Identification and natural antibiotic susceptibility of Morganella morganii.

Our data should elucidate whether or not natural antibiotic susceptibility can be used as an aid for subspecies or biovar discrimination of Morganella morganii (II). Furthermore, our goal was to create a database of the natural susceptibility of M. morganii (III) and we were interested in the relative frequency of the recently described subspecific taxa (I). On the basis of trehalose fermentation (TRE), ornithine decarboxylase (ODC), and lysine decarboxylase (LDC) activities, we determined the biovar for 90 clinical isolates of M. morganii. Within these strains we examined the natural antibiotic susceptibility of 53 morganellae to 70 antibiotics by determination of the MICs with a microdilution procedure. (I): 80 strains (89%) of all morganellae belonged to M. morganii ssp. morganii (TRE-), with biovar A (LDC-, ODC+) predominating (67 strains). The remaining strains of this subspecies were identified as biovar B (LDC+, ODC+; 12 strains) and biovar C (LDC-, ODC-, one strain). Ten strains of M. morganii ssp. sibonii (TRE+) were found: four strains belonged to biovar F (LDC variable, ODC-) and six strains to biovar G (LDC-, ODC). (II): With one exception we found no significant differences in antibiotic susceptibility between different biovars. M. morganii ssp. morganii strains are more susceptible to tetracycline than strains of M. morganii ssp. sibonii, but there is no evidence that this parameter could be useful to differentiate biovars within a subspecies. It could be shown that 8 of 30 strains of biovar A and 2 of 12 strains of biovar B were tetracycline resistant. However, one M. morganii ssp. sibonii strain was clinically susceptible to tetracycline according to French and American standards. (III): The natural population of M. morganii is primarily (naturally) resistant to certain penicillins like benzylpenicillin, oxacillin, and amoxicillin, first and second generation cephalosporins (excluding cefoxitin), cefpodoxime, all antibiotics of the ML group (macrolides and lincosamides), sulfamethoxazole, glycopeptides, fosfomycin, and fusidic acid, naturally sensitive to aminoglycosides, piperacillin, mezlocillin, ticarcillin, third and fourth generation cephalosporins, carbapenems, aztreonam, quinolones, trimethoprim, cotrimoxazole, and chloramphenicol. M. morganii is naturally resistant to a wide range of antibiotics. The natural resistance pattern is useful for validation of sensitivity tests. Susceptibility to antibiotics is an unsuitable parameter for the discrimination of the described subspecific taxa of M. morganii. M. morganii strains that do not belong to biovar A are rare.