Susceptibilities to cefotaxime (Ctx) and ceftazidime (Caz) were examined for 90 recent clinical isolates of from Greek hospitals. Most (68%) of the isolates were resistant to both drugs, and all were resistant to cefoxitin. β-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 β-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 β-lactamase. Iso-electric focusing revealed the presence of five forms of the chromosomal β-lactamase, randomly distributed amongst the Ctx-Cazresistant and -sensitive isolates. Plasmid-mediated β-lactamases of TEM and PSE types also were found in many isolates. These data indicate that the extremely high prevalence of Ctx-Caz-resistant isolates in Greek hospitals is attributed to the dissemination of mutants which constitutively overproduce the class-I chromosomal β-lactamase. Over 90% of these isolates exhibited cross-resistance to aminoglycosides, suggesting the accumulation of unrelated antibiotic resistance mechanisms.


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