When withdrawn on day 5, bacterial numbers rapidly fell, and were no longer detectable after 48 h, by day 7 post-colonisation. To investigate the impact of controlled colonisation on immunogenicity and protection, further groups of mice were colonised with

D39Δpab in the presence or absence of PABA for 5 days. Serum anti-D39 IgG level was assessed at 14 and 28 days, prior to pneumonia challenge with WT D39. By day 14 post-colonisation, mice receiving 5 days of PABA supplementation had approximately 10-fold greater median serum IgG against D39 than those not receiving PABA ( Fig. 7B). By day 28, levels were not significantly different between the groups, indicating more rapid development of an antibody response when growth of the auxotrophic bacteria was supported at this level. In mice colonised with D39Δpab alone, there was no evidence of protection (median survival time 3.00 days, overall see more survival 30%) compared to controls (median time 2.87 days, 20% survival) ( Fig. 7C). In mice where colonisation was

supported with PABA, there was a trend towards longer survival compared with controls (median survival time 6.90 days, overall survival 35%, P = 0.09). Thus, the enhanced immune kinetics suggested that the degree of nasopharyngeal bacterial exposure was directly impacting on subsequent immunogenicity, and MK-1775 price could make a contribution towards protection. Live attenuated vaccines must possess both antigens and adjuvants which persist in sufficient quantity in an appropriate location for

enough time to induce a protective response. We have investigated how multiple factors may contribute towards the immunogenicity of a colonising bacterial strain and determine whether the colonisation event is sufficient to induce protection. We have previously shown prior colonisation protects against invasive D39 pneumonia by preventing septicaemia Casein kinase 1 with no protection at the mucosal level and is dependent on serum antibody [5]. Hence, systemic IgG rather than local immunological responses to colonisation are likely to be the important protective response for this model of S. pneumoniae infection, and this was supported by the close correlation between the serum IgG response and protective efficacy for the different strains studied here. Compared to its WT parent strain, D39Δpab was poorly immunogenic following colonisation. Supplementation with PABA for 5 days restored the ability of D39Δpab to colonise, and enhanced the speed of anti-D39 IgG seroconversion. The majority of mice with PABA supplementation had high titres of anti-D39 IgG, whereas in mice without PABA titres were much more variable. This was associated with a strong trend towards protection. These data support the hypothesis that for a given strain of S. pneumoniae, the duration of colonisation is important in generating protective immunity. Whether the ‘area under the curve’ (reflecting total antigen present over time i.e.