In vitro activity of imipenem--a review

A review is given of the microbiological properties of imipenem, a new carbapenem antibiotic with an exceptionally broad spectrum of antibacterial activity. An evaluation of results of numerous in vitro studies reveals that imipenem effectively inhibited growth of 53 of 55 bacterial species, the mean MIC90 being less than 8 mg/l. The MIC90 for cocci, with the exception of Staphylococcus epidermidis, is in the range of 0.01-3.1 mg/l. The MIC90 for all Enterobacteriaceae is equal to or less than 8 mg/l. Pseudomonas aeruginosa and other non-fermentative gram-negative bacteria are generally susceptible to imipenem, only Pseudomonas maltophilia and Pseudomonas cepacia showing intrinsic resistance. Imipenem is currently the most active drug available against anaerobic bacteria, the MIC usually being below 1 mg/l even for Bacteroides fragilis. Rare bacteria such as Nocardia asteroides, Listeria monocytogenes or fast growing Mycobacterium spp. which cause difficult-to-treat infections are also susceptible to imipenem. Increases in inoculum size have only a minimal effect on activity of the drug. In most species the MBC only slightly exceeded the MIC; however in the case of Streptococcus faecalis the MBC value was many times the MIC value. Synergism has been observed in combinations of imipenem with aminoglycosides, and antagonism in combinations with other beta-lactam antibiotics against Pseudomonas aeruginosa and Serratia marcescens. Imipenem is stable in the presence of the common chromosomal and plasmid-mediated enzymes. Induction of inactivating enzymes was observed in staphylococci, Pseudomonas aeruginosa and Serratia marcescens.