In this month's Ask the Expert series, Chief Science Officer Elizabeth DuPriest, PhD explains exactly what lactobacilli are and their importance in maintaining vaginal health.
Q: I keep reading about lactobacilli. What are they?
A: For our purposes, the short answer is that lactobacilli are beneficial bacteria that live in the human gut and vagina, produce lactic acid, and protect against other, potentially damaging microbes. Vaginal species usually also produce hydrogen peroxide. There is also good evidence that they also interact with the immune system with anti-inflammatory effects. A healthy vaginal microbiome is one that is composed primarily, even almost exclusively, of lactobacilli. Lactobacilli are also used commercially to produce fermented dairy products such as yogurt and kefir, so our diet is one source of the lactobacilli our body hosts.
More specifically, the term “lactobacilli” relates to the Lactobacillaceae family of bacteria (in phylum Firmicutes, a clade that includes Streptococcus, Staphylococcus, Bacillus, Clostridium, and other related groups) as it was described prior to 2020. (It’s worth noting that there is constant reorganization of bacterial species into new groups based on new phylogenetic analyses.) These are non-spore-forming Gram-positive rods of varying size with low GC-content genomes. The term “lactobacilli” encompasses the genus Lactobacillus as well as several newly designated genera, such as Ligilactobacillus, Limosilactobacillus, Lacticaseibacillus, Lactiplantibacillus, and even somewhat less related groups such as Leuconostocaceae. Lactobacilli are part of a larger group called Lactic Acid Bacteria. This group includes non-bacillus genera such as Streptococcus; these still produce some lactic acid to some extent but do not have the same beneficial effects of the vaginal lactobacilli.
Lactobacilli are anaerobic, though usually somewhat aerotolerant, making the low-oxygen vaginal canal a perfect habitat for them. They produce energy primarily through fermentation of carbohydrates, with vaginal species depending mostly on metabolites derived from glycogen breakdown including maltose, maltotriose and dextrins1. The end result of fermentation of these sugars is lactic acid (for homofermentative species) plus possibly other compounds such as acetic acid, ethanol, and carbon dioxide (for heterofermentative species). This lactic acid – a waste product from the perspective of the bacteria – is critical for determining pH of the vaginal fluid2, and for inhibiting pathogens, including other bacteria (Gram-negative and Gram-positive), yeasts, protozoans, and even viruses3. Only certain species have co-evolved to be fully mutualistic in the human vagina; these include L. crispatus, L. gasseri, and L. jensenii. Other species are more adapted to the gut or even to other species, but may also be somewhat protective in the vagina, such as L. acidophilus. L. iners is a species that is specific to the human vagina but plays a yet-to-be-determined role4. It appears to be less protective than other vaginal lactobacilli, but healthier for vaginal epithelium than non-lactobacilli species. It often appears after an episode of bacterial vaginosis or other dysbioses.
Beyond lactic acid, the vaginal lactobacilli also produce hydrogen peroxide and anti-bacterial proteins called bacteriocins that help suppress pathogens. These, in addition to successfully competing for binding sites on sloughing vaginal epithelial cells, help to keep lactobacilli the dominant microbes in a healthy human vagina. The result of a lactobacilli-dominant vagina is a healthy, intact vaginal lining that is resistant to infection by bacteria and viruses and does not show indications of inflammation or immune system activation.
Amabebe E, Anumba DOC. The Vaginal Microenvironment: The Physiologic Role of Lactobacilli. Front Med. 2018;5:181. doi:10.3389/fmed.2018.00181
O’Hanlon DE, Moench TR, Cone RA. Vaginal pH and microbicidal lactic acid when lactobacilli dominate the microbiota. PloS One. 2013;8(11):e80074. doi:10.1371/journal.pone.0080074
O’Hanlon DE, Moench TR, Cone RA. In vaginal fluid, bacteria associated with bacterial vaginosis can be suppressed with lactic acid but not hydrogen peroxide. BMC Infect Dis. 2011;11:200. doi:10.1186/1471-2334-11-200
Petrova MI, Reid G, Vaneechoutte M, Lebeer S. Lactobacillus iners: Friend or Foe? Trends Microbiol. 2017;25(3):182-191. doi:10.1016/j.tim.2016.11.007