Antimicrobial susceptibility patterns (antibiograms) as an aid in differentiating citrobacter species

The hydrogen sulfide-negative Citrobacter group represents a taxonomic problem. Various investigators have proposed such designations as Padlewskia, Levinea, atypical Enterobacter cloacae, H₂S-negative variants of Citrobacter, Citrobacter koseri and Citrobacter diversus. This problem has been investigated with emphasis on antibiograms as a means of discrimination. Clinical isolates fitting the designation Citrobacter were studied and, using the criteria of Ewing and Davis, separated into two groups: C. diversus (40 strains) or C. freundii (25 strains). Susceptibilities to ampicillin, carbenicillin, cefazolin, cephaloridine and cephalothin were determined by the agar-dilution method. C. diversus strains were resistant to 8 μg/ml ampicillin (97.5%) and 32 μg/ml carbenicillin (87.5%), and were susceptible to 8 μg/ml cephalosporins (≥90%). C. freundii strains were moderately susceptible to 8 μg/ml ampicillin (25%) and susceptible to 8 μg/ml carbenicillin (92%), and were resistant to 8 μg/ml cephalosporins (≥92%). Using these data one can separate C. diversus from C. freundii with 90% accuracy.