association of agr gene expression with staphylococcus aureus virulence genes in bhi broth

abstract       background and objective: agr is the most important regulatory system for the expression of staphylococcus aureus virulence factors in different conditions. agr acts as a quorum sensing system in this bacterium which is activated by increased cell concentration during the transition from logarithmic growth phase to stationary phase. its role is to upregulate the secretory virulence factors such as alpha-hemolysin and inhibit the transcription of surface proteins including protein a-encoding gene. the aim of this study was to assess the relationship between the agr system expression and some virulence factors of staphylococcus aureus in brain-heart infusion (bhi) culture medium.      methods: the expression level of agra and rnaiii genes from the agr locus along with the expression of hla, spa and meca genes in bhi broth were assessed in different growth phases using real time-pcr. also, gyrb was used as an internal control in this study.      results: the growth curve of the five tested isolates in bhi broth at 24 hours showed that all the isolates had relatively similar growth patterns. agra gene expression in the stationary phase was decreased by 0.89-fold compared with the logarithmic phase. although the expression of rnaiii gene increased by 3-fold, hla expression decreased by 0.47-fold.      conclusion: an inactive agr system is observed in the bhi broth medium. bhi broth medium contains high amounts of suitable nutrients for the growth of staphylococcus aureus, thus the bacteria do not require the activity of the agr system for the regulation of the virulence genes in these conditions.