ESBLs Detection: Comparative Analysis of Different Methods in an Area with Uncommon Resistance Markers

QUINTEROS MIRTA; MOLLERACH MARTA; RADICE MARCELA; MATTEO MARIO; POWER PABLO; DI CONZA JOSÉ; ECHEVERRIA A.; COUTO ELSA; GUTKIND GABRIEL

California, USA.

Conferencia; 39th annual Interscience Conference on Antimicrobial Agents and Chemotherapy - ICAAC; 1999

ASM

  Seventy-two ESBL producing enterobacteria from different regions of our country were used for comparing detection abilities of the NCCLS recently recommended methods for E. coli, K. pneumoniae and K. oxytoca (screening and phenotypic confirmation) with the use of a single ceftazidime (CAZ) 5 mg disk. We included clinical isolates of E. coli (n=30), Salmonella Infantis (n=11), P. mirabilis (n=6), S. sonnei (n=1), and K. pneumoniae (n=24), previously characterized as resistant to cefotaxime (CTX) by disk diffusion tests. b-Lactamase production was analyzed by isoelectrofocusing, and codifying genes for the enzymes confirmed by colony blot hybridization. We compared the following methods: (I) Agar dilution with CTX and ceftazidime (CAZ), alone and in presence of 4 mg/ml clavulanic acid (CLA); (II) Disk diffusion with standard concentrations of CAZ and CTX, using the new NCCLS breakpoints; (III) Disk diffusion with 5 mg CAZ (susceptible ³ 22 mm); (IV) Disk diffusion with CTX/CLA (30/10 mg) and CAZ/CLA (30/10 mg), compared with the inhibition zones for CTX and CAZ, respectively. Resistance was successfully detected  (100 %) both by CTX agar dilution and diffusion, but only in two thirds of the isolates by the recommended CAZ breakpoints. The same results could be observed using the 5 mm increase by CLA. All (except for  one P. mirabilis) displayed inhibition zones £21 mm using the CAZ 5 mg disk. As expected, those methods using CTX were more appropriate for detection of ESBL in our country due to widespread of CTX M-2 (with very poor hydrolytic activity on CAZ). However, use of a 5 mg CAZ disk improved detection of resistance in microorganisms harboring these enzymes, except in a single P. mirabilis, possibly due to low level production.