Detection of Extended-Spectrum ß-lactamases in microorganisms harboring an AmpC ß-lactamase.

QUINTEROS M; RADICE M; POWER P; MATTEO M; MOLLERACH M; DI CONZA J; COSTA N; COUTO E; GUTKIND G

L’Aquilla, Italia

Congreso; International Congress on ß-lactamases.; 1999

Resistance of third generation cephalosporins in Citrobacter freundii, Serratia marcescens, Morganella morganii and Enterobacter cloacae is commonly due to the selection of derrepressed mutants on the cromosomally encoded AmpC B-lactamase, conferring resistance to cefoxitin (CXT) and both cefotaxime (CTX) and ceftazidime (CTZ). However, as CTXr/CTZs is a common fenotipe in other enterobacteria in our country, we have chosen 1 E. aerogenes, 2 C. freundii, 3 S. marcescens and 6 M. morganii resistant to CTX but not CTZ (accordingly to the recently modified NCCLS breakpoints) both by disk diffussion and MIC tests. Clavulanic acid (4 ug/ml) restores susceptibility in all cases. A 5 mm increment in the inhibitioon zone was always detected when 10 ug of clavulanic acid was added to CTX (30 ug) disks, but never with CTZ (30 ug), indicating the presence of an ESBL with poor activity on the later. Crude extracts presented a common band active both on ampicillin (100 ug/ml) and ceftriaxone (1,000 ug/ml) with an apparent pI of 8.2. Colony blot hybridization confirmed the presence of CTX-M-2 and AmpC B-lactamases in these microorganisms, although the amount of the cromosomal enzyme was to low for being detected after analytical isoelectric focusing of crude extracts. These results suggest both CTX and CTZ should be tested because the latter may not be able to detect resitance to third generation cephalosporins when the enzyme involved is CTX-M-2 (as in our case or any other putative enzyme with poor CTZ hydrolytic activities). Utilization of cefotaxime/clavulanate and ceftazidime/clavulanate disks has been recommended as a phenotypic confirming test for ESBLs in K. pneumoniae, K. oxytoca and E. coli. Our results suggest that this metodolgy may be also appropiate for the studied species.