CHAPTER 19 The Pros and Cons of Probiotics
The terms probiotics and antibiotics are derived from the Grecian word bios (life) and mean, respectively, “for life” and “against life.” Probiotics can be defined as viable microbes that, if delivered in sufficient amounts, act as active germs in the gut and yield positive effects on the host’s health. This definition is based on the work of Metchnikoff (1908), who first defined probiotics as “live micro-organisms, which exhibit a health-promoting effect”; Lilly and Stillwell (1965), who extended the definition by including substances produced by such microbes; Fuller (1989) and Gedek (1993), who suggested that probiotics are viable organisms that regulate the intestinal microbial balance and exhibit antagonistic properties; and the FAO/World Health Organization (2001), which emphasized that probiotics should be administered in adequate amounts to confer a health benefit on the host. In horses, animals known for having an especially sensitive gut, the possibility of optimizing digestive function to prevent or treat disorders with the use of probiotic microbes is of real interest. This chapter reviews the current information on the survival of probiotics in the equine gastrointestinal tract (GIT), and their modes of action, effects, risks, and regulation.
The probiotics Lactobacillus rhamnosus strain GG1 (LR), Escherichia coli strain Nissle 1917 (EC), and Saccharomyces cerevisiae CBS 493 94 (SC) can survive in the GIT of horses. None of these organisms is present in the feces of adult horses or foals before administration, and there is no permanent colonization or multiplication in the GIT, which justifies the current recommendation for daily administration of probiotics. These conclusions are supported by a number of observations.
When adult horses ingest doses of 1 × 1010 to 5 × 1011 colony-forming units (CFUs) of LR/500 kg body weight (BW) per day, they excrete low and variable fecal levels of LR except for when they ingest the probiotic at the highest dosage, in which case the peak intestinal colonization on day 3 averages 3 × 103 CFUs/g. Similarly, when horses receive a dose of 0.75 × 1010 CFUs/650 kg BW/day EC, only one third excrete viable EC 1 day after the 10-day supplementation period is finished.
When adult horses are supplemented with increasing daily doses of EC (0.75, 1.5, 2.5, and 5.0 × 1010 CFUs/500 kg BW/day), specific polymerase chain reaction (PCR) shows DNA from EC in the feces of only one third of the horses ingesting the probiotic at the lowest dose. However, this genetic material does not originate from a viable colony. When the feces are cultured, DNA from viable EC is found in all horses when they receive 1.5 × 1010 CFUs EC daily and with all higher doses.
After the first oral dose of 3 × 109 to 3 × 1010 CFUs/500 kgBW/day, SC appear in the colonic contents within 3 hours and in feces within 9 hours. Peak concentrations are observed within the first 12 hours and correlate with the dosage, varying from 5.7 × 103 to 1.9 × 105 CFUs/mL in colon contents and from 8 × 102 to 7.8 × 104 CFUs/g in feces.
Persistence of colonization is limited to 48 hours after cessation of administration for LR, which is similar to persistence patterns found for EC (24 hours) in adult horses. The behavior of LR differs between adults and foals, presumably because the immature nature of the GIT microflora in foals facilitates colonization. In foals supplemented with 2 × 1010 to 5 × 1011 CFUs/50 kg BW/day, LR concentrations in the feces are higher than those in adult horses, ranging from 4.5 × 103 to 3 × 107 CFUs/g in feces, and colonization peaks on day 3 for foals ingesting the lowest dosage and on day 7 for foals ingesting the highest dosage. Following cessation of administration, LR persists for a median time of 3 days and for as long as 9 days, suggesting a potential but transient colonization of the intestinal tract of foals.
Various modes of action have been reported for probiotic strains in the GIT ecosystem, including the effect on nutrient utilization or on competition among pathogens. Extending the results from one probiotic strain to another must be done with caution, because specific strains may act in different ways.
No studies have been undertaken that focus directly on the mode of action of probiotic bacteria in horses. Data from other species should be interpreted cautiously when applied to predicting or explaining effects in horses. For example, it must be clarified whether an induced increase in lactic acid production in the horse’s GIT is beneficial or, in particular situations, detrimental for the GIT environment. Further, individual probiotic bacterial species have different modes of action and thus need separate valuation.
Probiotic bacteria are expected to stabilize the desirable microbial community within the gut and its essential metabolic, trophic, and protective function. However, the mechanisms of action of probiotic bacteria are not fully understood. Suggested modes of action include competition for substrates and adherence sites on the epithelium, direct antagonistic effects against specific pathogenic microbes, influences on microbial metabolism, and stimulation of the immune system. Favorable survival characteristics and inhibition of the enteric pathogens Salmonella typhimurium, E. coli, and Clostridium perfringens have been demonstrated in vitro with potential probiotic bacteria (Bacillus subtilis, Lactobacillus salivarius