Comparative in vitro activities of apalcillin and piperacillin against gram-negative bacilli

The susceptibilities of 317 gram-negative bacilli to apalcillin and piperacillin were determined by standardized microdilution and disk diffusion tests. The respective percentages of strains susceptible to less than or equal to 64 micrograms of apalcillin and piperacillin per ml were as follows: members of the family Enterobacteriaceae, 90 and 88%; randomly selected Pseudomonas aeruginosa, 100 and 100%; multi-drug-resistant P. aeruginosa, 100 and 75%; and other nonfermenters and Aeromonas hydrophila, 99 and 97%. The drugs had equal activity against Enterobacteriaceae and A. hydrophila; apalcillin was more active against Pseudomonas species and Acinetobacter calcoaceticus, and piperacillin was more active against other nonfermenters. By using linear regression analysis, we found that the MICs of apalcillin and piperacillin were highly interrelated; the lines of best fit had slopes close to unity, and correlation coefficients (r) were greater than 0.90 for Enterobacteriaceae as a group and for other species individually. With disk diffusion testing, inhibition zone diameters around 100 micrograms apalcillin and 100 micrograms piperacillin disks correlated well with respective MICs for Enterobacteriaceae and P. aeruginosa (r = -0.93 to -0.96). Using an error rate-bound classification scheme, we determined breakpoints for apalcillin susceptibility and confirmed those previously established for piperacillin to be appropriate. The apalcillin disk, with modified breakpoints, could be used to predict piperacillin MICs for all organisms, but the piperacillin disk could not be used to predict apalcillin MICs for nonfermenters.