An investigation into host-defensive mechanisms by Lactobacillus species against pathogenic Trichomonas vaginalis

Abstract

Trichomonas vaginalis is a protozoan parasite that colonises the urogenital tract and is the causative agent of trichomoniasis, the most common non-viral sexually transmitted infection worldwide. Importantly, women have a higher disease burden than men. T. vaginalis is able to colonise the vagina extracellularly by adhering to the epithelial layer. This parasite alters the tight junctions of the host cells before damaging them by cytolysis. Upon entering the vaginal environment, it faces competition with the local microbiota. A microbiota comprised of mainly Lactobacillus is negatively correlated with T. vaginalis infection, with vaginal lactobacilli able to protect the host and mitigate infections. For T. vaginalis in particular, indigenous lactobacilli can decrease adherence to human epithelial cells. This work advances our understanding of the effect of Lactobacillus on T. vaginalis, more specifically in relation to its cytolytic properties. It further provides insights into the genomic differences between host-protective and non-protective strains of Lactobacillus gasseri, known to form the most stable population over time in the vagina. In order to assess the effect of different Lactobacillus species on the cytolysis of host cells promoted by T. vaginalis, multiple cytolysis assays were performed in which HeLa cells were incubated with the parasite, with and without lactobacilli. Levels of cytolysis were assessed by measuring lactate dehydrogenase release via a colorimetric assay. Further investigation into the effect of vaginal L. gasseri on the secretome of T. vaginalis identified differentially expressed pathogenicity factors involved or possibly involved in cytolysis and immune response modulation and evasion. The cytopathogenicity of Trichomonas was affected by lactobacilli in species- and strain-dependent manners. The vaginal strain of L. gasseri ATCC 9857 was substantially more protective against the T. vaginalis-induced cytolysis than the intestinal strain ATCC 33323, reproducing a niche-specific and host-protective feature observed previously by our group against parasite cytoadhesion. Hence, to assess possible genetic differences between these strains that could explain this protective phenotype, the genome of L. gasseri ATCC 9857 was sequenced. The genome is, with a size of around 2 Mbp and 2018 predicted coding sequences, of average size for lactobacilli. While subsequent genome comparison with the previously sequenced intestinal isolate ATCC 33323 on a gene presence/absence level identified differences between the two strains, particularly in their genes encoding bacteriocins and involved in the formation of exopolysaccharides, further studies will be necessary to evaluate how these genetic differences might impact on niche adaptation and/or host protection in the vagina. This work highlights the role of the protective vaginal microbiota against T. vaginalis, a prevalent and widely spread pathogen. Understanding the interplay between bacteria and this parasite, and factors involved in the defence of the host from the parasite, may help in developing new treatments against trichomoniasis.