The interactions among the members of microbial populations on skin are undoubtedly important, but poorly understood. Some may involve more or less direct interactions via competition for available nutrients. It is generally accepted that free fatty acids on the skin surface are products of microbial metabolism and that they are inhibitory to some organisms, particularly potential pathogens. Corynebacteria are among the most active lipase producers on skin (9), but micrococci have also been shown to be important contributors of lipolytic activity (10). It has been suggested that fatty acids are an important mechanism by which Gram-positive bacteria on skin exert an inhibitory effect over Gramnegative bacteria (11). Conversely, it has been shown that suppressing Grampositive skin populations with antibiotics can be followed by overgrowth of Gram-negative bacteria (12). Antagonism also can occur via excretion of bacterio-cins, which are a chemically diverse group of substances produced by many microorganisms that inhibit the growth of other species. Bacteriocins produced by Gram-positive organisms tend to have activity against closely related strains or species, whereas those produced by Gram-negative bacteria have broader activity. Bacterial interference is likely to be an important natural phenomenon that is helpful in understanding the forces that shape microbial populations, but this concept also has been applied in a clinical setting for infection control. For example, artificial colonization of nasal mucosa and umbilical sites with a nonvirulent strain of S. aureus has been shown to result in a decreased incidence of infection at those sites (13). Not all microbial interactions are inhibitory in nature. In vitro studies of growth enhancement or satellitism have been reported between bacterial isolates from normal healthy skin (14). The mechanism of satellitism is not clearly understood, but could involve production of growth factors by one organism that are stimulatory to another, or perhaps by destruction of inhibitory materials. All of these interactions contribute to the composition of the skin micro-biome, and it is apparent that the type and nature of the inhabitants, as well as the nature of the substrate, are important attributes that shape its composition.
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