The effect of the inoculum concentration on the in vitro sensitivity of Pseudomonas aeruginosa

Using a broth microtiter dilution method, the minimum inhibitory concentrations of antipseudomonal antibiotics were determined against 19 P. aeruginosa isolates. Two different concentration of inoculum, 10(5) and 10(8), were used to show the inoculum concentration effect of in vitro antibiotic susceptibility tests. On the basis of the MIC values and using Howard B.J. (1) breakpoints, the effect of inoculum density was most prominent for amikacin and aztreonam, intermediate for mezlocillin, ticarcillin, piperacillin, cefotaxime, cefoperazone, netilmicin, tobramycin, gentamicin, and least apparent for ciprofloxacin and carbenicillin respectively.