An evaluation of susceptibility testing methods for ampicillin-sulbactam using a panel of beta-lactamase-producing bacteria

Bacteria possessing TEM-1-like beta-lactamases are generally regarded as su sceptible to ampicillin-sulbactam (SAM), while those harboring OXA-1 enzyme s are considered resistant. The current study was undertaken to compare sus ceptibility testing using various combinations of ampicillin and sulbactam to improve clinical correlation. Members of the Enterobacteriaceae family h arboring TEM-1, SHV-1 or OXA-1-like beta-lactamases were tested using the a gar dilution method. A substantial proportion of strains harboring OXA-1-li ke beta-lactamases showed false susceptibility to SAM at the 1:1 ratio or f ixed sulbactam concentration of 8 mu g/ml. At a fixed sulbactam concentrati on of 4 mu g/ml, the activity of ampicillin-sulbactam appeared to be reduce d, with large numbers of TEM-1 producers becoming frankly resistant. Result s obtained with the 2:1 ratio exhibited the closest correlation with that o btained by the currently recommended disk diffusion test. However, very maj or errors were still found between the disk diffusion test and agar dilutio n test, suggesting the necessity for consideration of a change in criteria for interpretation of disk diffusion test results. In conclusion, SAM susce ptibility testing by agar dilution using other than a 2:1 ratio is not reco mmended and results should be interpreted with caution.