Probiotics are most commonly definined as "live microbial food supplement that affects beneficially the host animal by improving its intestinal microbial balance." This definition emphasizes the importance of living cells as essential components of probiotics.
The word “probiosis” comes from Greek: pro and BIOSIS (life), and therefore is opposed to antibiosis, as probiotics promote the proliferation of bacterial species within the gastrointestinal tract. Probiosis is defined as "the ability of the normal flora of the adult to resist the excessive growth and establishment of foreign components" which is enhanced or restored by probiotics. The concept of probiotics applied to preventive medicine emerged from the research of Dr. Metchnikoff. He postulated that the longevity observed in the Balkan people was due to regular consumption of fermented milk containing Lactobacillus bulgaricus.
The intestine of newborn animals is relatively sterile and therefore, is deficient in micro-organisms that provide resistance to disease. The intervention of a probiotic supplement provides the intestinal microflora.
It has been observed that probiotics act by the following mechanisms:
- Competition for nutrients:
Within the intestine, beneficial microorganisms and pathogens use the same types of nutrients. Thus there is a general competition for these nutrients to grow and reproduce. Therefore, the more you flood the gut with beneficial microorganisms, more competition is created between beneficial microorganisms and pathogens.
- Competition for adhesion sites:
The joining of adhesion sites along the intestinal wall is important for intestinal colonization and many pathogens rely on adherence to the intestinal wall to prevent being swept out by the peristaltic movements along the intestinal tract.
- Stimulating the immune system:
Stimulation of antibody production.
- Increased macrophagic activity:
Increased levels of interferon gamma.
- Direct antimicrobial effect:
This occurs via the bacteriocins which are produced by many species of lactic acid bacteria or by producing organic acids that can have a direct effect or operate by reducing the pH.
Probiotic microorganisms act as complements to the healthy microflora by producing enzymes that help the breakdown of the polysaccharide molecules and thus make better use of nutrients from aliments. The microflora also produces vitamins, thus providing the host with a secondary source of vitamins.