A probiotic is a live microorganism that offers considerable health benefits to the host when administered in sufficient quantities. Fermented milk which has been around since ancient times is possibly the first ever known probiotic. Otherwise a probiotic is a beneficial bacterium in gut microflora, but to be classified as such, it must be of human origin and should be suitable for human use as well.
One of the key characteristics of a probiotic is that it should be stable in acid and bile and must be compatible with intestinal mucosa. Some examples are, bacteria especially of the lactic acid strain (Lactobacillus acidophilus, L. casei, L. salivarius, L. johnsonii, L. fermentum--to name a few) Bifidobacterium bifidum, Saccharomyces boulardii, Streptococcus thermophilus,and Enterococcus faecium. In short a probiotic could be a bacterium, mold, or yeast---it could either be a single strain or a combination of different strains.

The popular book "The Prolongation of Life" by Metchnikoff probably generated much public interest in probiotics ---especially regarding the beneficial effects of fermented milk. This book expresses the view that people with regular intake of fermented milk are believed to have lived longer lives---especially as it could reverse the effects of autointoxication (self-poisoning due to reabsorption of metabolite wastes in the body- also called endogenic toxicosis) which was the cause of quick aging. The Turkish man Zaro Agha who went on to live for 162 years supposedly ate large quantities of yogurt throughout his life. Therefore lactic acid bacteria is reckoned as an important factor in both human and animal health.

Generally probiotics are used for:

1) Maintaining normal gut flora

When infection or disease occurs the gut microflora gets altered, in that, the growth of harmful pathogenic bacteria is predominant and the healthy nexus between host and microbe is lost. For example if probiotics like Lactobacillus acidophilus and Bifidobacterium longum are ingested it could augment the phagocytic function of human microphages

2) To counter microfloral imbalance (or what is called dysbiosis) especially when undergoing antibiotic therapy.

Sometimes when a person takes antibiotics for considerable length of time it destroys gastrointestinal flora. Probiotics that contains live non-pathogenic bacteria or yeast can be used as a dietary supplement to make the gastrointestinal flora normal. Therefore it follows that probiotics when used alongside antibiotics are helpful in dispelling some of the bad effects of antibiotics.
Even the therapeutic efficacy of some drugs can be increased and their side effects decreased if probiotics are administered alongside. For example researchers have confirmed that probiotics (L. acidophilus) when given alongside amoxicillin or clarithromycin had a better rate of success in derailing infections than when the antibiotics were given alone.

Often gastric mucosa could get inflamed or gastric barrier function could go haywire. In both instances probiotics could be helpful.

Probiotics have several other health benefits too. Here are a few of these applications.

Anti-mutagenic effect

Mutagenecity means the ability of certain substances to induce genetic mutation---which could prove harmful. Probiotics could exert potent Anti-mutagenic effect. For example, Lactobacilli strains in milk (milk cultured with these strains) could reduce the incidence of mutagenecity by binding with harmful chemicals and carcinogens in the gastric juice.

Lowering of serum cholesterol

Probiotic bacteria can lower serum cholesterol levels. Certain bacteria found in the gut may break down the bile acids that are secreted into the small intestine. This inhibits re-absorption of bile salts which in turn leads to reduction of liver cholesterol (liver needs cholesterol to make bile acids). Supposing the gut were more colonized with Lactobacillus acidophilus (that has greater propensity to break down bile acids) then obviously serum cholesterol levels could be lowered because there would be more inhibition of re-absorption of bile salts, less of cholesterol in the liver (as most of it would be used to make bile acids) and consequently less of cholesterol in the serum. For example it has been found that yoghurt fermented with L. acidophilus could reduce LDL cholesterol by 4.5%. But apart from theoretical paradigms it is not clear what level of probiotic dose is appropriate for lowering cholesterol and to what extent hypercholesterolemic patients could benefit from probiotic therapy.


Anti-hypertensive action

Hypertensive patients could benefit from consuming fermented dairy foods like fermented milk along with other foods that could possibly lower BP. But probiotics alone cannot significantly reduce BP on a long term basis.

Benefits to Immune system

The immune system produces Immunoglobulin A an antibody, which protects harmful microbes from binding and penetrating the gut wall. Yogurt and probiotics like Lactobacillus casei are capable of increasing sIgA levels by helping to produce more of IgA-producing plasma cells. For instance, people with regular intake of probiotic milk for up to six weeks had immune cells with better phagocytic capacity.

Antioxidant effects

Normal body metabolism produces free radicals especially the free radicals from oxygen. If these free radicals are not neutralized quickly enough, they can cause death of cells through oxidation of enzymes, proteins and lipids. Free radical induced cellular damage can lead to cancer, heart disease and other serious illnesses. Bacteria like Lactobacillus delbrueckii ssp. bulgaricus and Streptococcus thermophilus some found in yoghurt can successfully entrap reactive forms of oxygen (hydrogen peroxide and hydroxyl radical). Researchers are working on milk bacteria to be used as an antioxidant food supplement because milk bacteria can eliminate oxygen free radicals, and also in view of the fact that some lactobacilli have antioxidant effect in GI tract.


H.Pylori infection and gastritis

People with Helicobacter pylori bacterium (H. Pylori) infection could have peptic ulcer and/or bouts of gastritis together with melena. Potentially this could be a life threatening illness for the elderly, especially as internal gut hemmorrage could at times lead to sudden drop in systolic pressure. Probiotics with lactic acid bacteria (example isolated from yogurt) can be used to block the growth of h.pylori and the complications it could cause.
Swiss researchers have found that L. johnsonii is beneficial in combating gastritis due to H.pylori even if it were to be used alone and not in conjunction with other conventional medicines like omeprazole or ranitidine. Some researchers believe that h.pylori cannot be totally destroyed by probiotics alone, but in any case can help keep h.pylori levels lower

Diarrhea and Allergies

Diarrhoea which is a rotavirus infection is common in children especially under age five and for this category if probiotics like Lactobacillus GG, L. acidophilus or L. acidophilus combined with L. bulgaris are given it could be helpful in that, both the severity and duration of diarrhea are curbed. Antibiotic induced diarrhea can also be taken care of.

Often it has been noticed that the incidence of allergy is accompanied by changes in gut microflora. For example, just before an allergy the lactobacilli numbers are found to decrease. Probiotics if given can improve mucosal barrier function and diminish the intensity of allergic effects. For example food allergies linked to milk protein can be reduced by intake of Lactobacillus GG. For people susceptible to allergy, probiotics could be a key factor and in the case of young children it can help in the development of immune system.


Conclusion

At present there are several functional foods available that are in combination with probiotics like for example yogurts, beverages, and breakfast cereals. However more documentation needs to be done as regards the medicinal effect of probiotics such as lowering of cholesterol and BP. Moreover the dose and strains that could give optimal health benefits also needs to be researched further.

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