Characterization of Two Clinical, Multiple-Drug-Resistant Isolates of <i>Enterobacter cloacae</i>

Abstract

Two multiple drug resistant Enterobacter cloacae isolates (Nos. 460 and 493) varied phenotypically in bacteriocin susceptibility in the absence of significant·antigen variation. Both isolates were susceptible to chloramphenicol, nitrofurantoin, polymyxin B, nalidixic acid, norfloxacin, and enoxacin only. One isolate carried a non-conjugative resistance (R) plasmid, whereas the other isolate contained a conjugative, ‘curable’ R plasmid and a cryptic plasmid. Both wild-type isolates constitutively produced a chromosomal cephalosporinase (nitrocefin hydrolysis); ‘cured’ variants of E. cloacae isolate No. 493, which had become susceptible for lamoxactam, produced a cefazolin-inducible β-lactamase. The two E. cloacae isolates, including their ‘cured’ variants, were of low-grade virulence for outbred NMRI mice. Both isolates differed somewhat in susceptibility to defibrinated human blood. Inhibitory (0.25 μg/ml), but not subinhibitory (0.125 μg/ml) concentrations of norfloxacin and enoxacin combined with human blood yielded additive effects against both E. cloacae isolates.