Correlations between human skin bacteria and skin lipids.

Correlation matrices were constructed for various human skin variables (propionibacteria, micrococcaceae, sebum excretion rate, percentage of free fatty acids and free fatty acid production rate). Two independent populations were studied (n = 87 and n = 103), and a subpopulation (n = 30) was investigated on three occasions at intervals of I month. In this way consistently high correlations could be differentiated from 'fortuitous' correlations. Propionibacteria versus micrococcaceae showed the highest correlation coefficient and propionibacteria versus percentage free fatty acids was also highly correlated. A scatter diagram indicated a relationship between the lower threshold of microbial density and sebum excretion rate. An explanation based on relative duct and gland size is presented.