Abstract
Selection pressures exerted on Staphylococcus aureus by host factors during infection may lead to the emergence of regulatory phenotypes better adapted to the infection site. Traits convenient for persistence may be fixed by mutation thus turning these mutants into microevolution endpoints. The feasibility that stable, non-encapsulated S. aureus mutants can regain expression of key virulence factors for survival in the bloodstream was investigated. S. aureus agr mutant HU-14 (IS256 insertion in agrC) from a patient with chronic osteomyelitis was passed through the bloodstream using a bacteriemia mouse model and derivative P3.1 was obtained. Although IS256 remained inserted in agrC, P3.1 regained production of capsular polysaccharide type 5 (CP5) and staphyloxanthin. Furthermore, P3.1 expressed higher levels of asp23/SigB when compared with parental strain HU-14. Strain P3.1 displayed decreased osteoclastogenesis capacity, thus indicating decreased adaptability to bone compared with strain HU-14 and exhibited a trend to be more virulent than parental strain HU-14. Strain P3.1 exhibited the loss of one IS256 copy, which was originally located in the HU-14 noncoding region between dnaG (DNA primase) and rpoD (sigA). This loss may be associated with the observed phenotype change but the mechanism remains unknown. In conclusion, S. aureus organisms that escape the infected bone may recover the expression of key virulence factors through a rapid microevolution pathway involving SigB regulation of key virulence factors.
Highlights
Selection pressures exerted on Staphylococcus aureus by host factors during infection may lead to the emergence of regulatory phenotypes better adapted to the infection site
Strain HU-14 was passed through blood using the bacteremia mouse model and the emergence of colonies with positive reaction to anti-capsular polysaccharide type 5 (CP5) immune serum was monitored by colony immunoblot assay
Other suspicious colonies from the same passage were tested for capsular polysaccharide (CP) expression but only P3.1 was confirmed as CP5 positive (Fig. 1d)
Summary
Selection pressures exerted on Staphylococcus aureus by host factors during infection may lead to the emergence of regulatory phenotypes better adapted to the infection site. One of the strategies to investigate which factors may be up- or down-regulated during adaptation is to assess whether any of these factors is fixed by mutation during chronic infection Among these factors, loss of short sequence–repeats in the protein A Xr region, small colony variant (SCV) emergence and loss of capsular polysaccharide (CP) expression can be mentioned[12,13,14,15]. We were able to demonstrate that the loss of CP expression due to a mutation in the agr occurs during chronic o steomyelitis[9] This finding suggested that loss of RNAIII expression may yet be another endpoint in microevolution since agr regulates the expression of a vast number of virulence factors. We investigated whether S. aureus can regain the ability to produce CP, even in the presence of a mutated non-functional Agr, when it is transferred from infected bone to the bloodstream
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