Persistent individual differences in the bacterial flora of the skin of the forehead: numbers of propionibacteria.

The bacterial flora of the central area of the forehead of 48 adults was examined by a standardized swabbing procedure to determine the extent of individual variation. Propionibacteria were the most abundant organisms on most subjects. Their numbers per cm-2 of skin ranged from fewer than 6 to 1,100,000. Seventeen subjects had more than 100,000 and 16 subjects yielded fewer than 10,000. Six had fewer than 1,000. Differences with respect to sex and age were not statistically significant. Coagulase-negative cocci numbered from 25 to 75,000 per cm-2. There was no statistically significant correlation between numbers of propionibacteria and numbers of cocci. In order to distinguish trivial or transient individual differences from those of a more fundamental nature, 16 subjects were examined four times at intervals of several months with a total elapsed time of from 329 days to 523 dyas and a median of 385 days. Eight subjects were representative of the upper third and 8 the lower third of the population with respect to numbers of propionibacteria in the initial survey. Geometric mean values for propionibacteria per cm-2 of skin in the measurements of subjects with a rich population ranged from 152,686 to 280,867. Corresponding figures for the subjects with a sparse population were 1,215 to 3,639. Comparing geometric means by the t-test yielded p values of less than .001 in the second and fourth measurements and less than .01 in the third. It is concluded that a rich or a sparse population of propionibacteria is a stable individual characteristic of the skin of the forehead.