CONTINUOUS-CULTURE STUDIES O F INTERACTIONS AMONG HUMAN SKIN-COMMENSAL BACTERIA

THE study of interactions among skin commensals has been prompted by the recent finding that more than one-fifth of normal persons possess antibiotic-producing bacteria on their skin (Selwyn and Ellis, 1972). The antagonistic effects of a strain of Staphylococcus epidermidis against a variety of bacteria have been investigated both semi-quantitatively on solid media and quantitatively in liquid batch cultures (Marsh and Selwyn, 1977). However, batch cultures permit only short-term growth studies, and the organisms in such a system undergo unrealistically wide changes in population density in comparison with those observed in most in-vivo situations. Studies have therefore now been performed in a chemostat, which provides stable long-term growth conditions. MATERIALS AND METHODS Bacterial strains. These were strain S6 +, an antibiotic-producing strain of S. epidermidis, biotype 4 (Baird-Parker, 1974); strain S6-, an antibiotic non-producing S. epidermidis of biotype 4; and strain M7, a Micrococcus of biotype 7 (Baird-Paker, 1965). All three strains had been isolated from the forehead of one of us (S. S.). Previous studies on solid and in liquid media showed that strain M7 was antagonised by strain S6+ but not by strain S6-