R Factor-Mediated Antibiotic Resistance in Serratia marcescens

Abstract

Nineteen of 39 multiresistant strains of Serratia marcescens isolated from clinical sources transferred antibiotic resistance to Escherichia coli or Klebsiella pneumoniae recipients. Marcesins and/or phage prevented effective resistance transfer to E. coli and attempts to select marcescin-resistant mutants of the E. coli recipient strain were unsuccessful. Transfer of resistance was demonstrated for all drugs tested except nalidixic acid. Approximately 90% of donors resistant to tobramycin, ampicillin, or carbenicillin transferred resistance to these drugs. High levels of transferred resistance (minimal inhibitory concentration, >2,500 μg/ml) were demonstrated particularly for ampicillin, carbenicillin, and kanamycin. Transmissibility of Serratia R factors was greatest between isogeneic strains of E. coli K-12. Comparative rates of spontaneous loss of R factor-mediated resistance indicated that Serratia R factors are less stable in E. coli and K. pneumoniae transcipients than in the indigenous hosts.