated that
bacteria are the most numerous microbes present in the stom-
ach and gastrointestinal tract. The composition of the bacterial
populations varies from animal to animal, even within a
species. Sometimes the diet of an animal can select for the
dominance of one or a few bacteria over other species. The sit-
uation is similar in humans. Other factors that influence the
bacterial make up of the human stomach and gastrointestinal
tract include age, cultural conditions, and the use of antibi-
otics. In particular, the use of antibiotics can greatly change
the composition of the gastrointestinal flora.
Despite the variation in bacterial flora, the following
bacteria tend to be present in the gastrointestinal tract of
humans and many animals: Escherichia coli, Clostridium per-
fringens, Enterococci, Lactobacilli, and Bacteroides.
The esophagus is considered to be part of the gastroin-
testinal tract. In this region, the bacteria present are usually
those that have been swallowed with the food. These bacteria
do not normally survive the journey through the highly acidic
stomach. Only bacteria that can tolerate strongly acidic envi-
ronments are able to survive in the stomach. One bacterium
that has been shown to be present in the stomach of many peo-
ple is Helicobacter pylori. This bacterium is now known to be
the leading cause of stomach ulcers. In addition, very con-
vincing evidence is mounting that links the bacterium to the
development of stomach and intestinal cancers.
In humans, the small intestine contains low numbers of
bacteria, some 100,000 to 10 million bacteria per milliliter of
fluid. To put these numbers into perspective, a laboratory liq-
uid culturethat has attained maximum bacterial numbers will
contain 100 million to one billion bacteria per milliliter. The
bacterial flora of this region consists mostly of lactobacilli and
Enterococcus faecalis. The lower regions of the small intestine
contain more bacteria and a wider variety of species, includ-
ing coliform bacteria such as Escherichia coli.
In the large intestine, the bacterial numbers can reach 100
billion per milliliter of fluid. The predominant species are anaer-
obic bacteria, which do not grow in the presence of oxygen.
These include anaerobic lactic acid bacteria, Bacteroides, and
Bifidobacterium bifidum. The bacteria numbers and composi-
tion in the large intestine is effectively that of fecal material.
The massive numbers of bacteria in the large intestine
creates a great special variation in the flora. Sampling the
intestinal wall at different locations will reveal differences in
the species of bacteria present. As well, sampling any given
point in the intestine will reveal differences in the bacterial
population at various depths in the adherent growth on the
intestinal wall.
Some bacteria specifically associate with certain cells in
the gastrointestinal tract. Gram-positive bacteria such as strep-
tococciand lactobacilli often adhere to cells by means of cap-
sules surrounding the bacteria. Gram-negative bacteria such as
Escherichia colican adhere to receptors on the intestinal
epithelial cells by means of the bacterial appendage called
fimbriae.
The importance of the microbial flora of the stomach
and gastrointestinal tract has been demonstrated by compari-son of the structure and function of the digestive tracts of nor-
mal animals and notobiotic animals. The latter animals lack
bacteria. The altered structure of the intestinal tract in the
notobiotic animals is less efficient in terms of processing food
and absorbing nutrients. Additionally, in animals like cows
that consume cellulose, the fermentationactivity of intestinal
microorganisms is vital to digestion. Thus, the flora of the
stomach and intestinal tract is very important to the health of
animals including humans.See alsoEnterobacteriaceae; Probiotics; Salmonella food
poisoningMMicrobial geneticsICROBIAL GENETICS
Microbial genetics is a branch of genetics concerned with the
transmission of hereditary characters in microorganisms.
Within the usual definition, microorganisms include prokary-
otes like bacteria, unicellular or mycelial eukaryotese.g.,
yeasts and other fungi, and viruses, notably bacterial viruses
(bacteriophages). Microbial genetics has played a unique role
in developing the fields of molecular and cell biology and also
has found applications in medicine, agriculture, and the food
and pharmaceutical industries.
Because of their relative simplicity, microbes are ideally
suited for combined biochemical and genetic studies, and have
been successful in providing information on the genetic code
and the regulation of geneactivity. The operonmodel formu-
lated by French biologists François Jacob(1920– ) and
Jacques Monod(1910–1976) in 1961, is one well known
example. Based on studies on the induction of enzymesof lac-
tose catabolism in the bacterium Escherichia coli,the operon
has provided the groundwork for studies on gene expression
and regulation, even up to the present day. The many applica-
tions of microbial genetics in medicine and the pharmaceuti-
cal industry emerge from the fact that microbes are both the
causes of disease and the producers of antibiotics. Genetic
studies have been used to understand variation in pathogenic
microbes and also to increase the yield of antibiotics from
other microbes.
Hereditary processes in microorganisms are analogous
to those in multicellular organisms. In both prokaryotic and
eukaryotic microbes, the genetic material is DNA; the only
known exceptions to this rule are the RNAviruses. Mutations,
heritable changes in the DNA, occur spontaneously and the
rate of mutation can be increased by mutagenic agents. In
practice, the susceptibility of bacteria to mutagenic agents has
been used to identify potentially hazardous chemicals in the
environment. For example, the Ames test was developed to
evaluate the mutagenicity of a chemical in the following way.
Plates containing a medium lacking in, for example, the nutri-
ent histidine are inolculated with a histidine requiring strain of
the bacterium Salmonella typhimurium.Thus, only cells that
revert back to the wild type can grow on the medium. If plates
are exposed to a mutagenic agent, the increase in the number
of mutantscompared with unexposed plates can be observed
and a large number of revertants would indicate a strong muta-womi_M 5/7/03 7:52 AM Page 381