Submitted to: Infection and Immunity
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 6/15/2001
Publication Date: 11/12/2001
Interpretive Summary: Few bacteria contaminate eggs often enough to cause a public health problem, but the incidence of illness in people from Salmonella enteritidis rose sharply over the past 20 years even though chickens were not getting noticeably sick. One of the differences between S. enteritidis and pullorum is that S. enteritidis produces flagella, whereas S. pullorum doesn't. We were curious to know if loss of flagella from a highly virulent strain of S. enteritidis increased the ability of this organism to cause disease in chicks, because there is no known biological reason why S. pullorum lost its flagella. Because the presence of flagella on the cell surface is controlled by a large number of factors, we made a mutant that only removed flagella (fliC) and a regulatory mutant (flhD) that had the potential to affect other cell functions. We found that only the regulatory mutant was enhanced in its ability to cause disease when given by mouth to birds, but not when it was injected. In contrast, the presence of flagella increased disease if it was injected into the bird, but not when given by mouth. These results give a first indication of how the ability to turn flagella on and off in relationship to regulation of other cell functions is very important for maximizing virulence in poultry.
Technical Abstract: The importance of flagella in Salmonella pathogenesis has been the subject of some debate. We have observed that an efficient egg-contaminating strain of Salmonella enterica serovar Enteritidis (S. enteritidis), SE-HCD, produces copious amounts of flagella and autoinduces LuxR regulated genes at high cell density, but it is parenterally adapted. Using two different chick models of infection, we compared the pathogenicity of aflagellate mutants (fliC::Tn10 mutant and flhD::Tn10 mutant) with SE-HCD. Neither mutant was significantly attenuated in 5-day-old chicks by either oral or parenteral challenge. Surprisingly, the flhD mutant showed significantly increased organ invasion and severity of infection after oral administration of a low dose to older chicks, whereas both flagellar mutants given parenterally were attenuated. Immunization of birds with flagellin reduced yields of organisms from the intestinal tract only, whereas a whole cell commercial bacterin reduced the numbers in both the intestinal tract and organs. These investigations suggest that the ability to undergo on/off flagellar phase variation is an important virulence attribute for maintenance of oral invasion by egg-contaminating S. enteritidis and for the first time provide a concept of the evolutionary pressures behind emergence of avian-adapted D1 serotypes of Salmonella.