Abstract
An increasing number of severe infections caused by Staphylococcus aureus ST398 strains has been observed. However, it has not been elucidated whether all ST398 strains are equally virulent. We collected 13 strains from China and Canada to test in a Caenorhabditis elegans infection model and compared their whole genome sequences (WGS) to explore potential insights into their virulence. All isolates belonged to ST398-methicillin-susceptible S. aureus (MSSA) with variant spa types (t034, t571, t1451, t1250). Pulsed field gel electrophoresis (PFGE) and WGS analyses showed that the 13 isolates clustered into 3 genomic types (Types A-C). WGS and prophage phylogenetic analyses also revealed that the strains could be divided into 3 phage groups (Groups 1–3), which correlated with high-, moderate-, and low-nematocidal activities, with mean killing rates of 94, 67, and 40%, respectively. Group 1 carried ϕSa3-Group 1 (ϕSa3-G1), Group 2 carried ϕSa3-G2, and Group 3 lacked ϕSa3. Interestingly, strain GD1706 (that genetically clustered within Type C) and strain GD487 (within Type B) both carried ϕSa3-G1 like phages and killed 92% of the nematodes, similar to the Type A strains carrying ϕSa3-G1. This study demonstrated that different ST398 sub-lineages possess variable virulence capacities, depending on the presence or absence, as well as the structure of the prophage ϕSa3 that carries virulence factors.IMPORTANCESince first being reported in the early 2000s, Staphylococcus aureus ST398 has not only become recognized as a frequent colonizing strain in economically important livestock animals, but has also proven to be a concern for infection in humans and, in particular, has been linked to higher rates of severe invasive human infections. We collected ST398 strains from China and Canada to test in a worm (Caenorhabditis elegans) infection model and compared their whole genome sequences to gain insight into pathogenesis. We have shown that different ST398 sub-strains differ in their virulence potential based on the presence or absence and structure of prophage ϕSa3, which carries important virulence factors. Our observations suggest that ST398 strains are relatively heterogeneous from a clinical perspective, and more studies are needed to differentiate between virulent and non-virulent ST398 strains to determine the true global spread of relevant sub-strains.
Highlights
IntroductionSince first being reported in the early 2000s in association with livestock (LA), multilocus sequence type ST398 Staphylococcus aureus has become recognized as a significant colonizing strain in livestock animals and occasionally acting as a pathogen causing infections in livestock, and companion animals, with the majority of strains displaying a multi-resistant phenotype and reported as methicillin resistant (MRSA) (McNamee and Smyth, 2000; Menzies and Ramanoon, 2001; Bradley, 2002; Voss et al, 2005; Springer et al, 2009; Cuny et al, 2010; van Duijkeren et al, 2010; Weese, 2010; Graveland et al, 2011; Dorado-Garcia et al, 2013)
pulsed field gel electrophoresis (PFGE) typing and molecular analysis revealed that all of our ST398 isolates clustered apart from other dominant methicillin resistant (MRSA) strains in Canada and United States described to date, sharing only 40% similarity with these other strains (Figure 1)
We examined the genetic background and virulence of a collection of ST398-MSSA strains isolated from mainland China and Calgary, Canada, and found that there are differences in virulence noted between subgroups of the strain, and that carriage/structure of φSa3 appears to play a major role in that virulence
Summary
Since first being reported in the early 2000s in association with livestock (LA), multilocus sequence type ST398 Staphylococcus aureus has become recognized as a significant colonizing strain in livestock animals and occasionally acting as a pathogen causing infections in livestock, and companion animals, with the majority of strains displaying a multi-resistant phenotype and reported as methicillin resistant (MRSA) (McNamee and Smyth, 2000; Menzies and Ramanoon, 2001; Bradley, 2002; Voss et al, 2005; Springer et al, 2009; Cuny et al, 2010; van Duijkeren et al, 2010; Weese, 2010; Graveland et al, 2011; Dorado-Garcia et al, 2013). Human colonization and infection with LA-ST398-MRSA was initially reported among swine farmers in France and the Netherlands in the early 2000s, affecting people working in close contact with livestock and other farm animals, with the human isolates genetically linked to those collected from animals (Armand-Lefevre et al, 2005; Voss et al, 2005; Huijsdens et al, 2006; van Belkum et al, 2008; van Rijen et al, 2008; Grisold et al, 2010; Graveland et al, 2011) Since these initial reports, ST398 strains have subsequently been reported as causing infections in humans in the absence of livestock exposure, with isolates predominantly described as being methicillin-sensitive (MSSA) (Uhlemann et al, 2012a, 2013; Brunel et al, 2014). Whole genome sequence (WGS) analysis of the strains was done, with the aim of revealing insights into the virulence of this emerging pathogen
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