On the evolution of virulence during Staphylococcus aureus nasal carriage

Young, B. C. and D. J. Wilson (2012)
Virulence 3: 454-456. (pdf)

In a recent paper appearing in the Proceedings of the National Academy of Sciences of the United States of America, Young and colleagues report the case of a persistent carrier of Staphylococcus aureus who went on to develop a virulent bloodstream infection (Young et al., Proc Natl Acad Sci USA 2012). By sequencing the genomes of bacteria sampled from the nose and bloodstream over an extended period, they pieced together the evolutionary changes that accompanied the transition from asymptomatic carriage to invasive disease. Among an unusual cluster of knockout mutations that coincided with the beginning of a period of declining health in the patient, they identified the truncation of an AraC family transcriptional regulator as a likely modifier of virulence, demonstrating that genetic changes that occur in bacterial genomes during prolonged carriage could play a role in pathogenesis. Here we discuss the alternative evolutionary hypotheses for the observed excess of knockout mutations and review the evidence in favor of a causal role for regulatory gene dysfunction in bacterial virulence. We consider the prospects for systematic studies into the evolution of virulence in bacterial carriage populations.