Flagella of Salmonella typhimurium are a virulence factor in infected C57BL/6J mice

To determine whether flagella, chemotaxis, and motility of Salmonella typhimurium are virulence factors in infected C57BL/6J mice, we constructed isogenic pairs of derivatives of the nonfimbriated virulent strain SL3201. Of each pair, one member contained a mutation in a single gene that is required for expression of normal chemotactically directed motility, whereas the other member contained the wild-type form of the gene. No additional differences between the members of a pair were evident. The phenotypic parameters examined for all derivatives included in vitro growth rate, sensitivity to P22 phage, amino acid auxotrophy, and biotype. For a flagellated and nonflagellated pair, the electron microscopic appearance of each member was examined as well as its lipopolysaccharide and outer membrane profiles by sodium dodecyl sulfate-polyacrylamide gel electrophoresis. The virulence of the various derivatives was then assessed in mice challenged orally, intraperitoneally, or intravenously. The results established that flagella, whether functional or nonfunctional as organelles of motility, were S. typhimurium virulence factors and that neither chemotaxis nor motility was required for virulence.