Microbiome therapeutics and live biotherapeutics

A live biotherapeutic product is a drug whose active ingredient is a living organism or a defined consortium of them. That is a strange kind of drug: the dose administered is not the exposure delivered, because the organisms may grow, may fail to establish, and act mainly by changing the chemistry of a compartment rather than by binding a receptor.

Colonisation resistance, stated chemically

The one indication where the mechanism is well characterised is recurrent Clostridioides difficile infection, and the key chemistry is bile acids. The liver secretes primary bile acids, chiefly cholic and chenodeoxycholic acid, conjugated to glycine or taurine. Taurocholate is the physiological germinant for C. difficile spores: a spore encountering it in the small intestine germinates into the toxin-producing vegetative form. In a healthy colon a small guild of commensals, notably Clostridium scindens and relatives carrying the bile acid inducible operon, deconjugate and 7α-dehydroxylate those primary acids into secondary bile acids — deoxycholate and lithocholate — which inhibit both germination and vegetative outgrowth.

Broad-spectrum antibiotics remove that guild. The bile-acid pool stays primary, germination proceeds unopposed, and recurrence follows. Restoring the guild restores the conversion, and the effect is measurable as a shift in the faecal bile-acid profile. Two products approved in the United States exploit this: a rectally administered faecal microbiota suspension and an orally administered purified Firmicutes spore preparation, the latter deliberately built from spores because they survive ethanol treatment that removes vegetative pathogens and gastric transit that would destroy them.

Nutrient competition works alongside it. Antibiotic disruption liberates mucosal sugars — sialic acid and fucose released by residual bacterial sialidases and fucosidases — which C. difficile consumes; a restored community reoccupies that niche. Short-chain fatty acids from fibre fermentation, butyrate in particular, feed colonocytes and keep the epithelium hypoxic, which itself suppresses the facultative anaerobes that bloom in dysbiosis.

Why the transfer to other diseases is hard

Three constraints bound the field. First, engraftment. Whether an administered strain establishes depends on whether its metabolic niche is vacant, on the resident community, and on the host’s diet; recipients differ, and a fixed dose therefore produces variable colonisation. Antibiotic or bowel preparation before dosing is essentially niche-clearing, and it works better in a disrupted gut than in an intact one — which is precisely why C. difficile, a disease of a wrecked community, was the tractable first target.

Second, causality. Associations between community composition and disease are abundant and mostly not directional: inflammation reshapes the microbiota at least as readily as the microbiota drives inflammation. Interventional trials in inflammatory bowel disease and in immuno-oncology support have produced signals, but the field has not yet delivered a second indication with the mechanistic clarity of the bile-acid story.

Third, definition and manufacture. A consortium must be reproducible: strain-level identity, viable counts, strict anaerobic handling, and stability of a living product in storage. Regulators treat these as living drugs with genomic characterisation and antimicrobial-resistance screening obligations, not as food supplements, and that distinction is where much of the field’s cost sits.

See also the technology article at /technology/microbiome-therapeutics-lbps/.