Objective: To investigate the natural susceptibility to 69 antimicrobial agents of 107 Enterobacter strains comprising E. amnigenus (n = 18), E. cancerogenus (n = 26), E. gergoviae (n = 28) and E. sakazakii (n = 35).
Methods: Minimal inhibitory concentrations (MICs) were determined with a microdilution procedure in Isosensitest broth and cation-adjusted Mueller-Hinton broth.
Results: All the species were naturally sensitive or intermediate to tetracyclines, amino-glycosides, numerous beta-lactams (acylureidopenicillins, ticarcillin, ampicillin/sulbactam, several cephalosporins, carbapenems, aztreonam), quinolones, antifolates, chloramphenicol and nitrofurantoin. Natural resistance was found to penicillin G, oxacillin, several macrolides, lincosamides, streptogramins, glycopeptides, rifampicin and fusidic acid. Species-related differences in natural susceptibility were found to some beta-lactams, azithromycin and fosfomycin. Whereas E. gergoviae was the most susceptible species to azithromycin, E. cancerogenus was most susceptible to fosfomycin and was the only species showing natural resistance to amoxicillin, amoxicillin/clavulanic acid, cefaclor, cefazoline, loracarbef and cefoxitin. There were only minor medium-dependent differences in susceptibility to most antibiotics.
Conclusions: The present study establishes a database concerning the natural susceptibility of recently established Enterobacter species to a wide range of antibiotics, which can be applied for the validation of routine susceptibility test results. beta-Lactam susceptibility patterns indicate the expression of species-specific beta-lactamases expressed at high or low levels in all the species except E. sakazakii.