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Poultry meat is an important source of zoonotic  Salmonella  infection. Oral vaccination of chickens with live attenuated  Salmonella  during grow-out is an attractive approach to control  Salmonella  colonization in the chicken gastrointestinal tract. In this study, we report the construction of methionine-dependent and growth of  Salmonella  Typhimurium mutant strains with methionine auxotrophy (Δ metR  and ΔΔ metRmetD ) and survival in chicken feed and fecal matrices. The methionine auxotroph mutant ΔΔ metRmetD  grew slowly on L-methionine but failed to grow on D-methionine, as expected, and exhibited lower affinity for methionine compared with the isogenic parent strain (Δ metR  single mutant) in whole-cell affinity experiments. Preliminary data conducted as part of a previously published bird challenge study indicated that the methionine auxotroph was less effective at protection in chickens to a challenge with virulent wild-type parent strain but generated greater  Salmonella -specific serum IgG. Although the auxotroph could not sustain itself in minimal media it was able to survive when incubated in the presence of chicken and fecal material. The immune response appears promising but further work may be needed to alter low-affinity methionine transporters and methionine biosynthesis genes in combination with the knock-out of the high affinity transporter  metD  reported here to ensure timely clearance of the candidate vaccine strain.

Growth Characterization of Single and Double Salmonella Methionine Auxotroph Strains for Potential Vaccine Use in Poultry