Characterization of spontaneous resistant variants of Serratia marcescens selected in the presence of carbenicillin.

Spontaneous carbenicillin-resistant variants were isolated from pigmented Serratia marcescens GRI 2677 with frequencies as high as 1 in 7.4 x 10(4). One-hundred and thirteen such variants were characterized with respect to pigmentation, antimicrobial susceptibility and beta-lactamase production. Sixty-eight were less pigmented than the parent culture, 84 showed low level resistance to five aminoglycosides and 17 showed increased beta-lactamase activity. Thirty-three variants were unstable and reverted to the parental phenotype at high frequency. One designated GRI 2677-8, was additionally auxotrophic and was further characterized. On the basis of these criteria the variants were assigned to one of 21 sub-groups. These results are discussed in relation to resistant clinical isolates and a possible mechanism that explains the diversity of variants.