Expression of a pertussis toxin S1 fragment by inducible promoters in oral Streptococcus and the induction of immune responses during oral colonization in mice

Previous work aimed at developing a live oral vaccine expressing pertussis toxin Sl fragment on the surface of the bacterium Streptococcus gordonii elicited a lower than expected antibody response, perhaps because of low antigen expression. In this study, in-frame promoter fusions were constructed to investigate whether an increase in antigen production by the streptococcal vaccine strain results in a better antibody response. The promoters tested were (i) the Streptococcus mutans sucrose-inducible fructosyltransferase (ftf) promoter and (ii) the Bacillus subtilis/Escherichia coli chimeric tetracycline-inducible xyl/tetO promoter. Each of these two promoters was placed upstream of the spaP/s1 fusion gene to drive its expression. The constructs were introduced into S. gordonii DL1 and S. mutans 834. The inducibility of the promoters was confirmed through the determination of SpaP/S1 production via Western blottings. Induced production of SpaP/S1 was observed in S. gordonii and S. mutans with each of the promoters, but the level of expression was the highest in S. mutans, using the xyl/tetO promoter. Thus, S. mutans carrying the xyl/tetO/spaP/s1 construct (S. mutans PM14) was used in oral colonization studies in BALB/c mice. Streptococccus mutans PM14 was able to colonize the animals for the 14-week duration of experimentation. A mucosal IgA response was observed in all the treatment groups but was highest in mice receiving tetracycline induction. In the mouse model of Bordetella pertussis respiratory infection, animals colonized with S. mutans PM14 showed a decreased in B. pertussis lung colony count (P = 0.03) on day 3 compared with control mice colonized by the parent S. mutans 834.