Differential susceptibility of aeromonads and coliforms to cefsulodin

Cefsulodin was evaluated as a potential selective agent for aeromonads. Resistance of Aeromonas and coliform isolates was determined by using a standard disk diffusion technique, A total of 119 Aeromonas and 78 coliform strains were isolated, For 102 of 129 Aeromonas isolates (environmental and reference strains), the MIC of cefsulodin was <8 mu g/ml. Results of MIC tests by the agar dilution method showed that a concentration of cefsulodin of 10 mu g/ml or less inhibited the growth of 96% of isolates, In comparison, for 81 of 94 coliform isolates (environmental and reference strains), the MIC of cefsulodin was >32 mu g/ml. Because cefsulodin suppresses growth of Aeromonas and other oxidase-positive organisms, total coliform (TC) and Escherichia coli counts on Chromocult Coliform agar (CC agar) without cefsulodin and on CC agar with 10 mg of cefsulodin per liter (CC-CFS) were compared, Variance analysis of data from II sewage-polluted irrigation water specimens did not demonstrate any statistically significant difference in the enumeration off. coli with CC and CC-CFS media, On average, the CC agar recovered 2.46 times as many TCs as CC-CFS, However, Aeromonas colonies made up an average of 58.6% of the TC counts on CC agar. Because no Aeromonas spp, were recovered on CC-CFS, background interference was eliminated and the counts that were obtained reflected more accurately the number of TCs, Results of this study suggest that cefsulodin may be a useful selective agent against Aeromonas spp, which should be included in coliform chromogenic media when high levels of accompanying flora are expected.