The effect of adding calf serum on the virulence genes expression of Staphylococcus aureus

Staphylococcus aureus is an opportunistic pathogen and one of the most important health-threatening agents worldwide. Expression of bacterial virulence factors is not similar in laboratory medium conditions and in vivo. The aim of the present study was to evaluate the effects of adding 5% calf serum on the virulence genes expression of S. aureus.   The expression levels of agrA, RNAIII, hla, spa, and mecA genes were     determined during the growth of S. aureus isolates in BHI broth and BHI broth enriching with calf serum during different growth phases. Therefore, the growth curve of the five isolates of S. aureus in two different culture conditions was plotted. Subsequently, RNA was extracted from different phases of growth and the genes expression were analyzed by Real-time PCR.   In average, the expression levels of agrA and RNAIII from stationary phase to the exponential phase in BHI broth containing calf serum was increased 3.4 and 9.5-fold, respectively, while the agr system could not appropriately play its regulatory role in the expression of virulence genes. As a result, the expression of hla gene was decreased 0.81-fold and the expression levels of spa and mecA genes was only increased 1.25 and 1.03-fold, respectively.   Regardless of the growth phase, the expression of the whole genes in BHI broth containing calf serum were increased in comparison to BHI broth. Consequently, it appears that S. aureus is capable to induce virulence genes expression in the presence of calf serum factors similar to conditions available in vivo.