Dr Roy Fuller defined a probiotic as 'a live microbial feed supplement which beneficially affects the host animal by improving its intestinal microbial balance'. This definition emphasises the importance of live cells as essential components of probiotics.
The word probiosis originated from Greek: pro (for) and biosis (life), and is therefore opposite in meaning to antibiosis, promoting the proliferation of bacterial species within the gastrointestinal tract. Probiosis is defined as 'the property of the normal adult flora to resist the overgrowth of component strains and the establishment of foreign strains' and is reinforced or re-established by probiotics. The concept of probiotics originated from Metchnikoff. He postulated that the longevity observed in the Balkan people was due to the regular consumption of soured milk containing Lactobacillus bulgaricus.
Probiotics have been shown to work by the following mechanisms:
- Competition for nutrients
Within the gut, beneficial as well as pathogenic microorganisms will be utilising the same types of nutrients. Thus there will be a general competition for these nutrients to grow and reproduce. Hence, the more the gut is flooded with beneficial microorganisms, the more competition is created between beneficial and pathogenic microorganisms
- Competition for adhesion sites
Adhering to adhesion sites along the wall of the gut is an important colonisation factor and many intestinal pathogens rely on adhesion to the gut wall to prevent them being swept away by peristaltic of food along the intestinal tract.
Probiotic microorganisms act like and add to the healthy microflora by producing enzymes which aid the breakdown of polysaccharides molecules and hence utilise more nutrients form the diet. The microflora also produces vitamins which supply a secondary source to the host.
