In vitro antibacterial activity of imipenem against Pseudomonas and Staphylococcus species. Comparison with thirteen other antimicrobial agents.

The in vitro antimicrobial activity of imipenem against recent clinical isolates of Pseudomonas spp. (94 strains) and penicillin-resistant Staphylococcus spp. (50 Staph. aureus and 50 coagulase-negative Staphylococcus) was assessed using the Mueller-Hinton agar dilution method. Results were compared with those simultaneously obtained for amikacin, netilmicin, tobramycin, norfloxacin, piperacillin, ceftazidime, ceftriaxone and azthreonam against Pseudomonas spp., and for rifampicin, clindamycin, netilmicin and cefoxitin, besides penicillin and methacillin, against Staphylococcus spp. About 50 and 90% of 84 Pseudomonas aeruginosa isolates were inhibited by concentrations of imipenem equal to or less than 2 and 8 mg/l respectively. The in vitro activity of imipenem was comparable to that of ceftazidime and norfloxacin, but superior to that of the aminoglycosides and all the other antibiotics tested, in terms of potency by weight. Among other Pseudomonas spp. only P. malthophilia (2 strains) proved resistant to imipenem. Rifampicin was the most active antibiotic by weight against Staph. aureus but imipenem was more active than clindamycin and, especially, netilmicin and cefoxitin. Imipenem was highly active also against coagulase-negative staphylococci, with some differences related to the high incidence of methicillin-resistant strains. MICs of imipenem in Mueller-Hinton broth correlated with those obtained in agar, unlike the aminoglycosides. There were no significant inoculum effects on MICs of imipenem and MBCs were within one twofold dilution of MICs in over 75% of assays.