Problems in interpretation of piperacillin susceptibility of TEM-1 producing Escherichia coli in the disk diffusion test.

The susceptibility of 455 Escherichia coli blood culture isolate to piperacillin was tested with the disk diffusion test. The presence of different beta-lactamase genes in these strains was also studied using DNA hybridization. Of the TEM beta-lactamase producing isolates, 64% (61/95) were interpreted as intermediately susceptible to piperacillin. Because piperacillin is hydrolyzed by TEM-type beta-lactamases, we suggest that the intermediate susceptibility category should be reduced or omitted in testing piperacillin susceptibility of Escherichia coli isolates.