Impact of the regulatory loci agr, sarA and sae of Staphylococcus aureus on the induction of alpha-toxin during device-related infection resolved by direct quantitative transcript analysis

The cytotoxic alpha -toxin (encoded by hla) of Staphylococcus aureus is reg ulated by three loci, agr, sarA and sae, in vitro. Here, we assess the regu lation of hla in a guinea pig model of device-related infection by quantify ing RNAIII (the effector molecule of agr) and hla directly in exudates accu mulating in infected devices without subculturing of the bacteria. LightCyc ler reverse transcription-polymerase chain reaction (RT-PCR) was used to qu antify the transcripts. Strains RN6390 and Newman expressed considerably sm aller amounts of RNAIII in the guinea pig than during in vitro growth. The residual RNAIII expression decreased during the course of infection and was negatively correlated with bacterial densities. As with RNAIII, the highes t hla expression was detected in both strains early in infection. Even in s train Newman, a weak hla producer in vitro, a pronounced expression of hla was observed during infection. Likewise, four S. aureus isolates from cysti c fibrosis (CF) patients expressed Q1hla despite an inactive agr during dev ice-related infection as in the CF lung. Mutation of agr and sarA in strain Newman and RN6390 had no consequence for hla expression in vivo. In contra st, the mutation in sae resulted in severe downregulation of hla in vitro a s well as in vivo. In conclusion, S. aureus seems to be provided with regul atory circuits different from those characterized in vitro to ensure alpha -toxin synthesis during infections.