In 2012, I started a post-doctoral fellowship studying the comparative virulence of Salmonella serovars. Salmonella is a diverse bacterial genus with well over 2600 distinct serovars having been described. In our work, we have found that Salmonella serovars have significant variation in their in vitro and in vivo virulence capacities. This work has been published in both Frontiers in Microbiology and Applied and Environmental Microbiology. We have analysed whole genome sequences of selected Salmonella serovars and compared the variation that occurs amongst genes controlling bacterial invasion, intracellular replication and other disease processes. We have found substantial variation in the amino acid sequences between the selected serovars and are now in the process of characterising how this amino acid variability translates to functional variability.

I recently completed an epidemiology project tracking where Salmonella enters the egg laying production system. Egg laying hens typically become infected with Salmonella when they are transferred from rearing farms to production facilities at 15-18 weeks of age (McWhorter and Chousalkar, unpublished). Infection with Salmonella at this point typically does not cause disease symptoms in the birds because the dose is generally low and chickens, in general, are more resistance to non-species specific Salmonella than other vertebrates. My work has shown that the bacteria establish persistent infection and can be shed intermittently over the 80 week production lifespan of a layer hen. This leads to increased contamination of the farm environment and can then be a risk for egg contamination.

On farm Salmonella control programs are multifaceted involving strict biosecurity, cleaning and disinfecting. The use of a live, attenuated strain of Salmonella Typhimurium as a vaccine has been adopted in certain parts of the poultry industry. The vaccine has been shown to have efficacy in chicken meat birds but because of the extended productive lifespan of a layer hen, it was not known how effective the vaccine is over an extended period. We conducted both controlled experimental and on farm trials which demonstrated that the vaccine was not able to prevent infection or reduce the total amount of Salmonella shed in faeces or the rate of egg contamination. This outcome has led the further collaboration with the vaccine manufacturer, Bioproperties.

Most recently, I have commenced a project testing the efficacy of several commercial sanitizers used in the chicken meat industry to control Campylobacter and Salmonella. I am also working on studying the behaviour of Salmonella in raw egg-based foods. In artificially contaminated aioli, the number of culturable bacteria decreases over time. I established, however, that while the bacteria were non-culturable, they remained viable (viable-but-nonculturable, VBNC). This part of my research program is now aimed at understanding the risk that VBNC Salmonella in food represents for human disease.