Abstract
Salmonella enterica is a foodborne zoonotic pathogen of significant public health concern. We have characterized the virulence and antimicrobial resistance gene content of 95 Salmonella isolates from 11 serovars by DNA microarray recovered from UK livestock or imported meat. Genes encoding resistance to sulphonamides (sul1, sul2), tetracycline [tet(A), tet(B)], streptomycin (strA, strB), aminoglycoside (aadA1, aadA2), beta-lactam (blaTEM), and trimethoprim (dfrA17) were common. Virulence gene content differed between serovars; S. Typhimurium formed two subclades based on virulence plasmid presence. Thirteen isolates were selected by their virulence profile for pathotyping using the Galleria mellonella pathogenesis model. Infection with a chicken invasive S. Enteritidis or S. Gallinarum isolate, a multidrug resistant S. Kentucky, or a S. Typhimurium DT104 isolate resulted in high mortality of the larvae; notably presence of the virulence plasmid in S. Typhimurium was not associated with increased larvae mortality. Histopathological examination showed that infection caused severe damage to the Galleria gut structure. Enumeration of intracellular bacteria in the larvae 24 h post-infection showed increases of up to 7 log above the initial inoculum and transmission electron microscopy (TEM) showed bacterial replication in the haemolymph. TEM also revealed the presence of vacuoles containing bacteria in the haemocytes, similar to Salmonella containing vacuoles observed in mammalian macrophages; although there was no evidence from our work of bacterial replication within vacuoles. This work shows that microarrays can be used for rapid virulence genotyping of S. enterica and that the Galleria animal model replicates some aspects of Salmonella infection in mammals. These procedures can be used to help inform on the pathogenicity of isolates that may be antibiotic resistant and have scope to aid the assessment of their potential public and animal health risk.
Highlights
Salmonella enterica is a Gram-negative bacterium of significant public and animal health concern worldwide (Majowicz et al, 2010)
Sixty-four of the 95 field isolates were positive by microarray for one or more antimicrobial resistance (AMR) gene; no AMR genes were present in the remaining isolates including LT2 (Supplementary Table S1)
We have used virulence and AMR genotyping to provide an assessment of the pathogenic potential of a variety of S. enterica serovars and isolates and their susceptibility to antibiotic therapy
Summary
Salmonella enterica is a Gram-negative bacterium of significant public and animal health concern worldwide (Majowicz et al, 2010). S. enterica harbor a diverse assortment of virulence genes required for adhesion, invasion, intra-cellular survival, and replication. These genes are located on various elements of the genome including the chromosome, plasmids, integrated bacteriophage DNA, Salmonella pathogenicity islands (SPIs), and Salmonella genomic islands (SGIs). Some of these elements are conserved throughout the species, such as SPI1 (encoding factors important for cell adhesion) and SPI2 (encoding factors required for intracellular survival and replication); while other elements are serovar specific, e.g., SPI-7 in S. Tools that can determine the resistance profile and inform on the potential for pathogenicity of Salmonella isolated from food animals or food products can inform on the possible risk posed to public or animal health
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