Functional characterisation of superantigens in Staphylococcus aureus disease pathogenesis

Bacterial superantigens (SAgs) are virulence factors that induce nonspecific T-cell proliferation contributing to host immune avoidance, and occasionally severe life-threatening toxinoses such as toxic shock syndrome. In the current study, the multiple functions of 3 superantigens named staphylococcal enterotoxin-like toxins X, Y and Z are investigated. SElX and SElZ were non-emetic in a musk shrew model of emesis. SElX is structurally and phylogenetically related to staphylococcal superantigen-like proteins (SSls) which are non-mitogenic but exhibit a variety of immune modulatory properties. We carried out protein and gene expression analysis of mutants of different S. aureus gene regulators and demonstrated that selx expression is controlled by saeRS, a two-component regulator linked to the bacterial response to phagocytic signals. Considering the co-regulation of SElX with known mediators of innate immune evasion we investigated a potential role for SElX in both humoral and cellular innate immune modulation and discovered that SElX strongly binds to human, bovine, murine, and laprine neutrophils and interferes with IgG-mediated phagocytosis, independently of Fcγ receptor signalling. Bacterial survival assays with neutrophils demonstrated that the deletion of selx significantly reduced the ability of S. aureus to resist neutrophil killing. Site-directed mutagenesis in the conserved sialic acid-binding motif of SElX abolished its neutrophil binding capacity, which is consistent with a critical role for glycosylated receptors in this interaction. Importantly, the sialic-acid binding mutants of SElX retained the ability to induce T-cell proliferation demonstrating that the distinct functions of SElX are mechanistically independent. Affinity precipitation experiments identified potential glycoprotein receptors for SElX and the interaction with protein ICAM-3, an important ligand for MAC-1 integrins, was validated suggesting SElX may interfere with cell signalling. Taken together, we present the first example of a bi-functional SAg that can manipulate two distinct arms of the human immune system and contribute to S. aureus survival during infection

Recombination-mediated remodelling of host-pathogen interactions during Staphylococcus aureus niche adaptation

Large-scale recombination events have led to the emergence of epidemic clones of several major bacterial pathogens. However, the functional impact of the recombination on clonal success is not understood. Here, we identified a novel widespread hybrid clone (ST71) of livestock-associated Staphylococcus aureus that evolved from an ancestor belonging to the major bovine lineage CC97, through multiple large-scale recombination events with other S. aureus lineages occupying the same ruminant niche. The recombination events, affecting a 329 kb region of the chromosome spanning the origin of replication, resulted in allele replacement and loss or gain of an array of genes influencing host–pathogen interactions. Of note, molecular functional analyses revealed that the ST71 hybrid clone has acquired multiple novel pathogenic traits associated with acquired and innate immune evasion and bovine extracellular matrix adherence. These findings provide a paradigm for the impact of large-scale recombination events on the rapid evolution of bacterial pathogens within defined ecological niches

Recombination-mediated remodelling of host–pathogen interactions during staphylococcus aureus niche adaptation

Large-scale recombination events have led to the emergence of epidemic clones of several major bacterial pathogens. However, the functional impact of the recombination on clonal success is not understood. Here, we identified a novel widespread hybrid clone (ST71) of livestock-associated Staphylococcus aureus that evolved from an ancestor belonging to the major bovine lineage CC97, through multiple large-scale recombination events with other S. aureus lineages occupying the same ruminant niche. The recombination events, affecting a 329 kb region of the chromosome spanning the origin of replication, resulted in allele replacement and loss or gain of an array of genes influencing host-pathogen interactions. Of note, molecular functional analyses revealed that the ST71 hybrid clone has acquired multiple novel pathogenic traits associated with acquired and innate immune evasion and bovine extracellular matrix adherence. These findings provide a paradigm for the impact of large-scale recombination events on the rapid evolution of bacterial pathogens within defined ecological niches