Absence of all components of the flagellar export and synthesis machinery differentially alters virulence of Salmonella enterica serovar typhimurium in models of typhoid fever, survival in macrophages, tissue culture invasiveness, and calf enterocolitis

In this study, we constructed an flhD (the master flagellar regulator gene) mutant of Salmonella enterica serovar Typhimurium and compared the virulen ce of the strain to that of the wild-type strain in a series of assays that included the mouse model of typhoid fever, the mouse macrophage survival a ssay, an intestinal epithelial cell adherence and invasion assay, and the c alf model of enterocolitis. We found that the flhD mutant was more virulent than its parent in the mouse and displayed slightly faster net growth betw een 4 and 24 h of infection in mouse macrophages. Conversely, the flhD muta nt exhibited diminished invasiveness for human and mouse intestinal epithel ial cells, as well as a reduced capacity to induce fluid secretion and evok e a polymorphonuclear leukocyte response in the calf ligated-loop assay. Th ese findings, taken with the results from virulence assessment assays done on an fljB fliC mutant of serovar Typhimurium that does not produce flagell in but does synthesize the flagellar secretory apparatus, indicate that nei ther the presence of flagella (as previously reported) nor the synthesis of the flagellar export machinery are necessary for pathogenicity of the orga nism in the mouse. Conversely, the presence of flagella is required for the full invasive potential of the bacterium in tissue culture and for the inf lux of polymorphonuclear leukocytes in the calf intestine, while the flagel lar secretory components are also necessary for the induction of maximum fl uid secretion in that enterocolitis model. A corollary to this conclusion i s that, as has previously been surmised but not demonstrated in a comparati ve investigation of the same mutant strains, the mouse systemic infection a nd macrophage assays measure aspects of virulence different from those of t he tissue culture invasion assay, and the latter is more predictive of find ings in the calf enterocolitis model.