Differential quantitation of surface and subsurface bacteria of normal skin by the combined use of the cotton swab and the scrub methods

Abstract

By testing adjacent sites on the hypothenar eminence of the palm, enriched with bacteria by massaging the forehead, we found that the numbers of bacteria recovered from the skin surface by a wet cotton swab in 30 s were not significantly different from the numbers obtained by a brisk scrubbing with a blunted Teflon policeman for 120 s. This was true of aerobes (gram-positive cocci) and anaerobes (propionibacteria). If the same site on the palm was swabbed two times for 15 s each time, 67 to 94% of the total recovered bacteria were obtained on the first swab. Differential localization of bacteria into surface and subsurface populations was accomplished by first swabbing a test skin site to assay the surface flora and then scrubbing the same site to test for subsurface organisms. On the palm the swab yielded more aerobes and anaerobes than did the subsequent scrub. On the forehead the scrub yielded three to eight times as many anaerobes as the preceding swab. In some tests gram-positive cocci were distributed on the forehead like propionibacteria (large excess in scrub specimen); in other tests their numbers were similar in the swab and scrub specimens or there was a large excess in the swab specimen. These results indicate that there was no substantial subsurface flora on the palm. On the forehead propionibacteria were predominantly in deeper locations in all tests; gram-positive cocci were variable: in some test sites they were largely at the surface, whereas at other sites a predominance of cocci was in subsurface locations.