Probiotics, or direct fed microbials, are an entire class of live microorganisms with a very complicated but intriguing story (Krestel-Rickert and Kisic, 2003). These microbes aren't necessary to the diet for palatability or processing purposes, nor do they contribute directly to the nutrition of the animal. Rather, they are a natural complement to prebiotic soluble fibers like inulin, beet pulp or soy hulls, which are intended to support the health of the lower gastrointestinal (GI) tract.
Instead of providing substrate to the indigenous flora in the colon, probiotics represent the adding of beneficial microorganisms to the colon. The pay-off can be the prevention and treatment of such diseases as diarrhea (Sauter et al., 2006). However, describing their benefit to the animal can border on a medical claim rather than nutritional fortification. This can be problematic when it comes to crafting a marketing message necessary to justify their addition and cost in commercial petfood. Unfortunately, like a vaccination they don't offer much in the way of outward or observable results. Issues also extend to determining which probiotic organisms to use and how to get them into the animal.
How they work
The mode of action for probiotics in the gut is described by microbiologists as "colonization resistance," which in a nutshell is a "good versus bad" proposition. More specifically, the probiotic organisms add to the population of miroorganisms in the hindgut (colon) to a level sufficient to tip the balance in favor of the "good guys." The net result is that the colonic contents (digesta) become more acidic, with a better mix of short chain fatty acids (the fuel of the cells lining the colon).
In addition, the good guys inhabit the "hiding places" along the gut wall and generally create an unfriendly environment for "bad guy" pathogens like E. coli, Clostridium and Salmonella. There is also some suggestion that probiotics can stimulate immunity (IgG and IgA) at the gut level in both dogs and cats (Benyacoub et al., 2003; Marshall-Jones et al., 2006).
To be effective, probiotics must be live and viable. They are by design intended to be transient through the gut, not taking up full residence, lest they become pathogenic themselves. Thus, they must be provided orally on a regular basis, the sort of delivery that best fits with their addition to the animal's daily diet. Functionally, they must also be able to survive acidic digestion in the stomach, enzymatic/bile digestion in the small intestine and arrive intact in the large intestine to do their job.
For this reason, spore-forming anaerobes like Bacillus are often chosen (Biorge et al., 1998). Other organisms like Lactobacillus, Enterococcus, Bifidobacterium and Saccharomyces species are commonly proposed or tested. But this is an oversimplification of the dizzying array of candidate organisms that could qualify as probiotics.
Because food is provided daily, it makes good sense to add probiotics through this route. However, most gut-associated microorganisms and probiotic candidates are susceptible to heat treatment and, in general, petfoods involve some measure of sterilization and/or pasteurization. This makes it difficult to add them to foods with any hope of retaining them as live and viable. It certainly precludes mixing them with the raw ingredients prior to processing.
Canned food applications are disqualified due to the temperatures encountered during retorting. The elevated dry heat encountered with baking and the extremes in temperature and pressure of extrusion are too harsh to allow many, if any, probiotics to survive. While there have been several attempts to select probiotic organisms that will survive food processing (the so-called thermophiles), this has met with little real success. So, most petfood application is relegated to a post-processing coating of probiotics in a powder or liquid form to the outside of the kibble or treat.
This is fraught with problems as well, since adherence can be inconsistent and retention difficult. For example, in a recent report, 19 commercial petfoods purporting to contain probiotics failed one or more of the microbial guarantees (bacteria species, counts and/or contamination; Weese and Arroyo, 2003). Quality control practices are also challenging since few petfood production facilities are equipped with full-scale microbiology labs to validate the application of the probiotics. Further, efforts to understand the viability of these organisms for the full extent of a product's shelf life are lacking in the literature.
Supplements that contain the required amount and type of probiotics might be the answer and would certainly provide a more regulatory compliant vehicle for their delivery; however, everyday administration can be a problem.
Gaps and needs
Most of the probiotic research has been conducted with very small sub-populations of healthy dogs and cats. While deductive rationale for their benefit have been offered as a result of these trials, there is no clear-cut direct or clinical evidence that the constant use of probiotics reduces the incidence of disease in dogs or cats. A great deal of research is currently being published in an attempt to better define the right probiotics and their application for pets and man. However, while much of this is very good work, from a broader perspective the general direction is being driven by a "supply side" attempt to justify one microorganism over another.
The greater pet-owning community would benefit from a longer-view evaluation of the merits surrounding probiotics in pet diets, rather than getting bogged down in the short-term minutiae. This is because probiotics, though challenging to administer and to directly observe their effects, hold promise as a dietary component for the animal's GI health and stability.