COMPARISON OF FIXED CONCENTRATION AND FIXED-RATIO OPTIONS FOR DILUTION SUSCEPTIBILITY TESTING OF GRAM-NEGATIVE BACILLI TO AMPICILLIN AND AMPICILLIN-SULBACTAM

Ampicillin combined with sulbactam was tested at both fixed ratio (2:1 and 1:1) and fixed sulbactam concentrations (4 mug/ml, 8 mug/ml and 16 mug/ml) against 2440 consecutively isolated gram-negative bacilli. Sulbactam significantly enhanced the spectrum of ampicillin activity. Overall, at 8 mug/ml ampicillin inhibited 50 % of the Enterobacteriaceae isolates, whereas 69 % to 84 % of the isolates were inhibited by the various sulbactam combinations. The widest spectrum of activity for ampicillin/sulbactam was achieved by testing at a fixed sulbactam concentration of 16 mug/ml, followed by the 1:1 ratio and the fixed 8 mug/ml (84 %, 76 % and 74 % inhibited, respectively). The amount of sulbactam at the susceptible breakpoint concentrations of ampicillin markedly affected the percentage of susceptible strains. Combinations that include 8 mug/ml of sulbactam are suggested for consideration.