The generally accepted definition of a probiotic is 'a live microbial food supplement which beneficially affects the host animal by improving its intestinal microbial balance'. This definition is, however, rather limited as some probiotics are transient and do not take up residence in the intestinal tract. A better definition may be '[a] microbial dietary supplement that beneficially affect the host physiology by modulating mucosal and systemic immunity, as well as improving nutritional and microbial balance of the intestinal tract' (Salminen et al 1998).
The gastrointestinal tract is sterile at birth. Normal gut flora develops gradually over time and is influenced by factors such as composition of the maternal gut microflora, diet, degree of hygiene, use of antibiotics or other medication, the environment and possibly genetic aspects. Once established, a person's individual gut flora remains surprisingly constant throughout life. This is likely to be due to the fact that the gut immune system learns to recognise and tolerate those bacterial species acquired during early infancy. It is therefore very difficult to alter the composition of the gut flora after this time. Successful colonisation with probiotics is therefore most often transient, as the gastrointestinal tract has many defences that inhibit this process (Vanderhoof & Young 2002). The intestines are host to 1014 microbes representing 400-500 different species (Ouwehand et al 2002).
Clinical note— Prebiotics
There is another concept associated with the microflora and intestinal health:
prebiotics, which are compounds that modify the environment of the gastrointesti-© 2007 Elsevier Australia
nal tract to favour proliferation of the beneficial intestinal microflora (Gibson & Roberfroid 1995). Herbal and nutritional prebiotics include the fibre-supplement known as slippery elm (Ulmus fulva), oligofructose and inulin. The prebiotic approach, while promising, has not been thoroughly tested by controlled clinical trials.
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