Correlation of in vitro susceptibility testing results for amoxicillin- clavulanate and ampicillin-sulbactam using a panel of lactamase-producing Enterobacteriaceae

Abstract

Correlation between in vitro susceptibility results for amoxicillin- clavulanate (AMC) and ampicillinsulbactam (SAM) was studied using 136 clinical and control strains of Enterobacteriaceae harboring TEM-1, SHV-1 or OXA-1-like β-lactamases. Determination of minimal inhibitory concentration of antibiotics was performed by agar dilution. The β-lactamases were initially characterized using isoelectric focusing. Further identification was done by DNA hybridization with or without prior PCR amplification. All strains sensitive to SAM were found to be sensitive also to AMC. In contrast, among those susceptible to AMC, only 50% were sensitive to SAM while 36% gave intermediate results and 14% were resistant. Major differences were found solely among SHV-producers while minor differences occurred mostly among TEM- producers. This phenomenon is probably related to the differential activities of etavulanate and sulbactam against various β-lactamases. In conclusion, testing of Enterobacteriaceae isolates for susceptibility to AMC and SAM should be performed and reported individually to avoid erroneous designation of susceptibility.