ACTIVITIES OF BETA-LACTAMS AGAINST ACINETOBACTER GENOSPECIES AS DETERMINED BY AGAR DILUTION AND E-TEST MIC METHODS

The agar dilution MIC method was used to test activities of ticarcilli n, ticarcillin-clavulanate, amoxicillin, amoxicillin-clavulanate, ampi cillin, ampicillin-sulbactam, piperacillin, piperacillin-tazobactam, i nhibitors alone, ceftazidime, and imipenem against 237 Acinetobacter g enospecies. A total of 93.2% of strains were beta-lactamase positive b y the chromogenic cephalosporin method, Overall, ampicillin-sulbactam was the most active combination against all strains (MIG at which 50% of the isolates are inhibited [MIG(50)] and MIG(90), 4.0 and 32.0 mu g /ml; 86.9% susceptible at less than or equal to 16 mu g/ml), followed by ticarcillin-clavulanate (16.0 and 128.0 mu g/ml; 85.7% susceptible at less than or equal to 64 mu g/ml), piperacillin-tazobactam (16.0 an d 128.0 mu g/ml; 84.8% susceptible at less than or equal to 64 mu g/ml ), and amoxicillin-clavulanate (16.0 and 63.0 mu g/ml; 54.4% susceptib le at less than or equal to 16 mu g/ml). Ceftazidime and imipenem yiel ded MIC(50)s and MIC(90)s of 8.0 and 64.0 mu g/ml (ceftazidime) and 0. 5 and 1.0 mu g/ml (imipenem), respectively; 71.3% of strains were susc eptible to ceftazidime at less than or equal to 16 mu g/ml, and 99.2% were susceptible to imipenem at less than or equal to 8 mu g/ml. Sulba ctam was the most active beta-lactamase inhibitor alone (MIC(50) and M IC(90), 2.0 and 16.0 mu g/ml); clavulanate and tazobactam were less ac tive (16.0 and 32.0 mu g/ml for both compounds). Enhancement of beta-l actams by beta-lactamase inhibitors was not always seen in beta-lactam ase-positive strains, and activity of combinations such as ampicillin- sulbactam was due to the inhibitor alone. Acinetobacter baumannii was the most resistant genospecies. By contrast, Acinetobacter haemolyticu s, Acinetobacter calcoaceticus, Acinetobacter johnsonii, Acinetobacter junii, Acinetobacter radioresistens, and other non-Acinetobacter baum annii strains were more susceptible to all compounds tested, E-test MI Cs were within 1 dilution of agar dilution MICs in 38.4 to 89.6% of ca ses and within 2 dilutions in 61.6 to 98.6% of cases.