From: pautrey on
The Human Normal (Commensal) Flora

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Microorganisms in Health and Disease

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A wide variety of microorganisms interact with humans, exploiting a
wealth of micro-environments. Certain parts of the body are sterile.
These include the solid organs, blood, cerebrospinal fluid, and urine.
The lower respiratory tract in health is also sterile. This sterility
in an open system is due to the action of the "mucociliary escalator"
that sweeps foreign matter back out of the lungs. Exposed sites are
colonised by large numbers of microorganisms that are highly adapted
to their particular environment. These organisms constitute the body's
normal flora.




In diagnostic microbiology, it is important to distinguish between a
patient's normal flora, and the causative agent of the infection from
which they are suffering. This is not always an easy process, since in
some cases microbes are normal flora at certain anatomical locations,
but are considered pathogens when isolated from other sites.




Most of the microbes discussed are bacteria, although some fungi and
certain protozoa also contribute the human normal flora. Viruses are a
special case. There is still debate concerning the nature of viruses,
and certainly they are structures that are incapable of autonomous
existence. In many cases virus infection is entirely to the detriment
of the host. However, certain viruses have the ability to form latent
infections. A good example of a virus that causes a latent infection
is the herpesvirus herpes simplex 1 or HSV 1. This virus infects the
vast majority of humans some time during early childhood, and often
the primary infection is inapparent. It infects the mucous membranes
around the mouth, and the virus spreads to the trigeminal ganglion.
There it enters a latent phase. The virus DNA persists, but there is
little or no production of novel virus particles. When the host is
subjected to stresses of various kinds, the latent virus can undergo a
re-activation. New virus particles are produced, and these spread back
to the mucous membranes around the mouth, where they erupt to cause
the characteristic "cold sores", suffered by a number of people.
Certain individuals are more prone to develop cold sores than others
are, but the majority of people world-wide carry HSV 1 as a latent
infection. Because it is so widespread, and because, at least for the
majority of the time it causes so few problems, it has been argued
that HSV 1 and other latent viruses may be considered as "commensal"
with humans.


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Microorganisms in Health

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To help you to interpret the information below, there is an
explanation of the Gram staining technique and also a description of
how cultures are plated to separate bacterial colonies. You should now
try to discover what comprises the human normal flora at various body
sites.

Read More:
http://www.bmb.leeds.ac.uk/mbiology/ug/ugteach/icu8/flora/cases.html