Interactive regulatory pathways control virulence determinant production and stability in response to environmental conditions in Staphylococcus aureus

The accessory gene regulator (agr) and staphylococcal accessory regulator (sar) loci are important regulators of toxin production in Staphylococcus aureus. In this study we examined how environmental conditions - degree of aeration and salt concentration - affect the transcription and translation of mRNAs for alpha-haemolysin (Hla) and serine protease (Ssp) via these pathways and influence the stability of these proteins. Using Northern analysis, we have confirmed earlier observations that sarA is involved in the upregulation of RNAIII, the effector molecule encoded by the agr locus. However, this effect was abolished in highly aerated cultures. While sar A does appear to have an up-regulatory effect on hla transcription that is independent of agr, we propose that the PC1839 (sarA) mutant produces less alpha-haemolysin activity mainly as a result of post-translational inactivation by proteases. The most obvious phenotypic feature of PC1839 (sarA) is the upregulation of proteases. In this study we show that ssp is repressed by SarA at the transcriptional level. Western analysis using an anti-alpha-haemolysin antibody identified a major breakdown product that is only present in the supernatant of strains that are overexpressing serine protease. We have also confirmed that agr exerts a significant regulatory influence on hla at the level of translation, as well as transcription. Finally, the addition of salt upregulates ssp transcription and dramatically downregulates transcription of hla, and is an example of an environmental parameter that affects toxin production independently of agr and sarA. How environmental signals are transduced to control alpha-haemolysin and serine protease production, activity and stability at multiple levels are discussed.