Characterisation of probiotic properties in human vaginal lactobacilli strains

Background Vaginal lactobacilli offer protection against recurrent urinary infections, bacterial vaginosis, and vaginal candidiasis. Objective To characterise the isolated vaginal lactobacilli strains for their probiotic properties and to compare their probiotic potential. Methods The Lactobacillus strains were isolated from vaginal samples by conventional culturing and identified by sequencing of the 16S rDNA fragment. Several functional properties were detected (production of hydrogen peroxide and lactic acid; antagonistic activity against Escherichia coli, Candida albicans, Candida glabrata, and Gardnerella vaginalis; auto-aggregation and adhesiveness) as well as safety (haemolytic activity, antibiotic susceptibility, presence of transferrable resistance genes). Results A total of 135 vaginal lactobacilli strains of three species, Lactobacillus crispatus (56%), Lactobacillus jensenii (26%), and Lactobacillus gasseri (18%) were characterised using several functional and safety tests. Most of L. crispatus (89%) and L. jensenii (86%) strains produced H2O2. The best lactic acid producers were L. gasseri (18.2±2.2 mg/ml) compared to L. crispatus (15.6±2.8 mg/ml) and L. jensenii (11.6±2.6 mg/ml) (p<0.0001; p<0.0001, respectively). L. crispatus strains showed significantly higher anti-E. coli activity compared to L. jensenii. L. gasseri strains expressed significantly lower anticandidal activity compared to L. crispatus and L. jensenii (p<0.0001). There was no significant difference between the species in antagonistic activity against G. vaginalis. Nearly a third of the strains were able to auto-aggregate while all the tested strains showed a good ability to adhere to HeLa cells. None of the tested lactobacilli caused haemolysis. Although phenotypical resistance was not found to ampicillin, chloramphenicol, erythromycin, gentamycin, tetracycline, and vancomycin, the erm(B), tet(M), and tet(K) were detected in some strains. All strains were resistant to metronidazole, trimethoprim/sulfamethoxazole, and kanamycin. Conclusions Our study revealed that the production of different antimicrobial metabolites is highly strain-specific and that the metabolites are not correlated with each other. L. crispatus displays better antagonistic activity against E. coli and Candida spp. than L. gasseri and L. jensenii; therefore; a potential probiotic candidate could be found among L. crispatus strains.