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

Abstract

bIn Australia, the egg industry is periodically implicated during outbreaks of Salmonella food poisoning. Salmonella enterica serovar Typhimurium and other nontyphoidal Salmonella spp., in particular, are a major concern for Australian public health. Several definitive types of Salmonella Typhimurium strains, but primarily Salmonella Typhimurium 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 nontyphoidal Salmonella isolates obtained from Australian egg farms. To achieve this, we assessed the capacity of Salmonella isolates to cause gastrointestinal disease using both in vitro and in vivo model systems. Data from in vitro experiments demonstrated that the invasion capacity of Salmonella serovars 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 the in vivo infection trial, clinical signs of infection and mortality were observed only for mice infected with either 10 3 or 10 5 CFU of S. Typhimurium DT9. No mortality was observed for mice infected with Salmonella serovars 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 either in vitro invasive capacity or in vivo pathogenicity was detected. Our data expand the knowledge of strain-to-strain variation in the pathogenicity of Australian egg industry-related Salmonella spp. Salmonella is one of the most common causes of food-borne infection worldwide. In Australia, the egg industry is periodically implicated in cases of Salmonella food poisoning (1). Uncooked or partially cooked foods containing raw egg as an ingredient accounted for 14% of food-borne outbreaks in 2006, 13% in 2007, and 28% in the first quarter of 2008 (2). It has been shown that some Salmonella serovars, such as Salmonella enterica serovar Enteritidis, have the capacity to infect developing eggs within the oviduct, and therefore contaminated eggs act as an ecological amplifier (3). It is believed that this could then facilitate the dissemination of Salmonella into the food chain and its eventual transmission to humans. These studies, however, for the most part have been focused on Salmonella Enteritidis with limited investigation of S. enterica serovar Typhimurium. The dramatic increase in Salmonella Enteritidis infections occurring in overseas countries has not been observed in Australia (4). Salmonella Typhimurium and other nontyphoidal Salmonella spp., however, have become a major concern for the Australian egg industry.S. enterica serovars are a diverse group of pathogens that have evolved to survive in a wide range of environments and across multiple hosts (5). There are several definitive types of S...

Dynamics of Salmonella Shedding and Welfare of Hens in Free-Range Egg Production Systems

“…Although the MLVA profiles of S. Typhimurium isolated in this study and from egg human outbreaks were similar, further studies are required for understanding the invasion potential of these isolates. It was demonstrated that the invasive potential of S. Typhimurium increased after enrichment (31). A hypothesis might be that the invasive potential of S. Typhimurium is influenced by the available favorable environment at the time of invasion.…”

“…The prevalence of these serovars was low in this study. Our previous study indicated that the presence of virulence genes may not necessarily dictate the virulence/invasive ability of Salmonella (31). The PCR results indicated that most of the above-listed serovars harbored virulence genes, but in vivo studies are required to study their virulence potential.…”

“…It is possible that some Salmonella isolates either acquired virulence genes or evolved from a more virulent type (S. Typhimurium, in this case). Previous studies detected virulence genes in various Salmonella isolates collected from cage farms (31,34). However, there is limited information on virulence typing of Salmonella serovars collected from free-range environments.…”

In vitro invasiveness and intracellular survival of Salmonella strains isolated from the aquatic environment

“…The capacity of Salmonella to survive in different conditions is also based on their ability to use environmental metabolites outside a host (Medrano‐Félix et al, ). However, other studies have demonstrated that some Salmonella strains are invasive with active virulence genes regardless of the isolation origin (McWhorter et al, ).…”

In vitro invasion capacity of Salmonella Typhimurium DT9 isolates sourced from humans and layer hen environments

“…Salmonella infection involves colonization of the small intestine followed by invasion of intestinal epithelial cells (Wallis & Galyov, ). Non‐typhoidal Salmonella serovars exhibit a wide range of in vitro invasion capacity (McWhorter & Chousalkar, ; McWhorter, Davos, & Chousalkar, ; Pang, Lin, Tsai, & Tsen, ; Shah, Zhou, Addwebi, Davis, & Call, ; Shah et al., ) that correlate differences in overall virulence (McWhorter & Chousalkar, ; McWhorter et al., ; Shah et al., ). These differences are likely due to genetic variation within (McWhorter & Chousalkar, ) or differential expression of Salmonella virulence genes (Shah et al., ).…”