Fig. 5. Probiotic quorum sensing signal and secreted protease are critical for pathogen survival.
a, The NIP-producing SAL strains promote GAS survival during co-cultivation in human saliva. Equal inoculum (107 CFUs ml−1) of GAS and indicated SAL strains was co-inoculated in saliva. b, Increased GAS survival in the presence of SAL requires enzymatically active SpeB. The survival of WT GAS and GAS speB_C192S mutant in the presence of SAL in saliva was compared. Samples were collected at the indicated timepoints, and GAS survival was assessed by CFU analyses. c, Model for SIP pathway activation and SpeB production by SAL. NIP produced by SAL (left) is exploited by GAS to activate endogenous SIP pathway (right). The initial activation of sip expression leads to robust induction of sip and speB expression by a positive feedback mechanism. SpeB proteolytically degrades SAL-derived salivaricins and promotes GAS survival during dual species growth. In a, GAS + WT SAL group as reference, and statistical significance was analysed by Kruskal–Wallis test. ND indicates limit of detection that was set at 10. In b, statistical significance was assessed by two-tailed Mann–Whitney test. n.s., not significant.