The Normal Bacterial Flora of Humans (page 2)
(This chapter has 5 pages)
© Kenneth Todar, PhD
Between Humans and the Normal Flora
E. coli is the best known
bacterium that regularly associates itself with humans, being an
invariable component of the human intestinal tract. Even though E. coli is the
most studied of all bacteria, and we know the exact location and
sequence of 4,288 genes on its chromosome, we do not fully understand
its ecological relationship with humans.
In fact, not much is known about the
nature of the associations between
and their normal flora, but they are thought to be dynamic interactions
rather than associations of mutual indifference. Both host and
are thought to derive benefit from each other, and the
are, for the most part, mutualistic. The normal flora
from their host a steady supply of nutrients, a stable environment, and
protection and transport. The host obtains from the
flora certain nutritional and digestive benefits, stimulation of the
development and activity of immune system,
and protection against colonization and infection by pathogenic
While most of the activities of the normal flora benefit their host,
some of the normal flora are parasitic
(live at the expense of
their host), and some are pathogenic
(capable of producing disease). Diseases that are produced by the
normal flora in their host may be called endogenous diseases. Most
endogenous bacterial diseases are opportunistic
infections, meaning that the the organism must be given a
special opportunity of weakness or let-down in the host defenses in
order to infect. An example of an opportunistic infection is chronic
bronchitis in smokers wherein normal flora bacteria are able to invade
Sometimes the relationship between a member of the normal flora an its
host cannot be
deciphered. Such a relationship where there is no apparent benefit or
harm to either organism during their association is referred to as a commensal relationship. Many of the
normal flora that are not predominant in their habitat, even
always present in low numbers, are thought of as commensal bacteria.
However, if a presumed commensal relationship is studied in detail,
parasitic or mutualistic characteristics often emerge.
Most members of the normal bacterial flora prefer to colonize
certain tissues and not others. This "tissue specificity" is
usually due to properties of both the host and the bacterium. Usually,
specific bacteria colonize specific tissues by one or another of these
1. Tissue tropism is the
bacterial preference or predilection for certain tissues for growth.
tissue tropism is that the host provides essential nutrients and growth
factors for the bacterium, in addition to suitable
oxygen, pH, and temperature for growth.
acidophilus, informally known as "Doderlein's bacillus"
colonizes the vagina because glycogen is produced which provides the
bacteria with a source of sugar that they ferment to lactic acid.
2. Specific adherence
bacteria can colonize a specific
tissue or site because they can adhere to that tissue or site in a
specific manner that involves complementary chemical interactions
between the two surfaces. Specific adherence involves biochemical
interactions between bacterial surface
(ligands or adhesins) and host cell molecular receptors.
The bacterial components that provide adhesins are molecular parts of
their capsules, fimbriae, or cell walls. The receptors on human cells
or tissues are usually glycoprotein molecules located on the host
cell or tissue surface.
adherence involves complementary chemical interactions between the host
cell or tissue surface and the bacterial surface. In the language
of medical microbiologist, a bacterial "adhesin" attaches covalently to
a host "receptor" so that the bacterium "docks" itself on the host
surface. The adhesins of bacterial cells are chemical components of
capsules, cell walls, pili or fimbriae. The host receptors are usually
glycoproteins located on the cell membrane or tissue surface.
Some examples of adhesins and attachment sites used for specific
adherence to human tissues are described in the table below.
2. Examples of bacterial specific
adherence to host cells or tissue.
bound protein (Glycosyl transferase)
epithelium of tongue
protein (choline-binding protein)
or urethral epithelium
3. Biofilm formation
Some of the indigenous bacteria are able to construct biofilms on a tissue surface,
or they are able to colonize a
built by another bacterial species. Many biofilms are a mixture
microbes, although one member is responsible for maintaining the
and may predominate.
depicting biofilm formation. Biofilms usually occur when one
bacterial species attaches specifically or non specifically to a
surface, and then secretes carbohydrate slime (exopolymer) that
imbeds the bacteria and attracts other microbes to the biofilm for
protection or nutritional advantages.
The classic biofilm that involves components of the normal flora of the
oral cavity is the formation of dental plaque on the teeth. Plaque
is a naturally-constructed biofilm, in which the consortia of bacteria
may reach a thickness of 300-500 cells on the surfaces of the teeth.
accumulations subject the teeth and gingival tissues to high
of bacterial metabolites, which result in dental disease.