MadSci Network: Medicine

Re: How does the pH of the skin serve as a barrier against bateria?

Date: Thu Apr 8 00:01:00 1999
Posted By: Tinsley Davis, Grad student, Microbiology, University of Wisconsin Madison
Area of science: Medicine
ID: 922900456.Me

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!

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