A non-optimal vaginal microbiome, as observed with bacterial vaginosis (BV), increases the risk of HIV acquisition compared to women with an optimal Lactobacillus crispatus-dominated microbiome. Lactic acid (LA), a metabolite produced by Lactobacillus spp., potently inactivates HIV. Here we investigated the bactericidal activity of racemic LA (DL-LA), D- and L-LA isomers against vaginal bacteria.
L. iners, L. crispatus, and G. vaginalis (ATCC strains) were treated with 1% LA pH3.8 at 37°C for 1h and viability quantified in CFU/ml. Mixed cultures of G. vaginalis, L. iners, and L. crispatus were incubated at 37°C for 24h with 1% LA in S-Broth at pH4.5, and viable bacteria quantified using taxon-specific PMA-qPCR.
For proteomics, L. crispatus was incubated with 1% LA in MRS broth for 6h and washed bacterial pellets were subjected to LC-MS/MS and differential protein abundance quantified.
For morphological changes, L. crispatus and L. iners were treated with PBS pH7, PBS pH3.8, or 1% DL-LA pH3.8 for 1h at 37°C. Bacterial pellets were fixed and processed for TEM. Bacteria in this study were cultured anaerobically.
Treatment with 1% DL-LA pH3.8 for 1h selectively killed G. vaginalis (50,000-fold reduction, n=3, p<0.0001) and L. iners (128-fold reduction, n=6, p=0.003), but not L. crispatus vs. untreated controls. D-LA, L-LA and DL-LA demonstrated similar bactericidal activites. LA treatment of mixed bacterial cultures dramatically depleted G. vaginalis and L. iners, and increased L. crispatus copy numbers. Top differentially expressed L. crispatus proteins after LA treatment included D-lactate dehydrogenase, GroEL, and GreA. Morphological changes were observed when L. iners was treated with DL-LA compared low pH alone, while L. crispatus remained unaffected.
LA targets key BV-associated vaginal bacteria and the less stable L. iners, which persists after metronidazole treatment for BV, but not L. crispatus. These findings suggest a potential intervention to prevent BV recurrence.