Animal probiotics & postbiotics

Probiotics in animal production are live microorganisms fed to modify the gut community. Their commercial importance rose with restrictions on antibiotic growth promoters, which created demand for something that could occupy the same niche by a different route.

Colonisation resistance

A healthy gut microbiota resists invasion by pathogens, and the mechanisms are reasonably well understood. Resident organisms consume the available nutrients and occupy attachment sites, leaving little for an incomer — competitive exclusion. This is the basis of the classic observation that a newly hatched chick, with essentially no gut flora, can be protected against Salmonella colonisation by dosing it with material from an adult bird’s caecum.

Beyond simple competition, resident organisms acidify the lumen with short-chain fatty acids and produce bacteriocins — narrow-spectrum antimicrobial peptides that inhibit closely related species. Some strains also act on the host rather than on the competitor: strengthening tight junctions between epithelial cells, stimulating mucin secretion, and modulating local immune signalling. These host-directed effects are strain-specific and are the hardest to demonstrate.

Viability is the engineering problem

A probiotic must be alive at the point of consumption. Compound feed is typically pelleted, which involves steam conditioning and mechanical work, reaching temperatures that kill vegetative bacteria. It is then stored for weeks or months at ambient temperature and humidity.

This selects the market. Spore-forming Bacillus species dominate feed probiotics because endospores survive pelleting, storage and gastric acid, then germinate in the intestine. Non-spore-forming organisms such as lactobacilli need protection — encapsulation, post-pelleting liquid application, or inclusion only in unpelleted feed or drinking water. The choice of organism is therefore driven as much by process survival as by biological effect, which is a real constraint on what the field can use.

Yeasts, particularly Saccharomyces cerevisiae var. boulardii and live yeast for ruminants, occupy a separate position: in the rumen they consume oxygen and stabilise pH, favouring fibre-degrading bacteria rather than acting against a pathogen.

Postbiotics remove the constraint

A postbiotic is an inactivated microorganism or a preparation of microbial components and metabolites — cell-wall fragments, exopolysaccharides, peptidoglycan, short-chain fatty acids — delivered without any live cells.

The rationale is direct: if the benefit is mediated by molecules that interact with the host’s receptors rather than by colonisation, then viability is unnecessary, and abandoning it removes every process constraint at once. A postbiotic tolerates pelleting, stores predictably, has a definable composition, and raises no questions about live organisms establishing in the gut.

The trade is that any effect requiring the organism to grow, compete for nutrients, occupy sites or produce metabolites in situ is lost by definition. Postbiotics can plausibly deliver immune stimulation through recognised structures such as β-glucans and peptidoglycan; they cannot deliver competitive exclusion.

What to be careful about

Effects are strain-specific and often diet- and species-specific, so results do not generalise from one strain to another of the same species, let alone across the category. Published trials vary widely in outcome, and responses are typically largest under challenge or poor husbandry and smallest in clean, well-managed systems — which is informative about the mechanism, and means a result obtained in one production context does not transfer to another.