Antimicrobial Susceptibility Patterns (Antibiograms) as an Aid in Differentiating Citrobacter Species

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Abstract
Southern, Paul M., Jr., and Bagby, Mary K.: Antimicrobial susceptibility patterns (antibiograms) as an aid in differentiating Citrobacter species. Am J Clin Pathol 67: 187–189, 1977. The hydrogen sulfide-negative Citrobacter group represents a taxonomic problem. Various investigators have proposed such designations as Padlevvskia, 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.

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