The organisms used in probiotics are known to produce antimicrobial substances that might affect the colonic microflora balance. Mechanisms by which probiotics could improve health include [8]

While there is some evidence to indicate these processes occur in vitro and in animal models, the efficacy of probiotics for humans remains speculative because well-designed and controlled studies that yield more conclusive evidence with proper statistical analyses allowing claims to be incorporated into established nutritional pathways are often lacking. Notwithstanding a lack of clear-cut mechanisms, probiotics may provide health benefits under certain circumstances [9].

Probiotics are marketed as functional foods, whereby they are ingested for their purported positive advantages in the digestive tract and/or systemic areas like the liver, vagina or bloodstream. Consumers should be provided with an independent assessment of physiological, microbial and safety aspects of these live microbial products especially if they can improve health [10-12]. Probiotic trials should use the best methodologies available. For probiotics to exert beneficial properties, they must have a high viability in the product and have robust survival properties in the gut, which is their first point of contact. Moreover, they should not adversely affect immune up-regulation, produce toxins, disrupt colonocyte function or have the ability to transfer antibiotic resistance to the normal gut microflora. Food vehicles include live yoghurts, fermented dairy drinks, freeze-dried supplements (capsules, pills, liquid suspensions, sprays), cheese, fromage frais and fruit juices. Both single and multiple strain products are available [13].

An alternative, or additional, approach is the prebiotic concept. A prebiotic is ‘a non-digestible food ingredient that beneficially affects the host by selectively stimulating the growth and/or activity of one or a limited number of bacteria in the colon that can improve the host health. Thus, the prebiotic approach advocates the administration of non-viable entities. Dietary carbohydrates, such as fibres, are candidate prebiotics, but most promise has been realized with non-digestible oligosaccharides, because of their selective metabolism. In particular, the ingestion of fructo-oligosaccharides (FOS) has been shown to stimulate bifidobacteria in the lower gut. As prebiotics exploit non-viable food ingredients, their applicability in diets is wide ranging. A further approach is synbiotics, where probiotics and prebiotics are combined. The prebiotic activity of fructose-containing oligosaccharides has been confirmed in both laboratory and human trials [14].

Combined mixtures of probiotics and prebiotics are often used because their synergic effects are conferred onto food products. For this reason, such mixtures are called synbiotics. The synbiotic effect may be directed towards two different ‘‘target regions’’ of the GIT: i.e. both the small and the large intestines. In addition, if the prebiotic carbohydrate is utilised by a probiotic strain, its growth and proliferation in the gut will be selectively promoted.

Probiotics can be bacteria, moulds, yeast. But most probiotics are bacteria. Among bacteria, lactic acid bacteria are more popular. Lactobacillus acidophilus, L. casei, L. lactis, L. helviticus, L. salivarius, L. plantrum, L. bulgaricus, L.rhamnosus, L. johnsonii, L. reuteri, L. fermentum, L. del brueckii, Streptococcus thermophilus, Enterococcus faecium, E. faecalis, Bifidobacterium bifidum, B. breve, B.longum and Saccharomyces boulardii are commonly used bacterial probiotics. A probiotic may be made out of a single bacterial strain or it may be a consortium as well. Probiotics can be in powder form, liquid form, gel, paste, granules or available in the form of capsules, sachets [15].

Foods such as cheese, yogurt, sour cream, smoothies, cereal, fitness bars, infant formulas, asparagus, soybeans contain probiotics. Probiotics are used in general health, animal agriculture, fertility of soil and oral cavity.

A good probiotic agent needs to be non-pathogenic, nontoxic, resistant to gastric acid, adhere to gut epithelial tissue and produce antibacterial substances. It should persist, albeit for short periods in the gastro-intestinal tract influencing metabolic activities like cholesterol assimilation, lactose activity and vitamin production. The survival of probiotic organisms in the gut depends on the colonization factors that they possess, organelles which enable them to resist the antibacterial mechanisms that operate in the gut. In addition to the antibacterial mechanisms, they need to avoid the effects of peristalsis, which tend to flush out bacteria with food. This can be achieved either by immobilizing themselves or by growing at a much faster rate than the rate of removal by peristalsis. The probiotic strain needs to be resistant to the bile acid, e.g. Bifidobacteria strains proved significantly less acid-resistant than the Lactobacillus strains, when exposed to human gastric juice [16].

Attributes of lactic acid bacteria such as anti-microbial agent production and competition with potential pathogens in the gut provided the impetus for investigating a role for probiotics in diarrhoea. Acute gastroenteritis diarrhoea in infancy is normally due to rotavirus and a number of trials have shown a reduction in the duration and incidence of diarrhoea in response to probiotics.

The potent anti-inflammatory effects of probiotics have clearly emphasized how the therapeutic potential of these agents may extend beyond their ability to displace other organisms and has led to their evaluation in inflammatory bowel disease both a lactobacillus and a bifidobacterium produced a marked and parallel reduction in inflammation in the colon and cecum and in the production of the pro-inflammatory cytokines IFN- Ɣ, TNF-α, and IL-12, while levels of the anti-inflammatory cytokine TGF-β were maintained. Similar effects have been demonstrated for the probiotic cocktail VSL#3 (containing Bifidobacterium breve, Bifidobacterium longum, Bifidobacterium infantis, Lactobacillus acidophilus, Lactobacillus plantarum, Lactobacillus paracasei, Lactobacillus bulgaricus, Streptococcus thermophilus) [17].

Lactose intolerance is a physiological state in human beings where they lack the ability to produce an enzyme named lactase or B-galactosidase. This lactase is essential to assimilate the disaccharide in milk and needs to be split into glucose and galactose. Individuals lacking lactase will not be able to digest milk and it often poses a problem in newborn infants. People with lactose intolerance problem express abdominal discomfort, diarrhoea, cramps, flatulence, nausea, vomiting, etc. Another problem associated with lactose intolerance is calcium deficiency. Calcium malabsorption may be due to deletion of diets with milk to avoid the complications of lactose intolerance. Yogurt contains less lactose than milk and delays gastric emptying, which partly explains why lactose-intolerant individuals tolerate yogurt. However, yogurt tolerance is mainly due to the supply of lactase activity from the lactic acid bacteria present in the yogurt itself. Evidence shows that bacteria must be live and present in sufficient quantity to be of benefit; yogurts containing 10⁸bacteria/ml are required. Calcium absorption is better and more in acidic conditions; hence, if lactose is converted to lactic acid, pH of the gut decreases, i.e. it becomes acidic favouring enhanced absorption of calcium. So, if probiotics are fed to lactose intolerance patients, then milk lactose is hydrolysed by probiotic strains and lactose is assimilated and calcium absorption is also favoured [18].

Colon cancer is a multi-factorial and complex neoplasm involving both genetics and environmental factors. There seems to be a strong relationship between colon cancer, diet and intestinal microflora. Probiotic modulation of the intestinal microflora may affect the activity of one of the enzymes (7a-dehydroxylase) forming these toxic products but probiotics may also reduce the toxicity of bile salts binding to them. While growing in the colon, probiotics seem to exert different functions such as controlling the growth of potentially harmful bacteria, binding to mutagens, preventing harmful enzyme activity in the gut (b-glucoronidase, nitroreductase), interacting with the cells of the colon, forming conjugated linoleic acid, a compound with anti-inflammatory properties that may inhibiting the development of cancer. The dietary administration of B. longum (1x10¹⁰ live bacterial cells/d) completely suppressed colon tumours induced by the compound 2-amino, 3-methyl, 3-imidazol (4, 5-f) quinoline, which is a carcinogen found in human diet [19]. The changes in gut bacterial enzymes that generate carcinogens and tumour promoters such as NH₃ and secondary bile acids, stimulation of immune surveillance, suppression of inflammatory processes, binding of carcinogens in the gut will have various levels of scientific support.

Constipation is common especially in elderly people. An increase in the number of bowel movements or a decrease in transit time has been reported in controlled studies that employed probiotics for treating constipation. The widely used laxative lactulose is a prebiotic, as it is not attacked by human disaccharidases and is substrate for the bifidobacteria in the colonic flora, that catabolise it to smaller molecules, creating an osmotic effect [15].

Probiotic strains, especially lactic acid bacteria have a major role to play in the cholesterol lowering mechanism. As the cholesterol level keeps increasing in the serum, it leads to cardiac diseases. Probiotic strains assimilate the cholesterol for their own metabolism. Probiotic strains can get bound to the cholesterol molecule, and they are capable of degrading cholesterol to its catabolic products. The cholesterol level can be reduced indirectly by deconjugating the cholesterol to bile acids, thereby reducing the total body pool. The bile acids commonly occur in the form of bile salts with glycine and taurine [20].

Probiotics have been shown to reduce the incidence of childhood eczema by half, compared to placebo, when administered during pregnancy and up to 6 months postnatally. Probiotics may exert a beneficial effect on allergic reaction by improving mucosal barrier function. In addition, probiotics consumption by young children may beneficially affect immune system development. Probiotics such as Lactobacillus GG may be helpful in alleviating some of the symptoms of food allergies such as those associated with milk protein. Probiotics consumption may thus be a means for primary prevention of allergy in susceptible individuals [12].

Good nutrition during pregnancy improves the chances of having a healthy baby who will be at lower risk of diseases later in life. Bacterial vaginosis, has been suggested as a factor that increases risk of preterm labour and infant mortality and probiotics been shown to decrease risk of bacterial vaginosis and maintain normal lactobacilli vaginal flora. In animal studies, these strains were found to be safe during pregnancy and to enhance the health of mothers and newborns [21]. Another area of interest in the use of probiotics in pregnancy is to prevent allergic reactions. Studies using L rhamnosus GG and B lactis BB12 have shown that atopic dermatitis, a condition that causes severe skin rashes in up to 15% of babies, can be prevented in 50% of cases if mothers ingest probiotics during pregnancy and newborns ingest them during the first 6 months of life. Probiotics during pregnancy also have an excellent safety record.

Probiotic formulations have been widely studied for the treatment of atopic dermatitis (AD; a type of eczema), a pathology defined as an inflammatory, chronically relapsing, non-contagious and pruritic skin disease which is associated with elevated IgE levels and Th2 responses. AD in animal models and human studies has been investigated using different probiotic different strains such as Bifidobacterium, Lactobacillus, and Lactococcus [18].

A desirable ability for a probiotic is to exert competitive exclusion. A higher affinity for binding sites would allow them to exclude attached pathogens that could subsequently be eliminated through co-aggregation. On the other hand, strong adherence to host cells would allow the probiotic to occupy any free space on the epithelial surface. Thus, they could slow down the multiplication of the pathogens by competing with them for space and nutrients. In the case of URTIs, most clinical trials are destined to prevent disease in the healthy population or to prevent recurrence in disease-prone patients [22].

The prevention or resolution of bacterial vaginosis is particularly important in women at risk of human immunodeficiency virus (HIV) infection. Studies have shown that women with bacterial vaginosis (no lactobacilli) are at significantly increased risk of HIV. Thus treatment of bacterial vaginosis and promotion of vaginal lactobacilli may reduce a woman's risk of acquiring HIV-1, gonorrhea and trichomoniasis. Human vaginal probiotic strain (Lactobacillus reuteri RC-14) can express potent functional viral inhibitors which may potentially lower the sexual transmission of HIV [14].

Bacterial vaginosis, yeast vaginitis and recurrent urinary tract infections (UTIs) are common urogenital problems. The normal vaginal flora in premenopausal women consists primarily of lactobacilli, which are protective against infection, but many pathophysiologic factors can cause unstable vaginal flora that may result in infection. There are only few studies with the use of probiotics in vaginal infections. The overwhelming experiences have revealed a positive effect of L. acidophilus. Evidence from the available studies also suggests that probiotics can be beneficial for preventing recurrent UTIs in women.

Postoperative complications in GI surgery could involve bacteremia and infectious complications. The main causes could be the translocation of the GI bacteria or its toxins as a consequence of bacterial overgrowth, the loss of intestinal epithelial integrity and the immunologic compromising of the host. Probiotics could be good candidate to fight against these factors through the competition with potential pathogens for nutrients or enterocyte adhesion sites, degradation of toxins, production of antimicrobial factors, and local and systemic immunomodulation [6].

Some preliminary evidence suggests that food productsderived from probiotics bacteria could possibly contribute toblood pressure control. This antihypertensive effect hasbeen documented with studies in spontaneous hypertensiverats. Two tripeptides, valine- proline-proline andisoleucine-proline- proline, isolated from fermentation ofa milk-based medium by Saccharomyces cereviseae and Lactobacillus helveticus have been identified as the activecomponents. These tripeptides function as angiotensin-I-converting enzyme inhibitors and reduce blood pressure [11].

Probiotics for periodontal therapy have not been extensively studied. Clinical studies where probiotic species have been investigated specifically from a periodontal disease perspective are sparse. Lactobacillus reuteri and Lactobacillus brevi are among the species able to affect gingivitis and periodontitis [18].

Adding the prebiotic inulin to yoghurt boosted the growth of probiotic bacteria and, when used in a novel double-microencapsulation, extended the survival rates of the friendly bacteria. The various prebiotic fibres protect the stability and viability of probiotic Lactobacillus rhamnosus strains during freeze-drying, storage in freeze-dried form and after formulation into apple juice and chocolate-coated breakfast cereals. The prebiotics studied were: sorbitol, mannitol, lactulose, xylitol, inulin, fructooligosaccharide FOS and raffinose.

Alginate-coated gelatin microspheres were produced to encapsulate the probiotic Bifidobacterium adolescentis 15703T for enhancing survival during exposure to the adverse conditions of the gastro-intestinal tract. Gelatin microspheres were cross-linked with the non-cytotoxic genipin and coated with alginate cross-linked by Ca2+ from external or internal sources [23].

A novel encapsulation method for probiotics, which excludes the use of organic solvents. The efficiency/potential of this new method increases stability of sensitive probiotic cultures, specifically Bifidobacteria. The potential of interpolymer complex formation in scCO2 for the encapsulation of sensitive probiotic cultures was demonstrated for the first time [24].