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Literature Review


The concept of probiotics was first reported by Elie Metchnikoff in 1907. He theorized that the longevity of certain ethnic groups was due to ingestion of fermented milk products, which manipulated the intestinal microflora to maintain the normal balance between pathogenic and non-pathogenic bacteria. The term probiotic was proposed in 1965, by Lilley and Stillwell, to describe substances that favored the growth of microorganisms. Fuller (1989) later revised this definition to specifically include microbes that directly benefit an animal by improving its balance of intestinal microorganisms. The term “probiotic” has since been used to refer to viable bacteria and fungal cultures, enzyme preparations, culture extracts or any combination of the above. Because of the confusion surrounding the term probiotic, the US Food and Drug Administration (FDA) began requiring manufacturers of animal feeds to use the term direct-fed microbial (DFM), instead of probiotic, in 1989 (Yoon and Stern, 1995). The FDA defined the term DFM as “a source of live (viable) naturally-occurring microorganisms”. Within the context of this paper, probiotic and DFM will be used interchangeably to refer to Fuller’s definition.
One of the reasons that use of probiotics has been viewed with skepticism is that the mechanism of action is not fully understood. Theories have included production of anti-microbial substances (Naidu et al., 1999), competition for adhesion receptors (Spring, 2000), competition for nutrients (Montes and Pugh, 1993) and stimulation of immune responses (Perdignon, 1986). Individual probiotics may use unique mechanisms or the same probiotic may inhibit various pathogens by different mechanisms.
Depending on the strain and type of nutrients available, lactic acid bacteria (LAB) are known to produce various anti-microbial substances. Lactic and other volatile fatty acids, which are end products of carbohydrate metabolism, decrease luminal pH, resulting in broad spectrum inhibition of gram-positive and gram-negative bacteria (Naidu et al., 1999). Some strains of LAB, Lactobacillus acidophilus for example, produce hydrogen peroxide (H2O2), which lowers the oxidation-reduction potential and specifically inhibits the growth of aerobic organisms. Anti-microbial proteins called bacteriocins, such as acidophilin, lactolin and acidolin, have anti-microbial activity against some strains of Escherichia coli and Salmonella typimurium and are also produced by microbes in the gut (Naidu et al., 1999). Lactic acid bacteria can produce carbon dioxide (CO2) via a number of pathways (Naidu et al., 1999). Carbon dioxide has destructive effects on cell membranes and is also able to decrease luminal pH. Two additional products of metabolism by LAB are diacetyl (2,3 butanedione) and acetaldehyde. Both have shown bactericidal effects against E. coli and Salmonella (Jay, 1982; Kulshrestha and Marsh, 1974).
The needs of pathogenic bacteria are similar to the needs of probiotics in that they must be able to survive and adhere to the intestinal epithelia in order to effect the host animal. Lactobacilli may prevent this by occupying epithelial binding sites or by producing a biofilm that physically protects the cells (Montes and Pugh, 1993). Both specific and nonspecific mechanisms have been identified for L. acidophilus (Kleeman, 1982). Some strains may also cause a nutrient depletion effect by exhausting food sources that are essential for the growth of other microorganisms (Montes and Pugh, 1993).
Finally, immune stimulation has been proposed as a mechanism of action for probiotic bacteria. Studies in mice suggest that administration of L. acidophilus and S. thermophilus significantly enhances the enzymatic and phagocytic activity of peritoneal macrophages (Perdignon, 1986). L. casei has demonstrated the ability to increase mucosal immunity via local production of IgA against Salmonella typhimurium infection when orally administered to mice (Perdignon, 1990).
In order for microorganisms to have a “probiotic” effect, they must meet specific criteria. They must have the ability to survive transit through acidic portions of the gut, adhere to the intestinal epithelial cells, colonize the intestinal tract and inhibit the growth of pathogenic bacteria. If intended for use in commercial products, they must also withstand processing techniques. Various commercial preparations of probiotics are available for livestock species, including the horse. The primary microorganisms that have been used as DFM for ruminants are fungal cultures including Aspergillus oryzae and Saccharomyces cerevisiae and lactic acid bacteria (LAB) such as Lactobacillus or Streptococcus (Yoon and Stern, 1995). Preparations for horses are similar; strains of Lactobacillus and Bifidobacterium are the most commonly used LAB, while Saccharomyces cerevisiae is the most common strain of yeast (Weese, 2001). It is important to know specifically which strain of a species is being used since only certain strains of a species have positive health effects and have the ability to survive processing and storage. Numbers of viable beneficial microorganisms in a product are quantified by listing colony-forming units per g (CFU/g).
Despite both their availability and widespread use in the industry, there are some concerns regarding commercially prepared probiotics. The two most pressing concerns are quality control and recommended dosing. Probiotic organisms are classified by the FDA as generally regarded as safe (GRAS), a designation that has lead to frequent use without standard efficacy or safety trials. Misidentification of bacteria is common in both human and veterinary products (Weese, 2002). A majority of products tested in a recent study did not contain the claimed organisms (or they were not viable), contained additional species, or contained lower concentrations than stated (Weese, 2002). Effective doses for probiotics are often underestimated by commercial producers. The dose of viable organisms required to ensure colonization of the equine intestinal tract is unknown, but researchers extrapolating from human studies report an average (~450 kg) horse would require at least 1 X 1010 to 1 X 1011 CFU/day of viable organisms (Weese, 2001). In most cases probiotic bacteria must be dosed daily. For example, lactobacilli, which attach to the epithelial cells in the intestinal tract, form a coating on the villi, which is eventually shed. Due to this sloughing, high doses of lactobacilli must be administered regularly (Montes and Pugh, 1993).
Reported benefits of probiotic supplementation in farm animals include improved digestion and absorption of nutrients (Abe et al., 1995), increased growth rate (Topliff and Monin, 1990), milk yield and egg production (Hoyos et al., 1987), as well as greater resistance to infectious disease (Lema et al., 2001). Condition and management of the animal seems to affect results. Stressed animals and those in sub-optimal conditions have shown the greatest response (Fuller, 1999). Some studies have shown little or no benefit to supplementation, however researchers seldom verify the strains being tested or assess the bacteria’s ability to colonize the gut, and studies most often use different combinations of microorganisms, making it difficult to analyze results (Fuller, 1992). Sound, scientific research involving the use of probiotics in the horse is even scarcer than in production livestock. As in other species, the results for horses have been mixed. Experimental trials are often in-house experiments or contracted by commercial producers. Still, some reviews conclude that the right probiotic, given at the right time, in the right dose, has the potential to provide significant health benefits to horses and other livestock (Fuller, 1999).
Infection with Salmonella can be fatal to horses and can also cause costly outbreaks in veterinary hospitals. One survey reported 14 outbreaks of the disease between 1985-1996 with 6 of these resulting in hospital closure, and costs per outbreak ranged from $10,000-$420,000 (Parraga et al, 1997). The microbial population in a horse’s gut may play a vital role in the prevention of Salmonella infection. Anti-microbial therapy decreases the dose of Salmonella organisms required for infection. Among horses with colic, the incidence of Salmonella infection is higher in those cases where the hindgut, and presumably the bacterial micro flora, was affected, such as feed and sand impactions (Parraga et al., 1997).

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Table of Contents
Literature Review
Weaning Stress
High Fat, High Fiber Diets
Vitamin E
Materials and Methods
Table 1. Ingredient composition of the SS and FF feeds
Table 2. Nutrient content of pasture and feeds.
Results and Discussion
Literature Cited

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