Human non-typhoidal salmonellosis is frequently acquired from poultry, pigs and cattle where Salmonella often persist without pathology. Some serovars cause enteritis or typhoid in livestock and the factors underlying their differential virulence are ill-defined.
We applied transposon-directed insertion-site sequencing (TraDIS) to screen 8550 S. Typhimurium mutants for their ability to colonise the intestines of chickens, pigs and calves. TraDIS uses massively-parallel sequencing of transposon-flanking regions to simultaneously map insertion sites and quantify the abundance of the cognate mutants in complex pools. TraDIS assigned the identity and phenotype of >90% of mutants screened, defining roles for 2715 genes in reservoir hosts with minimal animal use. A core set of genes mediating colonisation of each host was identified but mutations in some loci produced host-specific phenotypes not observed in rodents. By signature-tagging of mutants we also assigned spatial and temporal roles to S. Dublin genes during systemic translocation in cattle following oral or intravenous dosing and surgical cannulation. This assigned phenotypes to Salmonella pathogenicity islands, S. Dublin-specific sequences and sensory systems and revealed that some genes play roles specific to anatomical niche.
Separately, we have identified avian genes associated with resistance to fowl typhoid using inbred chicken lines that are resistant or susceptible to S. Gallinarum infection. Analysis of the distribution of microsatellites and SNPs in the progeny of crosses between the lines associated resistance with a locus encoding the kinase Akt1. We observed that avian Akt1 is activated by the T3SS-1 effector SopB to promote net intracellular replication. Basal and activated levels of Akt1 differ in bone marrow-derived macrophages from the two lines, consistent with the outcome of S. Gallinarum infection. Differential resistance of the lines was diminished upon oral infection with a S. Gallinarum sopB mutant, indicating that distinct responses to wild-type Salmonella rely on Akt1 activation.