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)
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
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
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
What is this quiet, unassuming ingredient, and should it be there?
It's the finishing touch that can meet both owner and pet needs.
The question is whether they provide additional benefit to the dog or cat
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