The Role of Motility and Aerotaxis in the Selective Increase of Avirulent Bacteria in Still Broth Cultures of Pseudomonas solanacearum

Summary: A difference in relative motility between virulent and avirulent Pseudomonas solanacearum 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 P. solanacearum when oxygen became limiting in broth media, in which virulent bacteria were not actively motile.