The role played by the group A streptococcal negative regulator Nra on bacterial interactions with epithelial cells

Group A streptococci (GAS) specifically attach to and internalize into huma n epithelial host cells. In some GAS isolates, fibronectin-binding proteins were identified as being responsible for these virulence traits. In the pr esent study, the previously identified global negative regulator Nra was sh own to control the binding of soluble fibronectin probably via regulation o f protein F2 and/or Sfbll expression in the serotype M49 strain 591. Accord ing to results from a conventional invasion assay based on the recovery of viable intracellular bacteria, the increased fibronectin binding did not af fect bacterial adherence to HEp-2 epithelial cells, but was associated with a reduction in the internalization rates. However, when examined by confoc al and electron microscopy techniques, the nra-mutant bacteria were shown t o exhibit higher adherence and internalization rates than the corresponding wild type. The mutant bacteria escaped from the phagocytic vacuoles much f aster, promoting consistent morphological changes which resulted in severe host cell damage. The apoptotic and lytic processes observed in nra-mutant infected host cells were correlated with an increased expression of the gen es encoding superantigen SpeA, the cysteine protease SpeB, and streptolysin S in the nra-mutant bacteria. Adherence and internalization rates of a nra l speB-double mutant at wild-type levels indicated that the altered speB ex pression in the nra mutant contributed to the observed changes in both proc esses. The Nra-dependent effects on bacterial virulence were confined to in fections carried out with stationary growth phase bacteria. In conclusion, the obtained results demonstrated that the global GAS regulator Nra modulat es virulence genes, which are involved in host cell damage. Thus, by helpin g to achieve a critical balance of virulence factor expression that avoids the injury of target cells, Nra may facilitate GAS persistence in a safe in tracellular niche.