High prevalence of stably derepressed class-I -lactamase expression in multiresistant clinical isolates of Enterobacter cloacae from Greek hospitals

Abstract

Susceptibilities to cefotaxime (Ctx) and ceftazidime (Caz) were examined for 90 recent clinical isolates of Enterobacter cloacae from Greek hospitals. Most (68%) of the isolates were resistant to both drugs, and all were resistant to cefoxitin. beta-Lactamase activities against cephaloridine in crude extracts from Ctx-Caz-resistant isolates were high, irrespective of whether or not the cells were grown with cefoxitin as an inducer of the chromosomal beta-lactamase, indicating stable derepression of the gene for the enzyme. On the other hand, double disk antagonism tests showed that all the Ctx-Caz-sensitive isolates possessed inducible expression of this beta-lactamase. Iso-electric focusing revealed the presence of five forms of the chromosomal beta-lactamase, randomly distributed amongst the Ctx-Caz-resistant and -sensitive isolates. Plasmid-mediated beta-lactamases of TEM and PSE types also were found in many isolates. These data indicate that the extremely high prevalence of Ctx-Caz-resistant E. cloacae isolates in Greek hospitals is attributed to the dissemination of mutants which constitutively overproduce the class-I chromosomal beta-lactamase. Over 90% of these isolates exhibited cross-resistance to aminoglycosides, suggesting the accumulation of unrelated antibiotic resistance mechanisms.