Resident, transient microflora
Normal human skin is protected by secretions from sebaceous and sweat glands. However, human skin is capable of harboring
a large quantity and variety of noninfectious and infectious microbes. It has been estimated that one square inch of hand-surface
area on average has a bioburden of about 10,000 microbes, including gram-positive and gram-negative bacteria, fungi, yeasts
and viruses. To a casual observer, 10,000 microorganisms per square-inch of hand surface probably would not appear to be soiled.
The microbial flora of skin can be subdivided into two general groups: resident microbes and transient microbes. Healthy skin
has a relatively stable normal flora of resident microbes. Resident microbes are termed "colonizing" because they grow easily
in the skin and its adnexal structures. Most resident microbes are found in superficial layers of the skin, but about 10 percent
to 20 percent are located deep within the keratinized layers of stratum corneum and inside adnexal structures such as hair
follicles, sebaceous glands and sweat ducts. Compared to transient microbes found on the surface of the skin, resident microbes
are not easily removed by ordinary soaps and mechanical friction. Therefore they serve as a microbial reservoir for rapid
repopulation of the skin following hand washing. The normal resident skin flora consists primarily of bacteria (including
the genera Staphylococcus, Enterobacter, Proprionobacterium, and Corynebacterium), certain viruses and yeasts.
With some notable exceptions, resident microbes that survive and multiply on the skin are not highly virulent and are not
usually implicated in nosocomial infections. Unless introduced into body tissues by trauma or medical devices such as intravenous
catheters, the pathogenic potential of the resident flora is low. However, infections with some resident microbes can occur
in immunocompromised patients, and following surgery or other invasive procedures that allow these microbes to enter deep
In contrast to resident microbes, the transient microflora of the skin contains microbes deposited from contact with contaminated
surfaces. Because most of the transient microbes can survive for only a limited time, they are also termed "noncolonizing
microflora". They are abundant on exposed skin, under nails, etc., but are relatively scarce on clean, unexposed skin. Transient
microbes acquired by contact vary greatly in number and kind (Photo 1, p. 46). Examples of transient organisms include Escherichia coli, Enterococcus spp., and Pseudomonas spp. Transient microorganisms found on the hands of hospital personnel often include pathogens acquired from contact with infected
patients. These microbes are common causes of nosocomial infections.
Hand washing with plain soaps and detergents is effective in removing many transient micro flora. In contrast, resident microorganisms
located in the deep layers may not be removed by hand washing with plain soaps and detergents. However, their numbers usually
can be reduced with hand washing products that contain antimicrobial ingredients.
When should hands be washed?
The indications for hand washing in a hospital environment depend on the type, intensity, duration and sequence of activity
of hospital staff. In general, hand disinfection procedures in hospital settings can be separated into two general categories:
hygienic and surgical.
The primary purpose of hygienic hand washing is to block transmission of microbial pathogens from health-care personnel to
patients by quickly and effectively removing dirt, organic debris and transient microbes. Whereas surgical hand washing is
designed to kill or remove as many bacteria as possible (including resident bacteria), the principal focus of hygienic procedures
is directed against the transient skin flora.
A sustained antimicrobial action directed against the resident flora as required for surgical disinfection is of little significance
in context of the goals of hygienic hand-washing procedures.
Let us return to the question of when to wash your hands. Here are some generalities. (See checklist, p. 48.)
In context of these guidelines, how frequently during the day do you wash your hands? Would you let a health-care worker care
for you if he or she followed the same pattern of hand hygiene that you follow?
Hand washing should be encouraged when personnel are in doubt about the need to do so. However, to minimize nosocomial infections,
more is required than knowing when to wash your hands. Knowing how you wash your hands and what preparations to use is as
important as knowing when to wash.