Relation of Beta-Lactamase Activity to Antimicrobial Susceptibility in Serratia marcescens

One-hundred clinical isolates of Serratia marcescens were tested for susceptibility to cephalothin, carbenicillin, ticarcillin, ampicillin, and cefoxitin. The majority of the 100 isolates (≥70%) were susceptible to carbenicillin, ticarcillin, and cefoxitin; less than one-half were susceptible to ampicillin; none were susceptible to cephalothin. Ten isolates from the 100 organisms tested were selectively assayed for their β-lactamase activity. Enzyme activity was measured using either iodometric or spectrophotometric methods, and the microbiological assay technique. It was concluded that β-lactamase production was not the sole determinant in β-lactam antibiotic resistance. Resistance without demonstrable β-lactamase was evident in strains for cephalothin, ampicillin, and cefoxitin. In addition, one strain which was susceptible to all antibiotics except cephalothin, elaborated considerable β-lactamase activity.