Salmonella enterica (S. enterica) is a bacterial pathogen with a worldwide association with animal and human disease and is therefore of great importance to both medicine and veterinary medicine. Livestock species such as cattle, pigs, and poultry are both critical reservoirs of salmonellosis in humans and serve as hosts for interestingly host-adapted Salmonella serovars where the outcome of infection is dependent on host-serovar specificity. In pigs, an important example of host-adaptation is S. enterica serovar Choleraesuis, a serovar adapted to cause systemic typhoid-like disease in pigs but enteritis in cattle. S. Choleraesuis and serovar Typhimurium – which is an example of a serovar which causes self-limiting in a wide range of hosts including pigs, cattle, and chickens – both use type III secretion systems (T3SS) as critical virulence factors for the invasion and survival within a host. The T3SS has previously been hypothesised to have a strong impact on host adaptation of Salmonella.
We investigated and characterised the secretomes of two strains of well-defined virulence in pigs and cattle – S. Tyhimurium ST4/74 and S. Choleraesuis SCSA50 – under T3SS-inducing in vitro conditions using label-free quantitative proteomics. Our main finding was that not only does the repertoire of secreted proteins differ between ST4/74 and SCSA50, but the amount of protein secreted was also significantly different. These results could not have been inferred from the published genome sequences. Validation by immunoblotting using monoclonal antibodies has confirmed this result and we are now investigating the regulation of the T3SS in these strains at the transcriptional level, with the ultimate goal of clarifying links between protein secretion, host-adaptation, and the zoonotic potential of Salmonella serovars.