The Staphylococcus aureus LXG-domain toxins EsxX and SAR0287 do not promote virulence in a zebrafish larval infection model

The <i>Staphylococcus aureus</i> type VIIb secretion system (T7SSb) is a multiprotein secretion system that secretes toxins with antibacterial activity, but which is also required for full virulence in animal models of infection. <i>S. aureus</i> strains carry one of four T7SSb locus types, named <i>essC1</i> to <i>essC4</i>, each of which encodes a characteristic LXG-family substrate at the T7SS locus. In <i>essC2</i> strains, this LXG-domain protein is EsxX, which has a glycine zipper sequence in its C-terminus and has potent antibacterial, membrane-depolarizing activity. In this work, we recognize conserved features of the <i>essC2</i> and <i>essC3</i> systems, identifying the LXG protein SAR0287 as structurally and functionally similar to EsxX. Using a zebrafish larval hindbrain ventricle infection model, we demonstrate that the T7SSb of <i>essC2</i> and <i>essC3</i> representative strains contributes to bacterial replication and zebrafish mortality. However, there is no significant loss of virulence in the model system if EsxX or SAR0287 is absent. These findings indicate that there is no discernible role for either toxin in this virulence model.