Whether a short term probiotic application that women might find easy to administer monthly, could increase the lactobacilli count. In some individuals it was extremely clear when the probiotic strains had been applied. The reason for this not being universal is unclear, but could be due to a particularly resilient indigenous microbiota or women who were non-responders for unexplained reasons. Only the relative abundance of bacterial taxa are measured, and therefore absolute changes affected lactate levels cannot be directly observed. Certain LY2157299 species of Lactobacillus have been associated with lower pH and may produce more lactate compared to others. There was a trend towards an increase in lactate after probiotic and decrease after placebo intervention, but as there were only four women whom responded to treatment and from whose samples enough material for metabolite analysis could be extracted, the sample size is likely too small to reach significance. Still, the observation that probiotics could potentially increase lactate levels is promising as lactate has been shown to have many beneficial properties in the vaginal tract such as HIV inactivation. Overall, the administration of the two lactobacilli strains did not induce any more changes than placebo in the metabolome, but lack of material makes this result inconclusive. In our experience and that of others, the metabolomic patterns differ between health and BV, with the latter showing odorous compounds such as cadaverine and putrescine, but these trends were not clearly observed in this study. In addition to looking at the bacterial communities, we sought to examine host responses to probiotic treatment. It has been well established that probiotic strains can affect host transcription in the gut in a strain specific manor but this is the first time similar studies have been carried out in the vagina. Our findings show that L. rhamnosus GR-1 and L. reuteri RC-14 had an immunomodulatory effect working on important central inflammatory mediators complement receptor 1, toll like receptor 2 and IL-18. Though limited in the number of subjects available for analysis, the strength of this analysis is the use a paired study design. Interleukin 18 is a proinflammatory cytokine inducing cellmediated immunity via interferon gamma though it also has effects on B cells and IgE production. In the female reproductive tract it may be of relevance in preterm birth indicating microbial invasion of the amnion, but it also has protective roles against genital herpes simplex 2.