A Novel Type of Resistant Bacteria Induced by Gentamicin*

A bstract Following growth of Serratia marcescens in the presence of gentamicin, both a normal resistant mutant and a pleiotropic resistant variant, Serratia marcescens , were isolated. The pleiotropic variant strain differed from the parent strain in antibiotic susceptibility profile, colonial morphology, biochemical properties, and susceptibility to Serratia bacteriophages and bacteriocins but was identical in bacteriocin production. While the pleiotropic variant was stable in the absence of gentamicin, reversion to the parent was found in cultures grown aerobically, but not anaerobically. The normal resistant mutants, of apparent spontaneous origin, did not exhibit pleiotropic effects. The pleiotropic variant grew better under anaerobic conditions than it did under aerobic conditions, while the parent strain grew better aerobically. Both the parent strain and the variant were capable of adapting to either environment. The difficulty of identifying this variant in a clinical microbiology laboratory is discussed.