Natural antibiotic susceptibility of Enterobacter spp., with special reference to Enterobacter aerogenes and Enterobacter intermedius strains.

The natural susceptibility to 71 antibiotics of 44 strains of Enterobacter aerogenes and 12 strains of Enterobacter intermedius was examined using a microdilution procedure in Isosensitest broth (for all strains) and cation-adjusted Mueller Hinton broth (for some strains). Both species were naturally sensitive or sensitive and intermediate to tetracyclines, all tested aminoglycosides, several penicillins and cephalosporins, carbapenems, aztreonam, quinolones, folate-pathway inhibitors, chloramphenicol and nitrofurantoin. Uniform natural resistance was found to cefoxitin and to antibiotics to which most other Enterobacteriaceae are also intrinsically resistant, e.g. several macrolides, lincosamides, streptogramins and glycopeptides. Major species-specific differences in susceptibility affecting clinical assessment criteria were found with amoxycillin, amoxycillin-clavulanate, some narrow-spectrum cephalosporins and fosfomycin. With the exception of penicillin G, oxacillin and cefoxitin, E. intermedius was naturally sensitive or naturally sensitive and intermediate (azlocillin) to all beta-lactams tested. Natural antibiotic susceptibility patterns of E. aerogenes and E. intermedius were analyzed with regard to the underlying mechanisms. The data were compared with the results from two recent studies dealing with natural susceptibilities of other clinically-relevant Enterobacter spp. With reference to beta-lactam susceptibility patterns, it can be assumed that all human-affecting Enterobacter species examined produce species-specific, chromosomally-encoded beta-lactamases of the AmpC type. The naturally-expressed amount of enzyme depends on the species.