Fig. 4. Pathogen exploits probiotic quorum sensing signal to activate secreted toxin production.
a–c, SAL induces early onset and robust induction of speB expression (a) and secreted SpeB protease production (b and c) in the sip* mutant during co-cultivation in vitro in transwells. The sip* mutant in the bottom well was grown in the presence of the indicated strains in the top well. speB expression (a) and secreted SpeB protease levels (b and c) were assessed by qRT–PCR and immunoblotting, respectively. In b and c, the sip* mutant in the bottom well was grown in the presence of WT GAS and WT SAL, respectively. In b and c, SpeBz indicates the enzymatically inactive higher molecular mass (~40 kDa) SpeB zymogen, whereas SpeBM is the enzymatically active, lower molecular weight mature SpeB (~25 kDa). d, SAL induces early onset and robust induction of speB expression in WT GAS during co-cultivation in human saliva. Samples were collected at the indicated timepoints and analysed for speB expression by qRT–PCR. Data graphed are from three biological replicates that are analysed in duplicate. In a and d, data are presented as mean ± s.e.m. e, SAL induces speB expression during co-infection in mouse nasopharynx. Mice (n = 5 per group) were infected intranasally with GAS (108 CFUs) in the presence or absence of SAL (108 CFUs). The NALT was collected at 24 hpi, and fold change in speB transcript levels relative to late exponential (LE) phase of GAS growth in vitro is shown. ND, not detected. The low quality and reduced yield of bacterial RNA from tissues from mice infected with GAS alone prevented reliable measurement of speB transcript levels. Bars represent mean ± s.e.m.