|MadSci Network: Medicine|
Dear Donnamarie, Our skin is one of the best defenses against germs for several reasons. The top layers of cells are dry and densely packed. The dryness and close quarters of this first line of defense make it inhospitable to many bacteria. Salty secretions from sweat glands create an environment that is hyperosmotic and thus discouraging to bacteria because the high salt concentrations pull water from inside the bacteria, dehydrating them. However, some bacteria do naturally associate with skin. Rather than harming us these bacteria actually help protect us and are referred to as the microflora. First, colonization of the skin by harmless bacteria means that there are few resources available for pathogenic bacteria because the microflora outcompete the incoming pathogen. Second, and in response to your question, some resident microflora help lower the pH of skin. A slightly acidic pH (4-6) helps deter colonization by non-resident bacteria and pathogens because many bacteria can survive only in a narrow pH range near neutral. The acidic conditions of the skin are caused by secretions from sweat glands, skin oil, and the breakdown of fatty acids by Staphylococcus epidermis. Thus a resident microflora species is partly responsible for the acidic pH of skin. The pH of the skin certainly differs throughout parts of the body; you would expect that regions with higher S. epidermis concentrations would be slightly more acidic. Slight skin pH differences are also present in individuals because not everyone’s skin is exposed to the same conditions such as weather and harsh detergents. A recent review (Dikstein and Zlotogorski, 1994) reported that there is no difference in skin pH between Caucasian males and females. The report also acknowledges that very little research has been done on non-Caucasian skin pH. However, the report does list one study on men and women of India that suggests Indian skin has a slightly more alkaline (basic range), though the data are not definitive because the groups tested were small. Read the following reference for detailed info on what is known about skin pH (The lingo is very manageable): Dikstein, S. and A. Zlotogorski. 1994. Measurement of skin pH. Acta Derm.Venereol (Suppl.) 185:18-20. Good luck! -Tinsley
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