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Unformatted text preview: e skin is a very inhospitable environment. It is composed of a tightly packed
continuous layer of cells impregnated with the protein keratin which is hard to penetrate.
In addition, the constant production of new skin cells and the sloughing off of old dead
ones (desquamation) makes maintaining a presence on the surface of the skin difficult for
bacteria. However, the normal bacterial flora of the skin is adapted to life on the skin and
can maintain a presence even in spite of constant losses due to desquamation.
Sweat and oil glands secrete sweat and oil to surface of skin. Sweat contains salt at FOOD-302 concentrations that can inhibit growth of many organisms; it also contains the lysozyme
which can degrade peptidoglycan leading to cell lysis. Recently it has been found that
sweat glands produce small peptides called cathelcidins ( Latin catharticus, to purge,
and cida to kill). These molecules help protect against infections by inserting into
microbial cytoplasmic membranes forming pores which leads to cell lysis.
Over most of the body, sweat evaporates quickly so most of the skin is a very dry
place. However, moisture is retained between the legs, under arms and in the genital and
anal regions etc. This is also where oil glands associated with hair follicles are found.
Glandular secretions associated with hair follicles contain urea, amino acids, salts,
lactic acid and oils (types of lipids) at a pH between 4-6. These moist/oily areas of
skin are where the majority of normal flora is located. There can be 200,000 to 1 million
organisms/cm2 in the moist/oily skin areas in contrast to 100-10,000 organisms/cm2 in the
Some bacteria can live in the dry areas of the skin, eg. the Gram-positive bacteria
Streptococcus epidermidis and Staphylococcus aureus. In fact, Gram-positive bacteria
are generally more resistant to adverse drying conditions and are more salt-tolerant than
Gram-negative bacteria; this allows these bacteria to live in the high salt concentration in
sweat. Both these organisms metabolize the proteins from dead epidermal cells.
As mentioned above, the skin pH is slightly acidic. This is partly due to the metabolism
of the normal flora. For example, consider the bacterium Propionibacterium acne which
is associated with hair follicles and oil glands. It is an aerotolerant anaerobe which
catabolizes oil secretions by fermentation, lowering the skin pH through production of
short chain volatile fatty acids like propionic acid. Since most bacteria not normally found
on skin will be neutralophiles, this acid pH helps to inhibit growth of other
microorganisms that could potentially cause skin infections. Products of skin oil
degradation by P. acne also include longer chain fatty acids. These fatty acids inhibit the
growth of other bacteria because they act like detergents (disrupt membranes). Thus, the
normal flora of the skin modify the skin environment through their metabolism which can
have the beneficial effect of protecting the body against infection by harmful
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This note was uploaded on 10/25/2013 for the course MICB 201 taught by Professor Davidturner during the Fall '12 term at The University of British Columbia.
- Fall '12