Microbiome science

Lactobacillus

Also called: lactobacilli, lactic acid bacteria

Lactobacillus is a genus of bacteria that produces lactic acid by fermenting carbohydrates. It is the most common bacterial group in fermented foods (yoghurt, kefir, sauerkraut) and probiotic supplements. The evidence is strain-specific, not genus-specific, which means a label that just says 'Lactobacillus' is not enough. Specific strains have good evidence for antibiotic-related diarrhoea, IBS, infant colic, and vaginal health.

What Lactobacillus does

Lactobacillus species ferment carbohydrates into lactic acid, lowering local pH. This acidification protects against pathogens, supports nutrient absorption, and shapes the broader microbial community. Different species have evolved for different niches: some for the small intestine, some for the colon, some for the vagina, some for fermenting milk.

Strains with the strongest evidence

  • Lactobacillus rhamnosus GG: reduces antibiotic-associated diarrhoea, shortens infectious diarrhoea in children. The most studied probiotic strain in the world.
  • Lactobacillus reuteri DSM 17938: reduces crying time in colicky breastfed babies, improves regularity in some adults.
  • Lactobacillus acidophilus NCFM: reduces flatulence and bloating in IBS-D in some trials.
  • Lactobacillus crispatus: dominant species in healthy vaginal microbiome, used in probiotics for bacterial vaginosis recurrence.
  • Lactobacillus casei Shirota (Yakult): immunomodulation and bowel regularity in older adults.

Lactobacillus in food

  • Yoghurt (live cultures): the Codex Alimentarius definition of yoghurt requires Lactobacillus bulgaricus + Streptococcus thermophilus. Some 'bio' or probiotic yoghurts add L. acidophilus, Bifidobacterium, or other strains on top of the two-organism base.
  • Kefir: more diverse than yoghurt, often 30+ strains across Lactobacillus and Bifidobacterium genera.
  • Sauerkraut, kimchi: L. plantarum dominant.
  • Sourdough: L. sanfranciscensis is the classic, contributes flavour as well as fermentation.
  • Most cheese: lactobacilli are part of the starter culture, especially in aged hard cheeses.

What they don't do

Lactobacilli rarely colonise the gut long-term. Most strains in supplements pass through within a week of stopping. They work mainly through transient effects: producing lactic acid as they pass, releasing bioactive compounds, and modulating immune signalling. They don't 'reset' or 'rebuild' the microbiome the way marketing copy suggests. Diversity comes from feeding the bacteria you already have.

Recent reclassification

In 2020 the Lactobacillus genus was split on DNA evidence, 23 new genera (Lactiplantibacillus, Limosilactobacillus, Lacticaseibacillus, etc.) plus an emended Lactobacillus, for 25 in total. Most consumer-facing labels still say 'Lactobacillus' for familiarity. The strains and their evidence haven't changed; just the taxonomic name. So Lactobacillus rhamnosus GG is now technically Lacticaseibacillus rhamnosus GG, but the evidence base is the same.

Common questions

Is more Lactobacillus better?
Not necessarily. Healthy adult guts have moderate Lactobacillus levels in the small intestine. Excess in the small intestine can be a sign of SIBO. The vagina is the one site where high Lactobacillus dominance is consistently good.
How many CFUs do I need?
Strain-dependent. Most evidence sits in the 1 to 50 billion CFU per day range. Higher isn't always better. Look for the CFU count at end of shelf life, not at manufacture, since viability drops over time.
Can I get enough Lactobacillus from food?
For general gut diversity, yes. For specific clinical effects (treating C. difficile, antibiotic-associated diarrhoea, infant colic) the dose in food is too low and inconsistent. Use food for maintenance, supplements for specific situations.
What's the difference between Lactobacillus and Bifidobacterium?
Different genera, different niches. Lactobacillus species are mostly small intestine and produce lactic acid. Bifidobacterium species are mostly colon and produce acetate and lactate. Both are common in probiotics; both are useful for different reasons.

Sources