Enumeration of beta-haemolytic streptococci on normal skin by direct agar contact.

Normal skin sites in children from a population in which streptococcal impetigo is endemic were examined for the presence of beta-haemolytic streptococci by a direct agar-contact technique. Ninety-eight of 554 samples (18%) were positive for these organisms. Penicillin prophylaxis reduced the frequency of isolation of streptococci from the normal skin for a period of 3 weeks, perhaps accounting in part for the lower isolation rate in this than in earlier studies. Numbers of streptococcal colony-forming units in positive samples were generally low, both in terms of absolute numbers isolated from the surface area sampled and in comparison with numbers of other aerobic flora recovered. The presence of streptococcal pyoderma at the time of agar contact was not necessarily associated with the presence of or with increased numbers of streptococci on samples obtained from normal skin sites. Low counts were consistently found in early summer and higher counts in some samples in late summer. In a simultaneous comparison of paired samples taken from adjacent sites, the frequency of detection of streptococci by direct agar contact compared favourably with that obtained with a moist-swab method. The increased frequency of detection by the agar-contact method appeared to be related to an increased sensitivity for the detection of low numbers of streptococcal colony-forming units on the normal skin.