In vitro Evaluation of Small Molecule Inhibitors and Probiotic Byproducts on Growth and Viability of Vaginal Microorganisms

Background: Interest in avoidance of antibiotic resistance development through non-antibiotic therapeutics prompts development of topical vaginal preparations that may inhibit vaginal microorganisms. Prior data on boric acid, zinc salts and probiotic microorganisms suggests a potential role in female genital tract infections.
Aims: Determine antimicrobial effects of boric acid, zinc sulfate and metabolites of probiotic lactobacilli alone or in combination on Candida albicans, Gardnerella vaginalis and Streptococcus agalactiae.
Methodology: Absolute counts of test organisms cultivated with or without inhibitory compounds were determined by a flow cytometric method and compromise of microbial cell integrity was demonstrated by propidium iodide staining, also determined by flow cytometry. All microbial count growth experiments were conducted in triplicate and averages were reported.
Results: The three microbial species challenged with boric acid, zinc sulfate or spent medium from probiotic lactobacilli showed varying degrees of susceptibility to these inhibitors, with boric acid showing consistent activity against C. albicans and also showing yeast cell damage demonstrated by propidium iodide uptake. Boric acid showed greater antibacterial activity against S. agalactiae than against G. vaginalis and neither of the bacterial organisms showed propidium iodide staining. Zinc sulfate inhibition was greatest for Candida and least for Gardnerella. Probiotic Lactobacillus spent media was also inhibitory toward the three test organisms with Gardnerella being slightly less susceptible than the other two test organisms. When binary combinations were tested, the combination of boric acid and Lactobacillus spent medium was the most effective in vitro against all three organisms.
Conclusion: Zinc sulfate did not prove any more effective in vitro against the three test organisms than did boric acid or Lactobacillus spent media. The most potent binary combination against the three test microorganisms was boric acid plus spent media from probiotic Lactobacilli.