Natural Colistin Resistance in Edwardsiella Tarda

All of 37 recent human isolates of Edwardsiella tarda were resistant to colistin. All strains were resistant to 6.3 mug of sodium colistimethate per ml; 82% of them were resistant to 100 mug/ml. With 34 Salmonella strains, the average minimal inhibitory concentration (MIC) was 1.6 mug of sodium colistimethate per ml. There was virtually no overlapping of MIC values between both groups of bacteria. Using the Kirby-Bauer method and 10-mug colistin disks, inhibition zone diameters were less than 11 mm for all Edwardsiella strains, and larger for all Salmonella strains. An atypical, mannitol-fermenting strain of E. tarda was resistant to colistin. It is concluded that E. tarda is naturally resistant to colistin, that this property may be helpful in differentiation, and that colistin should be tried in enrichment and selective media for the isolation of E. tarda. It was also observed that, under conditions as described, all strains of E. tarda were susceptible to streptomycin and to novobiocin, whereas all strains of Salmonella were resistant.