Comparison of Inhibitory and Bactericidal Activity of Antipseudomonal Antibiotics against Pseudomonas aeruginosa Isolates from Cystic Fibrosis Patients

Using a broth microtiter dilution method, the minimum inhibitory (MIC) and minimum bactericidal concentrations (MBC) of so-called antipseudomonal antibiotics were determined against 79 Pseudomonas aeruginosa isolates from cystic fibrosis patients. On the basis of the MIC values and using DIN breakpoints, the percentual susceptibilities led to the following rank order: imipenem (91%), ciprofloxacin (90%), tobramycin (87%), amikacin (87%), ceftazidime (82%), cefsulodin (76%), piperacillin (71%), azlocillin (62%), followed by gentamicin, ceftriaxone, mezlocillin, netilmicin, and cefotaxime (less than 50%). However, evaluating MBC values according to DIN breakpoints, only ciprofloxacin (82%), tobramycin (77%), amikacin (58%), and imipenem (57%) showed a pronounced antipseudomonal effectiveness. The data indicate that MBC determinations are necessary to evaluate the antibiotic susceptibility of P. aeruginosa rather than MIC determinations, at least in patients with impaired defence mechanisms which require bactericidal therapy.