We know that probiotics are live active cultures that when administered in adequate amounts confer a health benefit to the host. But what essentially makes a probiotic a probiotic? To be labeled as a probiotic, the product should be capable of surviving the irritant effects of both the stomach acid and bile to arrive at their destination in the colon where they exert their effects.
How do probiotics work?
The probiotics work through a number of mechanisms to strengthen our immune defenses and prevent us from sickness.
It is recognized for decades that the restoration of the balance in the microbial gut population is crucial for our health, and its disruption precipitates several GI and non-GI disease states. Depending upon the duration of their effects, probiotics can be classified as transient versus colonizing. Transient probiotics travel to the gut but are incapable of making a permanent home in the microbiome. They work as long as they are taken. In contrast, the colonizing probiotics (as the name implies) tend to colonize in the gut permanently; their effects last even after discontinuing them. Both kinds of probiotics stick to the gut wall to fend off the harmful invaders from adhering and exerting their deleterious effects. They block the growth of the bad pathogens and therefore, serve to boost the immune defenses both locally within and outside the gut.
Probiotics Induce the Production of Protective Mucin
The probiotic supplements stimulate the cells of the intestine to produce mucus that coats the intestine forming a protective barrier. This powerful gut barrier prevents the foreign attackers from invading. The favorable flora, primarily lactobacilli can compete for the binding sites of viruses (such as rotavirus) causing diarrhea in children.
The Probiotic Communicate with the Elements of the Immune System
Moreover, underneath the gut lining are elements of the immune system. The genetic material of colonizing probiotics has the ability to interact with these elements of the immune system. How is this accomplished? It is through the Toll-like receptors (TLRs), which are the key molecules of the immune system involved in identifying and guarding against the intruding bad guys – the so-called “bacterial-epithelial crosstalk.” Moving ahead, this cellular level crosstalk results in a series of reactions that leads to the formation of inflammatory signaling molecules called cytokines. These cytokines have several immunological functions including the movement of white blood cells towards the harmful pathogen that is then engulfed by these blood cells. Our gut immune defenses have a capacity to turn off this inflammatory response. In the absence of this turn-off mechanism, our gut becomes vulnerable to chronic GI ailments like inflammatory bowel disease. 
Colonizing Probiotics avert unnecessary Immunologic Reactions
The colonizing bacteria can induce oral tolerance through TLRs. Oral tolerance is the process by which the immune system does not respond to an innocuous agent that enters via the mouth. This, in turn, prevents excessive immunologic responses, including food allergies. 
Probiotics Release a variety of Anti-Infective Agents
Current experts suggest that probiotics themselves release bacteria-killing substances like hydrogen peroxide (H202) and bacteriocins. Bacteriocins are proteins that are toxic to other unfriendly bacterial strains but do not harm the friendly bacteria themselves. Hydrogen peroxide is a potent oxidizing agent that is particularly toxic to unfriendly microbes that lack catalase. Catalase is an enzyme that scavenges H2O2. Deficiency of this enzyme makes the bad bacteria susceptible to destruction. 
Written by: Dr. Rasheed Huma
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