Ampicillin-sulbactam and amoxicillin-clavulanate susceptibility testing ofEscherichia coli isolates with different beta-lactam resistance phenotypes

The activities of ampicillin-sulbactam and amoxicillin-clavulanate were stu died,vith 100 selected clinical Escherichia coli isolates with different be ta-lactam susceptibility phenotypes by standard agar dilution and disk diff usion techniques and with a commercial microdilution system (PASCO). A Bred ratio (2:1) and a fixed concentration (clavulanate, 2 and 4 mu g/ml; sulba ctam, 8 mu g/ml) were used in the agar dilution technique. The resistance f requencies for amoxicillin-clavulanate with different techniques were as fo llows: fixed ratio agar dilution, 12%; fixed concentration 4-mu g/ml agar d ilution, 17%; fixed ratio microdilution, 9%; and disk diffusion, 9%. Marked discrepancies were found when these results were compared,vith those obtai ned,vith ampicillin-sulbactam (26 to 52% resistance), showing that suscepti bility to amoxicillin clavulanic acid cannot be predicted by testing the is olate against ampicillin-sulbactam. Interestingly, the discrimination betwe en susceptible and intermediate isolates was better achieved with 4 mu g of clavulanate per mi than with the fixed ratio. In contrast, amoxicillin sus ceptibility was not sufficiently restored when 2 mu g of clavulanate per mi was used, particularly in moderate (mean beta-lactamase activity, 50.8 mU/ mg of protein) and high-level (215 mU/mg) TEM-1 beta-lactamase producer iso lates. Four micrograms of clavulanate per milliliter could be a reasonable alternative to the 2:1 fixed ratio, because most high-level beta-lactamase- hyperproducing isolates would be categorized as nonsusceptible, and low- an d moderate-level beta-lactamase-producing isolates would be categorized as nonresistant, This approach cannot be applied to sulbactam, either with the fixed 2:1 ratio or with the 8-mu g/ml fixed concentration, because many lo w-level beta-lactamase-producing isolates would be classified in the resist ant category. These findings call for a review of breakpoints for beta-lact am-S lactamase inhibitor combinations.