Robert J. Van Saun, DVM, MS, PhD, DACT, DACVN
The enteric environment is populated by a complex ecosystem of microbial organisms with diverse metabolic capabilities and range from being benign to pathogenic in the host animal. Prolonged antibiotic therapy, digestive diseases, prolonged inappetance and toxic indigestion among other conditions can result in depletion or alteration of the enteric microfloral populations. In such conditions, adjunct therapy often includes some form of dietary supplement that would either promote enteric bacterial growth or supplement bacterial numbers. Beyond adjunct therapeutic roles, there is interest in potential health promoting effects of such dietary supplements, especially in the human, equine and companion animal fields. The objectives of this presentation are to define differing groups of dietary microbial supplements and characterize known or implied mechanisms of action. Application of microbial supplements to equine nutrition will be reviewed based on the body of published studies in horses.
Definition and Proposed Actions
Prebiotic is a term used to define dietary fiber supplements that are intended to stimulate non-pathogenic enteric bacterial growth.1 Prebiotics are a diverse group of complex oligosaccharides that contain one or more of a variety of sugar moieties. Common sugar moieties include fructose (fructooligosacchrides, FOS), mannose (mannooligosaccharides, MOS), isomaltose, xylose (xylooligosaccharide, XOS) and arabinose (arabinooligosaccharide, AOS). Oligosaccharides are resistant to mammalian enzymatic digestion, but are readily fermented by enteric bacteria. Dietary oligosaccharide supplementation has been suggested to promote nonpathogenic colonic bacterial growth and maintain colonic health by production of beneficial short chain fatty acids (SCFA).1,2 Glycosidic linkages within specific oligosaccharides will stimulate growth of specific enteric bacteria as a result of their ability to hydrolyze the specific bonds. Using in vitro techniques, bacterial population growth was characterized for different commercial prebiotic oligosaccharides. Understanding the specific bacterial population stimulated with a given oligosaccharide allows for targeted manipulation of bacterial populations within the enteric environment in an effort to mitigate or prevent disease conditions. Our knowledge of the colonic environment and its influence on health is still in a state of infancy. Supplementation of XOS or lactulose maximally stimulated bifidobacteria growth while FOS stimulated lactobacilli numbers.3 A reduction in clostridia numbers was found with galactooligosaccharide fermentation.
As oligosaccharides have been shown to promote growth of specific nonpathogenic bacteria, one of the proposed mechanisms of action is a competitive exclusion of pathogenic bacteria. Fermentation production of VFA is believed to decrease intestinal pH adversely altering the environment for pathogenic bacteria. Oligosaccharides of mannose are thought to saturate the enteric environment and inhibit adherence of pathogenic bacteria to enterocytes by mannose-specific lectins.4 Glucomannans may also play a protective role in preventing absorption of mycotoxins in horses.5,6
Limited studies have addressed potential benefits of oligosaccharide supplements for horses. Yearling Quarter horses supplemented with 0, 8, 24 g FOS/day in a 3 x 3 Latin square study design showed a linear dose effect of lower fecal pH and increased short-chain VFA concentrations, consistent with increased bacterial fermentation.7 In this study fecal E. coli population was reduced in the 8 g treatment, with no difference between 0 and 24 g treatment groups. Other bacteria were not different across treatment groups. Supplementation of FOS seems contradictory given the current concerns over its potential role in laminitis (Bailey, Crawford).8,9 Studies feeding large molecular weight FOS (inulin) found dramatic declines in cecal and colonic pH a result of increased lactic acid production and an associated increase in toxic amines, which may be linked to the pathogenesis of laminitis.8,9 In contrast to these studies, feeding of short chain FOS prevented a decline in colonic or cecal pH associated with a starch overload from barley grain feeding compared to unsupplemented horses.10 Differences in the response between these experiments may be a result of anatomic location where the FOS supplement is fermented. Larger molecular weight compounds may be less degraded in the small intestine, resulting in more material being delivered to the large intestine.
Foals drenched with either 10 g arabinogalactan for the first 14 days of life or MOS supplement provided to the pregnant and lactating mare showed improvement in diarrhea susceptibility and severity.11,12 No or minimal differences were seen between treated and untreated foal groups on serum immunoglobulin concentrations or hematology parameters. Another study also did not find MOS supplementation to influence immunologic parameters in various age horses.13 These observations would suggest the health effects of prebiotics is not mediated through altered immunologic responses. A concern with the use of oligosaccharides, especially FOS, is targeting the desired enteric site for stimulating bacterial growth. Although most studies provide generally positive results with prebiotic supplementation, especially in young foals, questions concerning the interaction between site of action and oligosaccharide type suggest caution when feeding to older animals.
Probiotics--Direct Fed Microbials
Probiotics or direct fed microbials (DFM) are live microbial colonies of lactic acid bacteria, namely Lactobacilli, Bifidobacteria, and entercoli, typically present in the intestinal lumen of healthy animals and are perceived to exert beneficial properties such as improved lactose digestion and resistance to pathogens. Provision of live bacteria is believed to exclude or reduce growth of potential pathogenic bacteria by competitive inhibition, production of inhibitory substances, promotion of localized immune responses, or alteration of the luminal environment.14 Other proposed benefits of these bacteria to the host animal include production of vitamins, enzymes and volatile fatty acids (VFA) that may provide nutritional value, aid digestion and benefit gastrointestinal health. Questions have been raised relative to microbial viability and concentration within commercial human and veterinary probiotic products.15,16 An additional concern of these products relates to the applicability of cultured bacterial organisms to colonize in specific host animals.17 Some newer products entering the market combine prebiotics and DFM in what is termed synbiotics; attempting to take advantage of a potential interactive effect.
In determining efficacy of probiotic products, administration of live bacterial cultures should be able to colonize the intestine of the host animal. Similar to prebiotic supplements, site of colonization may be important to success of overall effect. The equine hindgut is somewhat similar to the rumen from a bacterial ecosystem perspective. Lactobacillus species with their ability to produce d- and l-lactate may not be desirable bacteria to add to this ecosystem. In contrast, these bacteria may be beneficial in colonizing the small intestine if they can truly competitively eliminate pathogenic species. Colonization of the equine gut with human strain Lactobacillus rhamnosus strain GG found low colonization in adults, even at a very high dose, and consistent colonization in foals with administration over 5 days.18 An equine-specific organism, Lactobacillus pentosus WE7, was identified as having good inhibitory activity against enteric pathogens and colonization ability.17 Unfortunately, when this organism was used as a probiotic agent in neonatal foals it induced clinical disease and diarrhea.19
Other studies also found minimal effects to the hindgut environment (pH, bacterial populations, and volatile fatty acid production) with administration of Lactobacillus acidophilus to cecally cannulated geldings.20 Supplementing a commercial product of mixed lactobacillus bacteria to foals had no effect on foals fed either a starch- or fiber-based diet at weaning.21 Use of commercial probiotics in sick horses has resulted in opposing responses relative to Salmonella shedding, diarrhea prevalence and disease recovery.22,23 Use of probiotics might be best targeted to younger animals. Concerns about safety and utility and limited number of clinical studies in horses require further evaluation of probiotic products.
Yeast Culture or Extract
Definitions and Mechanisms
Yeast products used as feed ingredients include dried yeast (active or nonfermentative), yeast culture, and yeast extract. Most yeast products are derived from Saccharomyces cultures, primarily S. cerevisiae, or Aspergillus oryzae. Active dry yeast contains a minimum of 15 billion live yeast cells per gram. Yeast culture is a dried product containing viable yeast cells and the yeast growth culture media. Yeast extract is a dried or concentrated product of cell contents from mechanically ruptured Saccharomyces cerevisiae cells. Dried yeast must contain a minimum of 40 percent crude protein, whereas yeast extract contains a minimum of 9 percent crude protein.
Various yeast products are commonly used in ruminant diets and have been fairly intensively studied relative to animal performance. Results of these studies suggest yeast additives directly facilitate fiber digestion and dry matter intake (active cultures). Yeast products may also contain metabolites or compounds having stimulatory properties on bacterial growth to facilitate fermentation and animal performance (active or nonfermentative). Supplemented live yeast cultures showed most pronounced effects on collectively increasing rumen bacterial growth. Modest changes were reported for measures of rumen fermentation, dry matter intake, growth, and feed efficiency with live yeast cultures.
Live yeast cultures have been most studied in horses. Most of these studies evaluated the potential of enhanced fermentation and fiber or nutrient digestibility in the horse. Unlike observed effects in ruminant studies, supplementation of yeast in horse diets tended to show some beneficial effects on fermentation, but results were consistent across studies. Both in vitro and in vivo studies have been completed in horses to address the effects of yeast on colonic microbial activity and growth. Results of in vitro studies have been inconclusive relative to effects on stimulating bacterial growth with more showing no impact on bacterial numbers.24-26 Effects on fermentation activity ranged from minimal augmentation (shorter time for gas production)27 to no effects.28,29 Other studies found effects on fermentation to depend upon type of diet. Lower lactate and higher acetate production was observed when feeding high starch or low forage diets, respectively.25,26 Studies measuring cecal and colonic pH found minimal effect of supplemental yeast.26,28,29 Some of these studies may not have challenged the microbial environment adequately. Other studies feeding either high starch or low forage diets to horses observed higher cecal pH with yeast supplementation compared to unsupplemented horses receiving similar diets.24,25,31
Besides affects on fermentation characteristics, other studies attempted to define the role of yeast supplements on nutrient digestibility. Again, study results have been mixed with no improvements found,32,33 but others have reported improved digestibility for one or more nutrients with yeast culture feeding. Improvement in dry matter, NDF, and ADF digestibility with yeast supplementation was reported for mature34,35 and yearling36 horses. Another yearly study found only hemicellulose digestibility improved with yeast supplementation.37 Improved nitrogen digestibility and retention with yeast supplementation was the most consistently reported response to yeast supplementation.35-39 Two studies reported improved digestibility with magnesium and phosphorus.34,39 Phosphorus digestibility, independent of source, was increased 22.3 percent across two different whole collection trials feeding forage (66 percent) and sweet feed.34
A final method of evaluating effect of yeast supplementation was assessment of overall animal performance. Weanlings fed yeast supplements showed improved weight gain, height, and feed efficiency,40 though no growth advantage was observed with yearlings.41 One research group observed improved digestibility of dietary energy, protein, and fiber resulting in greater milk production and improved foal growth when pregnant mares were fed live yeast 4 weeks prior to foaling.35,38,42 A health effect was observed with repeated supplementation of 10 x 109 live Saccharomyces boulardii every 12 hours for 14 days. Treated horses had reduced severity and duration of clinical signs associated with enterocolitis compared to placebo-treated horses.43 In this study, supplemented yeast could be found in feces during supplementation, but there was no ability to determine colonization potential of the colon.
Like studies completed with ruminants, variation in responses between studies was commonly observed. This variation may be attributed to differences in the amount of yeast supplement being fed, composition and treatment diet interaction, and diet adaptation time. Results across the studies would suggest some potential benefits to feeding yeast when high-starch or low-fiber diets are being fed.25,31 Improvements in nutrient digestibility, especially nitrogen, are suggested, but the exact mechanism for this response is not evident. Constant feeding of yeast may have greater potential for mediating an effect as compared to sporadic supplementation. Given most reported studies used live yeast cultures, application of nonfermentative yeast cultures requires further study in horses though similar responses are seen with different yeast products in ruminants.
Interest in treating infectious disease without chemotherapeutic means, promotion of health as well as what is considered more "natural" therapy has increased interest in the use of microbial supplements of some form. The enteric microbial population is a complex environment that is not completely understood. Evidence would suggest that some manipulations of the microbial populations can bring about improvements in animal health and welfare. Three types of microbial supplements were described; namely prebiotics, probiotics and yeast. Prebiotics, through their physical and chemical properties, promote growth of specific bacterial populations that seemingly out-compete potential pathogenic bacterium species. Probiotics provide a source of live bacteria that hopefully colonize the gut and competitively exclude pathogens. Though not completely defined, yeast supplements seemingly stimulate bacterial growth. One problem with microbial supplements is the inability to specifically target the anatomic region of the gut for which the effect is intended. There are some concerns with the use of FOS prebiotics and lactobacillus probiotics in older horses compared to young horses. Observed age-based responses to microbial supplements may be explained by differences in enteric maturity and predominate microbial populations. This is evidenced by generally positive response to probiotics and prebiotics in young animals compared to mature horses. Yeast products are the more extensively studied supplements and show positive effects in both young and mature animals. Evidence does not suggest blanket application of these products, but with an understanding of their potential mechanisms of action and an appreciation of the underlying disease process or intended action, these microbial supplements may provide appropriate adjunct therapy.
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