The effect of cell density and specific growth rate on accessory gene regulator and toxic shock syndrome toxin-1 gene expression in Staphylococcus aureus.

A continuous culture technique utilising a variant of Staphylococcus aureus 8325-4 containing transcriptional gene fusions was used to investigate the relationships between cell density (OD(600)), steady-state specific growth rate (mu) and expression of both agr (accessory gene regulator) and tst (toxic shock syndrome toxin-1). The expression of these genes was assessed by two single-copy independently arranged chromosomal-based reporter systems, beta-galactosidase agr-P3 promoter fusion and a lux-tst promoter fusion. Cell density and specific agr expression were found to be positively correlated. In the model, the minimum cell density predicted to promote specific agr expression was an OD(600) of 0.14, equivalent to 1.2x10(8) CFU ml(-1). No direct relationship between cell density and specific tst expression was detected. Specific expressions of agr and tst were not correlated with specific growth rate and there appeared to be no direct link between agr and tst specific expression. The results support the hypothesis that agr is a functional unit of quorum sensing and that the amount of specific expression of tst is modulated independently of agr.