beta-Lactamases and resistance to penicillins and cephalosporins in Serratia marcescens.

Strains of Serratia marcescens fall into one of two groups with respect to their resistance to to beta-lactum antibiotics. Most strains are highly resistant to cephalosporins but are significantly more susceptible to ampicillin and carbenicillin, whereas other strains are highly resistant to both penicillins and cephalosporins. Strains in the former category produce small amounts of an inducible cephalosporinase, which appears to be chromosomally mediated. Strains in the latter class also elaborate large amounts of a noninducible penicillinase-cephalosporinase, which is plasmidmediated. Ability to produce this type of enzyme can be transferred to Klebsiella pneumoniae or Escherichia coli and may be lost spontaneously or after exposure of S. marcescens to "curing" agents.