Normal Skin Flora
The skin contains normal microflora that
are universally found on all humans. It has been estimated that the number of
bacteria on the surface of the human skin is greater than the number of cells
in the human body. The normal skin flora include the bacteria Staphylococcus
epidermidis, Corynebacterium species, Propionobacterium acnes,
Micrococcus species, and Acetobacter species. The demodex mites are
the only parasites considered to be part of the normal flora. Pityrosporum species
are the only fungi that are considered to be normal skin flora. The microbes
that make up the normal skin flora under most circumstances do not cause any
type of disease. They are able to reproduce and maintain viable populations,
living in harmony with the host. In stark contrast, transient skin flora can
sustain growth only in certain skin environments. Transient microbes are not
able to produce long-lasting, viable reproductive populations and therefore are
unable to maintain a permanent residence. Some examples of transient skin flora
are Staphylococcus aureus, including methicillin-resistant S. aureus (MRSA), Enterobacter
coli, Pseudomonas aeruginosa, Streptococcus pyogenes, and some Bacillus species.
Normal and transient flora can become pathogenic under the correct
environmental conditions.
Normal bacterial colonization begins immediately after birth. Once
newborns are exposed to the external environment, they are quickly colonized
with bacteria. S. epidermidis is
often the first colonizing species, and it is the one most commonly cultured in
neonates.
The innate ability of certain bacteria to colonize the human skin is
dependent on a host of contributing factors. Availability of nutrients, pH,
hydration, temperature, and ultraviolet radiation exposure all play a role in
allowing certain bacteria to develop a synergistic balance. The normal skin
flora use these factors to their survival advantage and live in a symbiotic
relationship with the human skin. These microbes have evolved a competitive
advantage over the transient skin flora.
Under certain circumstances, normal skin flora can become pathogenic and
cause overt skin disease. Over- growth of Pityrosporum ovale (Malassezia
furfur) causes tinea versicolor, an exceedingly common superficial fungal
infection. Warm and humid environments are believed to be factors in the
pathogenesis. Tinea versicolor manifests as fine, scaly patches with hyperpigmentation
and hypopigmentation. Other Malassezia species have been implicated in
causing neonatal cephalic pustulosis, pityrosporum folliculitis, and seborrheic
dermatitis.
The common skin bacterium, S. epidermidis, is a grampositive
coccus that can become a pathogenic microbe under certain circumstances.
Conditions that increase the chance that this bacterium will cause pathogenic
skin disease include use of immunosuppressive medications, immunocompromised
state (e.g., human immunodeficiency virus infection), and presence of a chronic
indwelling intravenous catheter. S. epidermidis creates a biofilm on
indwelling catheters, which can lead to transient bacteremia and sepsis in
immunocompromised patients and occasionally in the immunocompetent.
P. acnes is a gram-positive organism that is found within the
pilosebaceous unit. These bacteria occur in high densities in the sebum-rich
regions of the face, back, and chest. It is the major species implicated in the
pathogenesis of acne vulgaris. In immunocompromised individuals, it has been
reported to cause abscesses.
Corynebacterium species, when in an environment of moisture and
warmth, can produce an overgrowth on the
terminal hairs of the axilla and groin regions, resulting in the condition
known as trichomycosis axillaris. Different colonies of this bacterium can
produce superficial red, yellow, or black nodules along the terminal hair
shafts. Corynebacteria can also cause pitted keratolysis, a superficial
infection of the outer layers of the epidermis on the soles.
The only parasites that can be found normally on human skin are the
demodex mites, which live in various regions of the pilosebaceous unit. Demodex
brevis lives within the sebaceous gland ducts, whereas Demodex folliculorum lives in the hair
follicle infundibulum. Demodex mites can cause demodex folliculitis. an infection
of the hair follicles that manifests as superficial, follicle-based pustules.
The most important skin microbes, based on their ability to cause
pathology, are the transient microbes. The best-known species is S. aureus. The
ability of S. aureus to cause folliculitis, boils, abscesses, and
bacterial sepsis is well documented and is a major cause of morbidity and
mortality.
