EFFECTS OF FOLLOWING NATIONAL COMMITTEE FOR CLINICAL LABORATORY STANDARDS AND DEUTSCHE-INDUSTRIE-NORM-MEDIZINISCHE-MIKROBIOLOGIE GUIDELINES, COUNTRY OF ISOLATE ORIGIN, AND SITE OF INFECTION ON SUSCEPTIBILITY OF ESCHERICHIA-COLI TO AMOXICILLIN-CLAVULANATE (AUGMENTIN)

Amoxicillin-clavulanate (Augmentin), as a combination of two active ag ents, poses extra challenges over single agents in establishing clinic ally relevant breakpoints for in vitro susceptibility tests. Hence, re ported differences in amoxicillin-clavulanate percent susceptibilities among Escherichia coli isolates may reflect localized resistance prob lems and/or methodological differences in susceptibility testing and b reakpoint criteria. The objectives of the present study were to determ ine the effects of (i) methodology, e.g., those of the National Commit tee for Clinical Laboratory Standards (NCCLS) and the Deutsche Industr ie Norm-Medizinische Mikrobiologie (DIN), (ii) country of origin (Spai n, France, and Germany), and (iii) site of infection (urinary tract, i ntra-abdominal sepsis, or other site[s]) upon the incidence of suscept ibility to amoxicillin-clavulanate in 185 clinical isolates of E. coli , Cefuroxime and cefotaxime were included for comparison. The use of N CCLS methodology resulted in different distribution of amoxicillin-cla vulanate MICs than that obtained with the DIN methodology, a differenc e highlighted by the 10% more strains found to be within the 8- to 32- mu g/ml MIC range. This difference reflects the differing amounts of c lavulanic acid present. NCCLS and DIN methodologies also produce diffe rent MIC distributions for cefotaxime but not for cefuroxime, Implemen tation of NCCLS and DIN breakpoints produced markedly different incide nces of strains that were found to be susceptible, intermediate or res istant to amoxicillin-clavulanate. A total of 86.5% strains were found to be susceptible to amoxicillin-clavulanate by the NCCLS methodology , whereas only 43.8% were found to be susceptible by the DIN methodolo gy. Similarly, 4.3% of the strains were found to be resistant by NCCLS guidelines compared to 21.1% by the DIN guidelines. The use of DIN br eakpoints resulted in a fivefold-higher incidence of strains categoriz ed as resistant to cefuroxime, There were no marked differences due to country of origin upon the MIC distributions for amoxicillin-clavulan ate, cefuroxime, or cefotaxime, as determined with the NCCLS guideline s. Isolates from urinary tract and intra-abdominal infections were gen erally more resistant to amoxicillin-clavulanate than were isolates fr om other sites of infection.