Colistin-sensitive Proteus organisms: including indole-negative Proteus vulgaris, non-swarming on first isolation.

Forty-eight isolates of and 4 isolates of were found by disc diffusion to be sensitive to colistin sulphomethate. The minimum inhibitory concentration (MIC) of colistin sulphomethate of the isolates ranged from 3·125-200 μg. per ml. The MIC was lower with a smaller inoculum, and for most strains the MIC in agar was lower than the MIC in broth. On first isolation only 48 per cent of the colistin-sensitive strains exhibited swarming at 37°, and after passage through semi-solid agar 25 per cent of the strains still did not swarm. None of the strains produced indole.