Postbiotics (Latin “post” term means “after” and “biotic”, life; also known as “tyndallized bacteria”, “paraprobiotic”, “heat-killed probiotics”, “inactivated probiotics”) are bioactive compounds for which to give a consensual definition is still difficult. The term postbiotic was defined by the International Scientific Association of Probiotics and Prebiotics (ISAPP) as “a preparation of inanimate microorganisms and/or their components that confers a health benefit on the host” (1). More precisely, postbiotics are deliberately inactivated microbial cells (yeast or bacteria) with or without metabolites or cell components (referring to cellular biomass) that contribute to demonstrated health benefits. Although the ISAPP definition is widely cited, six previous definitions of postbiotics have been proposed, including metabolites secreted by probiotics (1; 2; 3), or produced after the beneficial gut bacteria metabolize prebiotics components (4), or metabolites generated by the microbiota (5). Depending on the terms and definitions used, confusion may appear, and comparison between products is almost impossible. For a sake of clarity, the term postbiotic here refers to the definition of the ISAPP panel, as I believe that this is, to date, the best way to define it.Postbiotics as metabolites produced after metabolization of prebiotics or probiotic (4), or as metabolites generated by the microbiota (5) are not considered as appropriate definitions because they require other regulatory considerations and they describe physiological processes in situ, while biotic substances concept deals with voluntary nutritional interventions. Postbiotics field is seen here as an emerging field of research that extends the established domains of prebiotics and probiotics. We will specifically focus on the use of postbiotics to impact microbiome-gut-brain axis by showing their potential, but also what is still required to use them accurately.

Postbiotics are interesting ingredients to use in the petfood industry for various practical reasons: their safety, stability (6), their simple storage and transportation (7), and their resilience to gastric juices, digestive enzymes, and bile are few of them. In terms of bioactivity, they are made of various components among which teichoic acid, peptidoglycan, exopolysaccharides, surface layer proteins, bacterial metabolites (1) and of structures like extra-vesicles (8), all with strain-specific motifs. The main mode of action of a postbiotic is based on the fact that those components exert beneficial effects on the gut health, mainly through the inhibition of pathogens, the reinforcement of the gut barrier function, and immunoregulation mechanisms (1, 9). Indeed, interactions with host cells still can occur regardless of probiotic viability through cell signal receptors. So far, this remains a new field of investigation for pets. Wang et al. (2024; 10) reported that a blend of inactivated Bifidobacterium longum CECT-7347 and a prebiotic fibre (0.16% blend, no precision of the postbiotic quantity) improved the intestinal health (measured via a decrease in serum lipopolysaccharide levels, fecal pH, an increase in fecal sIgA contents and a modulation in the fecal bacterial community) of adult cats subjected to abrupt dietary change from a 33% crude protein diet (corn/chicken-based) to another one with 40% crude protein. Another in vitro study (11), reported that inactivated Limosilactobacillus reuteri NBF 1 with a prebiotic resulted in change in short-chain fatty acids (SCFA) and microbiota. In humans (12), zebrafish (8) and rodents (13), more evidence confirms the ability of using a postbiotic to exert an immunomodulatory effect and an effect on gut barrier. Although those results are promising, further research in pets to demonstrate bacterial strain-specific and animal species-specific effects of postbiotics on gut health is still needed.

Maintaining gut homeostasis may result in direct benefits not only for gut health, but also for systemic health. In humans, a new field of development is the gut-brain axis which refers to the bidirectional communication between the gastrointestinal system and the central nervous system through neuronal, neuroendocrine and neuroimmune pathways (14). In this context, many studies suggested a capacity of the gut microbiome to interfere in this bidirectional communication: modulation of various neurotransmitters levels, production of SCFA that may cross the blood-brain barrier and impacts neuronal and immune cells in brain, influence of immune cells to produce cytokines, … Emerging research indicates this connection is just as important for pets as it is for humans. Anxiety troubles, epilepsy, or loss of cognitive functions with aging are traits shared by both humans and pets (15) and represent a significant issue in veterinary behaviour, posing medical challenges and affecting both pets and pet parent’s quality of life. However, research specifically focused on pets is still in its early stages: evidences that support the influence of postbiotics on gut-brain connection are scarce and often involved blends of various types of nutraceuticals (16). Several studies in mice and humans showed promising results (17, 18, 19, 20). As an example, the postbiotic Lactobacillus gasseri CP2305 used at the dose of 10¹⁰ cells/day for 24 w in humans resulted in significantly reducing anxiety and sleep disturbance compared to placebo (double-blind, placebo-controlled, parallel-group clinical trial involving 60 subjects (18). Anti-anxiety effects of postbiotics have also been observed in healthy rodents (19, 20). In one case, authors reported a modified gene expression profile in the prefrontal cortex, and increased relative abundance of gut Butyricicoccus and Enterococcus concomitantly to the reduced anxiety-like effects (20); while the second showed a decrease in the baseline corticosterone level (considered as a biomarker of the hypothalamus-pituitary-adrenal axis), a distinct gut microbiota composition and change in behaviour (19).

Using postbiotics in pets is still at its infancy but first proofs of concept make this new technology interesting, and represents a way to foster microbial innovation to sustain pets’ health. In addition, the use of inactivated microbes paves the way of developing new microorganisms, including strict anaerobes, with new functional benefits, although some precautions of use have to be considered. A well-designed postbiotic shall be thought from the live form of the microorganisms via accurate screening of various microorganisms, for their functional benefits, but also for their safety and efficacy, requiring to identify novel biotics. Potential side-effects, accurate dosage and long-term safety effects must be evaluated. Also, growth media as well as process of inactivation have to optimize, then keep the compounds bioactive (20) and avoid deleterious metabolites production while microorganisms are stressed. The producers need to develop and provide quantitative methods to analyse the recovery of the postbiotics in the final products. Importantly, each microorganism has to be characterized at a strain level, as, like for probiotics, the bioactive effect is strain-dependent. The characterization of a specific strain could include a characterization of the cell wall components, and, if relevant, of the associated metabolites. Establishing a true causation between the postbiotic and the benefit using in vitro or in vivo models is of paramount importance: the efficacy of inactivated bacteria is not predicted by the efficacy of its live counterpart, and knowing the modes of action is key to predict the accuracy of using a microorganism inactivated rather than alive.

The journey to use postbiotics has to include dedicated research on pets; as pets are obviously not humans, we cannot assume that if a postbiotic works in humans, it works in pets without testing. Furthermore, the impact of postbiotics on gut-brain axis may vary depending on the breed, age, health status (i.e., suffering from various diseases) and diet, opening the field of customized nutrition by crossing the best microbial strain with the target host benefits, mixing down-town and bottom-up approaches. Thus, well-designed and controlled studies are needed to assess the benefits of strain-specific postbiotics for pets at the targeted physiological stage. These studies should focus on the mechanisms and on the clinical effects, requiring the selection of accurate biomarkers to decipher a “healthy microbiome and gut” and the link with the behaviour effects. We recommend, on top of clinical signs, to associate relevant blood biomarkers (e.g., cytokines, neurotransmitters, metabolites, hormones…) that might allow making a “physiological bridge” between the given postbiotic, the microbiota, the behaviour. Of note, this requires an accurate characterization of the target behaviour/cognitive function that is addressed. Such work will allow avoiding marketing hype and over-promising their health benefits without sufficient scientific backing. Also, educating vet practitioners and pet parents about the terminology and the mode of action of postbiotics is key. At the same time, clear labelling and regulation have to be set, and this is possible if biotics producers work together with the petfood industry and the regulation authorities to provide detailed information about product ingredients (cell biomass, presence of metabolites, which ones, quantitative methods, dosage, administration duration, …). Microbiome science advances, more powerful laboratory assays, robotics, and equipment, and the availability of different postbiotic derivatives in the market; all will influence market extension, efficacy and competitiveness. This holistic and proactive approach will ensure no confusion between biotic products and will offer a successful future in using postbiotics to sustain health of our companion animals, and paves the way of new nutritional approaches to cope with brain-related issues.