PLoS Biology: Staphylococcus aureus invading the blood are less toxic

Toxicity in nose, blood and skin bacteria.

Collaborative work with Ruth Massey's group at the University of Bath taking forward a study of within-host evolution of Staphylococcus aureus during infection has been published in PLoS Biology. Previously we reported in PNAS that in one patient, bacteria causing a serious bloodstream infection differed by just eight mutations from a persistently carried nose population. We identified one of those mutations as playing a potentially causative role in transforming the nose bacteria into a form capable of bloodstream infection - a regulatory protein called rsp. To investigate further, Ruth applied a number of tests to characterize bacteria taken prior to and during infection. In this new paper, we report the surprising result that the bloodstream isolates show reduced toxicity and that rsp is the responsible for this change.

The notion that isolates responsible for serious human infection are less toxic challenges some long-held beliefs about the mechanism of disease in Staphylococcus aureus infections. Most models of disease assume a straightforward relationship between increased toxicity and greater virulence - the propensity to cause, or severity of, disease.

To test her observation, Ruth collaborated with groups from New York and Cambridge to investigate whether the pattern observed in one patient held more generally across 134 Staphylococcus aureus belonging to the notorious USA300 strain. It did.

Curiously, bacteria isolated from the skin and from superficial infections were equally toxic to nose bacteria. These findings raise new questions about the role of toxicity in colonization, transmission and serious infections of Staphylococcus aureus. One possibility that we wish to investigate further is whether toxicity might be required for the usual transmission of Staphylococcus aureus populations in the nose, skin or superficial infections (such as impetigo), whereas loss of toxicity may promote transition to deep tissue and bloodstream infections by evading immune defences.