Pathogenicity of Salmonella Strains Isolated from Egg Shells and the Layer Farm Environment in Australia

Abstract

ABSTRACTIn Australia, the egg industry is periodically implicated during outbreaks ofSalmonellafood poisoning.Salmonella entericaserovar Typhimurium and other nontyphoidalSalmonellaspp., in particular, are a major concern for Australian public health. Several definitive types ofSalmonellaTyphimurium strains, but primarilySalmonellaTyphimurium definitive type 9 (DT9), have been frequently reported during egg-related food poisoning outbreaks in Australia. The aim of the present study was to generate a pathogenicity profile of nontyphoidalSalmonellaisolates obtained from Australian egg farms. To achieve this, we assessed the capacity ofSalmonellaisolates to cause gastrointestinal disease using bothin vitroandin vivomodel systems. Data fromin vitroexperiments demonstrated that the invasion capacity ofSalmonellaserovars cultured to stationary phase (liquid phase) in LB medium was between 90- and 300-fold higher than bacterial suspensions in normal saline (cultured in solid phase). During thein vivoinfection trial, clinical signs of infection and mortality were observed only for mice infected with either 103or 105CFU ofS. Typhimurium DT9. No mortality was observed for mice infected withSalmonellaserovars with medium or low invasive capacity in Caco-2 cells. Pathogenicity gene profiles were also generated for all serovars included in this study. The majority of serovars tested were positive for selected virulence genes. No relationship between the presence or absence of virulence genes by PCR and eitherin vitroinvasive capacity orin vivopathogenicity was detected. Our data expand the knowledge of strain-to-strain variation in the pathogenicity of Australian egg industry-relatedSalmonellaspp.