Summary: A difference in relative motility between virulent and avirulent was a possible factor in the shift from virulent to avirulent populations in still broth cultures. Virulent isolates grown on solid media or in tryptone yeast-extract glucose or glycerol broth for 24 to 48 h were mainly non-flagellated or non-motile, whereas avirulent isolates grown under the same conditions were usually flagellated and highly motile. Fimbriae were observed in electron photomicrographs of both types. A rapid preferential increase of avirulent bacteria occurred when mixtures were grown in still, but not in shaken, broth cultures; this relative increase was greatest close to the surface of the medium. Rapid aerotaxis of avirulent bacteria was demonstrated in mixtures of virulent and avirulent bacteria in a semisolid motility agar medium. Positive aerotaxis and high motility apparently favoured rapid increase of avirulent when oxygen became limiting in broth media, in which virulent bacteria were not actively motile.


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