By Dr Louise Maré, Agricultural Research Council – Livestock Business Division: Animal Production
The concept of probiotics evolved at the beginning of the 20th century from a hypothesis first proposed by the Nobel Prize winning Russian scientist Elie Metchnikoff. He suggested that the long and healthy life span of Bulgarian peasants was due to the consumption of fermented milk products (Metchnikoff, 1908). During the last few decades, research on probiotics has expanded beyond bacteria isolated from fermented dairy products to normal microbiota of the intestinal tract (Sanders and Huis in’t Veld, 1999).Vanbelle et al. (1990) defined probiotics as natural intestinal bacteria that, after oral administration in effective doses, are able to colonize the animal digestive tract, thus keeping or increasing the natural flora, preventing colonization of pathogenic organisms and securing optimal utility of the feed. Prebiotics are defined as non-digestible food ingredients that affect the host beneficially by selectively stimulating the growth and/or activity of bacteria in the colon (Gibson and Roberfroid, 1995).
This definition was recently amended to ‘A prebiotic is a selectively fermented ingredient that allows specific changes, both in the composition and/or activity in the gastrointestinal microbiota that confers benefits upon host well-being and health.’ In practice, the beneficial bacteria that serve as targets for prebiotics are mostly lactobacilli and bifidobacteria (Gibson et al., 1999; Bouhnik et al., 2004). Unlike probiotics were allochthonous microorganisms are introduced in the gut, and have to compete against established colonic communities, an advantage of using prebiotics to modify gut function is that the target bacteria are already commensal to the large intestine (Macfarlane et al., 2008). However, if for any reason like disease, ageing, antibiotic or drug therapy, the appropriate health-promoting bacteria are not present in the bowel, prebiotics are not likely to be effective. Combinations of prebiotics and probiotics are referred to as synbiotics.
Commercial probiotic products often do not meet expected standards in that the composition and viability of the strains may differ from information on the label (Hamilton-Miller et al., 1999; Hamilton-Miller and Shah et al., 2002; Weese, 2002; Fasoli et al., 2003). Another major issue in relation to the application of probiotics is the poor evidence for efficacy based on clinical trials (Klaenhammer and Kullen, 1999).
Three issues interfere with the identification of specific health effects of probiotics (Klaenhammer and Kullen, 1999). Firstly, the complexity and variability of the gastro-intestinal environment in relation to gastro-intestinal diseases make it difficult to determine the effect probiotics have on health and disease. Secondly, confusion as to the identity, viability and properties of probiotics lead to strains being incorrectly identified.
Lastly, single probiotic strains induce a multitude of effects among different hosts in a test population. A mono-strain probiotic is defined as containing one strain of a certain species whereas multi-strain probiotics contain more than one strain of the same species or genus. The term multi-species probiotics is used for preparations containing strains that belong to one or preferably more genera (Timmerman et al., 2004). Multi-species preparations have an advantage when compared to mono- and multi-strain probiotics (Timmerman et al., 2004). Multi-species probiotics benefit from a certain amount of synergism due to the combination of characteristics from different species.
The concept of probiotics plays an important role in animal health. Pig rearing has become an intensive commercial industry. Economic losses due to decreased health and performance brought about by intensive farming practices focused on increased production and low costs, are very important. Major efforts have been made to find different ways to improve the rearing of pigs. Antibiotics have been used successfully for more than 50 years to enhance growth performance and control the spread of disease (Gustafson and Bowen, 1997). Antibiotic resistance is as ancient as antibiotics, protecting antibiotic producing organisms from their own products (Phillips et al., 2004). Antibiotic resistant variants and species that are inherently resistant can dominate and populate host animals.
Increased concern exists about the potential of antibiotics in animal feed and their contribution to the growing list of antibiotic-resistant human pathogens (Corpet, 1996; Williams and Heymann, 1998). Although the use of antibiotics for growth promotion is still allowed in certain countries, including the United States, Australia and South Africa, several European countries have implemented strict legislation to prevent the incorporation of antibiotics in animal feed (Ratcliff, 2000). In 1986 Sweden was one of the first countries to ban the incorporation of low-dose antibiotics into animal feed.
The question remains, does the use of antibiotics in production animals pose a risk to human health? In a recent review, it was stated that the actual danger appears small and the low dosages used for growth promotion (generally below 0.2% per ton feed) could not be regarded as a hazard (Phillips et al., 2004). Antibiotics are used in animals and humans, and most of the resistance problem in humans arises from medicinal use.
Resistance may develop in bacterial populations present in production animals, and resistant bacteria can contaminate animal-derived food, but adequate cooking destroys most bacteria. Growth-promoting antibiotics predominantly active against Gram-positive bacteria have very little or no effect on the antibiotic resistance of salmonellae and consequently on infections caused by salmonellae. In some parts of the world, antibiotics used to treat animals and added to feed as growth promoters may have adverse effects when associated resistance is taken into account.
The same antibiotics are often used to treat humans (Phillips et al., 2004). In contrast, Piddock (2002) could not find clear evidence that antibiotic-resistant bacteria isolated from animals, cause infections in humans, for example quinolone-resistant strains of Salmonella serovar Typhimurium DT104 are not transmitted through production animals. The flouroquinolones used therapeutically in animals appear to pose little threat to human health. Flouroquinolone resistance was recorded in bacteria isolated from humans, in countries where the use of this growth promoter is banned such as Sweden, Finland and Canada (Rautelin et al., 1993; Sjögren et al., 1993; Gaudreau and Gilbert, 1998).
Faecal flora isolated from a healthy person may contain antibiotic resistant enterococci, but most enterococci isolated from animals do not colonize the human intestine (Dupont and Steele, 1987; SCAN, 1996, 1998; Bezoen et al., 1999; Butaye et al., 1999; Acar et al., 2000). E. coli resistance is more likely to be driven by antibiotic use in humans, although an animal origin for at least some clinical isolates cannot be excluded (Gulliver et al., 1999). The banning of antibiotic usage in animal feed remains a controversial issue especially in the way that it affects farming with production animals.
Many natural substances have been investigated as alternatives to conventional chemotherapeutic agents (Turner et al., 2002). Probiotics are one approach used to improve piglet health and deal with intestinal problems encountered during rearing (Vanbelle et al., 1990). Other approaches include acidification of feed or water (Chapman, 1988), altering dietary formulations for small piglets, the development of feeds with lower protein content (Lawrence, 1983), and vaccination with attenuated pathogens or with strains genetically modified (Greenwood and Tzipori, 1987; Trevallyn-Jones, 1987). The administration of growth hormones, somatostatin immunization and enzyme supplementation were also considered as alternatives to antibiotic treatment (Thacker, 1988).
Treatment with psychopharmacological drugs (Björk et al., 1987), utilization of the lacto-peroxidase system (Reiter, 1985) and stimulation of hormone-like proteins (anti-secretory factors) capable of reversing intestinal hyper secretion to reduce symptoms of diarrhoea (Lönnroth et al., 1988) were proposed. Some esoteric substances such as zeolite have reduced diarrhoea in piglets and increased feed efficiency (Mumpton and Fishman, 1977). Natural substances that enhance growth performance and immune function in pigs include plant products such as seaweed, saponins extracted from certain desert plants, spices and herbs (Turner et al., 2002). Probiotic preparations may be incorporated in prophylactic agents and it is important to know the mode of action to anticipate the dosage levels (Jonsson and Conway, 1992). The use of probiotics should not exclude other alternatives and a combination of treatments may be complimentary and more effective.
ThThe large population of LAB present in the digestive tract of a healthy animal makes piglets ideal candidates for probiotic dosage. The genetic background, physiological health status and diet of the animal may influence the effectiveness of probiotic preparations (Jonsson and Conway, 1992). It appears difficult to establish a probiotic permanently in the digestive tract of the host animal. Most studies indicate that the indigenous microflora are very efficient in preventing new organisms from establishing permanently (Jonsson and Conway, 1992). Other general disadvantages of probiotics include their high price and the fact that a high dosage of administration is required (Guerra et al., 2007). More basic knowledge of the digestive ecosystem is needed to obtain consistent effects from probiotics. With the introduction of molecular based techniques such as FISH, this might be achieved in the future.