Abstract
Salmonella can survive and persist in the environment, acting as a reservoir for infection of animals and ultimately humans through the food chain. Biofilm formation contributes to Salmonella survival and compromises cleaning and disinfection. The overall objective of the research detailed in this thesis was to further understand biofilm formation by Salmonella in pig and poultry production environments.
Biofilm formation by 132 Salmonella enterica subspecies enterica environmental isolates was studied by phenotypic characterisation, genomic analysis, and representative in vitro biofilm models. Biofilm-forming ability was assessed using the crystal violet microplate assay at environmentally relevant temperatures, with analysis of the influence of serovar, culture conditions and source environment. Bacterial motility, growth kinetics, antimicrobial resistance (AMR) profiles and colony morphology were also studied. A subset of isolates were analysed using whole-genome sequencing and then screened for genes associated with biofilm formation, AMR, and tolerance to quaternary ammonium compounds (QACs), with comparative genomic analysis of phenotype-genotype associations. A subset of S. 13,23:i:- isolates were evaluated for their tolerance to commercial disinfectants using modified coupon-based and metabolic microplate-based assays. Finally, biofilm formation was assessed in a developed in vitro model of a poultry drinking water system (DWS).
Most Salmonella isolates (130/132), representing a range of serovars, formed in vitro biofilms to varying degrees. Differences were observed, particularly between serovars, in planktonic and biofilm phenotypes and the prevalence of biofilm-associated genes and AMR determinants. However, any definitive conclusions regarding the factors influencing in vitro biofilm formation could not be reached. The findings suggest that the persistence of S. 13,23:i:- in the hatchery environment and its observed tolerance to Benzalkonium-chloride and QAC-/glutaraldehyde-based disinfectants may be facilitated by biofilm formation and/or the presence of QAC tolerance genes. In the poultry DWS model, biofilms were formed and quantified successfully, despite inoculation with low levels of bacteria.
The studies presented in this thesis provide new information about biofilm formation by Salmonella originating from pig and poultry production environments, and the poultry DWS provides a novel in vitro method for the study of biofilm formation under realistic conditions. These findings may be used to inform improved Salmonella control during primary production of food-producing animals.