Salmonella is a globally significant food-borne pathogen capable of persisting on-farm, posing a significant risk to the food chain through contamination of subsequent production stages. On-farm persistence varies between Salmonella serovars, but the underlying reasons for this remain poorly understood.

This thesis aimed to investigate differences in susceptibility between Salmonella serovars and isolates to commercially available disinfectants, to identify genetic factors conferring reduced susceptibility, and to assess the potential of efflux pump inhibitors (EPIs) to enhance disinfectant efficacy.

Minimum inhibitory concentration (MIC) and minimum bactericidal concentration (MBC) assays were used to screen nine serovars. Salmonella 13,23:i:- isolates (n=20) demonstrated significantly reduced susceptibility to two aldehyde and two Quaternary Ammonium Compound (QAC) products (p<0.001) in MIC and/or MBC assays. Growth kinetics assays identified ten S. 13,23:i:- isolates  with decreased susceptibility to the aldehyde-based disinfectant Viroshield. Genomic analysis identified that all ten isolates harboured an IncHI1 plasmid (p<0.001).

Curing the IncHI1 plasmid from one S. 13,23:i:- isolate resulted in significantly increased susceptibility to Viroshield (p<0.001). Moreover, conjugation of the plasmid back into the cured isolate restored pre-curing tolerance to the aldehyde disinfectant. These findings demonstrate the reduced susceptibility of S. 13,23:i:- isolates was due to the presence of the IncHI1 plasmid.

Pan-genome analysis identified two families of efflux pumps present on the plasmid. In vitro investigation into enhancing disinfectant efficacy with the EPI 1-(1-naphthylmethyl)-piperazine (NMP) in combination with a QAC and aldehyde-based disinfectant showed that NMP at 250µg/ml could potentially enhance disinfectant efficacy (p<0.05).

Importantly, this thesis refined three in vitro assays that will aid the study of susceptibility of Salmonella  isolates  to disinfectants (MIC, MBC, and growth kinetics assays). These will be used in future studies to improve food safety by enabling targeted cleaning and disinfection strategies on-farm where the use of disinfectants is particularly important considering growing concerns over AMR.