The small regulatory RNA FasX enhances group A S treptococcus virulence and inhibits pilus expression via serotype‐specific targets

Summary Bacterial pathogens commonly show intra‐species variation in virulence factor expression and often this correlates with pathogenic potential. The group A S treptococcus ( GAS ) produces a small regulatory RNA (s RNA ), FasX , which regulates the expression of pili and the thrombolytic agent streptokinase. As GAS serotypes are polymorphic regarding (a) FasX abundance, (b) the fibronectin, collagen, T ‐antigen ( FCT ) region of the genome, which contains the pilus genes (nine different FCT ‐types), and (c) the streptokinase‐encoding gene ( ska ) sequence (two different alleles), we sought to test whether FasX regulates pilus and streptokinase expression in a serotype‐specific manner. Parental, fasX mutant and complemented derivatives of serotype M 1 ( ska ‐2, FCT ‐2), M 2 ( ska ‐1, FCT ‐6), M6 ( ska ‐2, FCT ‐1) and M 28 ( ska ‐1, FCT ‐4) isolates were compared. While FasX reduced pilus expression in each serotype, the molecular basis differed, as FasX bound, and inhibited the translation of, different FCT ‐region m RNA s. FasX enhanced streptokinase expression in each serotype, although the degree of regulation varied. Finally, we established that the regulation afforded by FasX enhances GAS virulence, assessed by a model of bacteremia using human plasminogen‐expressing mice. Our data are the first to identify and characterize serotype‐specific regulation by an s RNA in GAS , and to show an s RNA directly contributes to GAS virulence.