In a recent study published in the Cell, a group of researchers investigated oleic acid (OA) as a potential treatment to inhibit Lactobacillus iners (L. iners) and promote Lactobacillus crispatus (L. crispatus) dominance in bacterial vaginosis (BV).
Study: Vaginal Lactobacillus fatty acid response mechanisms reveal a metabolite-targeted strategy for bacterial vaginosis treatment. Image Credit: Kateryna Kon/Shutterstock.com
Background
Specific female genital tract (FGT) microbiota are linked to adverse health outcomes, including infertility, preterm birth, and infections like human immunodeficiency virus (HIV). BV, associated with these outcomes, affects up to 58% of women globally and is marked by a lack of lactobacilli and dominance of anaerobic bacteria.
Standard antibiotic treatment with metronidazole often leads to recurrence, possibly due to a shift towards L.iners instead of L. crispatus, which is associated with better health outcomes. Further research is needed to validate the efficacy and safety of OA as a targeted treatment for promoting L. crispatus dominance and preventing BV recurrence.
About the study
The present study investigated bacterial growth inhibition, enhancement, and killing using various media formulations and additives. Bacterial isolates were revived on solid media, with starter cultures grown in liquid media for 48 hours. The optical density at 600 nm (OD600) was measured, and cultures were diluted to 40 times the starting OD600 for experiments. Growth enhancement involved pelleting and washing starter cultures to prevent nutrient carryover before inoculation.
For bactericidal activity, a minimum bactericidal concentration (MBC) assay assessed OA effects, with colony-forming units (CFU) counted after 48-72 hours. Competition and mock community experiments involved mixing bacterial strains in defined ratios, inoculating them into different media conditions, and measuring CFU titers and 16S ribosomal Ribonucleic Acid (rRNA) sequencing after 48 or 72 hours.
Transmission electron microscopy (TEM) imaging examined bacterial structures post-OA treatment. Adenosine Triphosphate (ATP) release assays quantified OA's impact on bacterial ATP levels. RNA sequencing analyzed gene expression in Lactobacillus strains treated with OA. Lipidomics and isotopic tracing studied metabolic changes in cultures treated with labeled OA. Gene knockouts in Lactobacillus gasseri were generated and verified using plasmid construction and whole-genome sequencing. Enzyme product characterization and targeted lipidomics analyzed fatty acid metabolism and hydroxylated fatty acids in cervicovaginal lavage samples, with 16S rRNA sequencing determining microbiota composition.
Study results
Cis-9-unsaturated long-chain fatty acids (cis-9-uLCFAs) have distinct effects on different species of Lactobacillus in the FGT. When cultured with cis-9-uLCFAs, including OA, linoleic acid (LOA), and palmitoleic acid (POA), L. iners was significantly inhibited, while L. gasseri, L. crispatus, L. jensenii, and L. mulieris exhibited little to no inhibition. This selective inhibition of L. iners was confirmed across multiple strains and media types, with OA displaying a bactericidal effect on L. iners at a concentration of 400 μM. In contrast, non-iners Lactobacillus species were not only resistant to these fatty acids but also experienced enhanced growth when supplemented with OA.
Transcriptomic analysis revealed that non-iners Lactobacillus species share a conserved set of genes, including an oleate hydratase (ohyA) and a fatty acid efflux pump (farE), which are absent in L. iners. The presence of these genes in non-iners species allows them to metabolize and detoxify cis-9-uLCFAs, conferring a competitive advantage in environments rich in these fatty acids. Further genomic and enzymatic studies confirmed that the OhyA enzymes in non-iners Lactobacillus are functionally active, enabling these species to utilize OA for membrane synthesis and growth.
In contrast, L. iners, which lack these protective genes, are susceptible to membrane disruption and cell death when exposed to cis-9-uLCFAs. The study also found that 10-hydroxystearic acid (10-HSA), a metabolite produced by OhyA, could further enhance the growth of non-iners Lactobacillus species while inhibiting L. iners.
Conclusions
To summarize, this study highlights species-specific differences in fatty acid metabolism among Lactobacillus species in the FGT, revealing potential therapeutic strategies for BV. cis-9-uLCFAs like OA, selectively inhibit L. iners and certain BV-associated bacteria while promoting the growth of beneficial species such as L. crispatus. The absence of key genes (ohyA and farE) in L. iners makes it susceptible to OA, whereas non-iners Lactobacillus species use these genes to resist OA and convert it for growth. These findings suggest metabolite-based approaches could enhance BV treatment by favoring L. crispatus dominance.
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