GI flora: Understanding the role of probiotics in veterinary medicine (Sponsored by Nestle Purina)


Part of the 2009 Nestlé Purina Veterinary Symposium publication
Jun 01, 2009

The mammalian intestinal tract contains a complex, dynamic, and diverse society of pathogenic and nonpathogenic bacteria. Researchers have estimated that the human body contains 1 014 cells—only 1 0 percent of which are not bacteria and belong to the human body proper.1 A large body of research has focused on the mechanisms by which pathogenic bacteria influence intestinal function and induce disease; however, recent attention has been focused on the indigenous nonpathogenic microorganisms and how they may benefit the host. Initial colonization of the sterile newborn intestine occurs with maternal vaginal and fecal bacterial flora. The first colonizers can include species of enterobacteria, streptococci, and staphylococci. These bacteria metabolize oxygen and favor the growth of anaerobic bacteria, including lactobacilli and bifi-dobacteria. This overview will introduce the history of probiotics, their safety and regulatory background, provide information on their mechanisms of action, and introduce possible ways probiotics may be used to treat clinical diseases.

History of probiotic health claims

Documentation of the healthy consumption of bacteria in food dates back to the Old Testament (Gen. 18:8). Plinius, a Roman historian in 76 B.C., recommended the use of fermented milk products for the treatment of gastroenteritis.2 In the early 20th century, Elie Metchnikoff, a Nobel Prize-winning Russian scientist, suggested that the ingestion of Lactobacillus-containing yogurt contributed to the longevity of Bulgarian peasants by decreasing the pathogenic bacteria in the intestine.3 These observations led to the concept of a "probiotic," derived from the Greek, meaning "for life." The term probiotic was first used in 1965 to define substances secreted by one microorganism that stimulates the growth of another. The meaning of the word probiotics subsequently evolved to apply to those bacteria that contribute to intestinal balance. The World Health Organization defines probiotics as "live microorganisms, which when administered in adequate amounts, confer a heath benefit on the host."4

Different strains of probiotic bacteria may exert different effects based on specific capabilities and enzymatic activities—even within one species.5, 6 Different microorganisms express habitat preferences that may differ in various host species. The four habitats in the gastrointestinal tract are: the surface of epithelial cells; the crypts of the ileum, cecum, and colon; the mucus gel that overlays the epithelium; and the lumen of the intestine.7 The luminal content of bacteria depends greatly on bowel transit, resulting in a relatively low microbial density in the small bowel.

Figure 1. Fortiflora for dogs and cats
Because probiotics have multiple mechanisms of action, many have potential applications for managing various diseases. Those probiotics that have undergone the most clinical testing in people and livestock and are used widely include Lactobacillus species (such as L. acidophilus, L. rhamnosus, L. delbrueckii ssp. bulgaricus, L. reuteri, and L. casei); Bifidobacterium species; Enterococcus faecium and Saccharomyces boulardii, which is a nonpathogenic yeast. In dogs and cats, lactobacilli and bifidobacte-ria have also been used clinically. However, Enterococcus faecium has also garnered attention in clinical use in Europe and in the United States (see Figure 1). Despite the interest in probiotics for clinical use, understanding their clinical application in veterinary medicine has been limited by the paucity of well-designed laboratory, translational, and clinical studies.

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