Dr. Jehan Abdul Sattar Probiotics

Dr. Jehan Abdul Sattar Probiotics

I. INTRODUCTION

In Western countries, the beneficial role of fermented milk (yogurt) in the prolongation of life was first advocated by Metchnikoff in 1907. He suggested that the bacteria and their metabolites in yogurt neutralized the harmful products yielded from foods in the GI tract and provided protection to human health. This group of bacteria was thought to reduce production of toxic compounds that adversely affect the human body, thus enabling humans to live longer . Research done since Metchnikoff’s period has shown that Lb. delbrueckii subsp. bulgaricus neither survives nor establishes itself in the gastrointestinal tract. However, other species of lactobacilli have been reported to provide some beneficial effects through growth and action in the gastrointestinaltract. This group of bacteria and others are now often referred to asprobiotics. Although there are other possibilities, cultures most often mentioned as probiotics for humans include Lb. acidophilus, Lb. casei, and Bifidobacteriumspecies. Thesespecies along with Propionibacterium species and Lb. reuteri are the ones most often considered for use as probiotics for livestock. All these species can survive and grow in the intestinal tract, and thus have the potential to provide benefits. Certain yeast cultures also are considered as being probiotics for livestock even though the yeasts are not expected to survive and grow in the gastrointestinal tract.

Bacteria normally used as starter cultures for some fermented milk products, such as Lb. delbrueckii subsp. bulgaricus and Streptococcus thermophilusused to manufacture yogurt, also may provide benefits, but not through the ability to survive and grow in the intestinal tract. Benefits they provide come primarilyfrom serving as a source of enzymes needed to improve digestion of nutrientin the gut. For example, β-galactosidase is needed for hydrolysis of lactose in the small intestine.

Whereas some reports indicate that the nutritional value of milk can beimproved by certain fermentations . Several benefits are possible from such microorganisms, including control of intestinal infections, control of serum cholesterol levels, beneficial influences on the immune system, improvement of lactose utilization in persons who are classified as being lactose maldigestors, and anticarcinogenic action.

The term probiotics was initially used with no specific definition. In 1989, Fullerdefined the term as products containing living microorganisms, which on ingestion(by humans, animals, and birds) in certain numbers exert health benefits beyondinherent general nutrition. In this statement, several requirements were included,such as the microbial cells should be alive and consumed in high numbers (usually109 cells/d). But it did not include which microorganisms (only intestinal or others)should be used and consumed for how long (daily as a preventative or for 2 or moreweeks as a therapeutical. Also Probiotics have been defined as ‘a preparation of or a product containing viable, defined micro-organisms in sufficient numbers, which alter the microbiota (by implantation or colonisation) in a compartment of the host and by that exert beneficial health effects in this host’. It is currently accepted the probiotic definition formulated in 2001 by FAO/WHO “Live microorganisms which when administered in adequate amount confera health benefit to the host”.

-Prebioticis ‘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’.

-Synbioticis a synergistic combination of a probiotic and prebiotic. Synbiotic supplements not only given as feed supplements or pharmaceuticals but increasingly in suitable food specimens such as dairy products, fruit juices and chocolates. The synbiotic concept has been widely used by European dairy drink and yoghurt manufacturers .Administration of synbiotics as a food supplement is safe, simple, and convenient.

Probiotics, prebiotics, and synbiotics that may be suitable for human consumption are listed below:

-Postbiotics refers to the metabolic by products like enzymes, peptides, teichoic acid, peptidoglycan, exopolysaccharides, cell surface and secreted proteins, bacteriocins and organic aids generated by a probiotic organism during its lifespan. Postbiotics have advantage due to their clear chemical structure, safety dose parameters and longer shelf life which can influence physiological function of host . Postbiotic substances contain various metabolites and signaling molecules which display broad antibacterial spectrum and immunomodulatory actions.

-Biogenics

The health benefit from the consumption of fermented dairy products is attributedto the lactic acid bacteria used in fermentation as well as the by-products of metabolismof milk nutrients by them. The latter aspect becomes important to explainhealth benefits attributed to fermented products in which nongut bacteria, such asLactococcus lactis, Str. thermophilus, and Lb. delbrueckii ssp. bulgaricus, are used in fermentation. These products have recently been termed biogenics and includecomponents in food that are derived through the microbial metabolic activity of foodnutrients and have health benefits. One such group is the peptide produced by theexoproteinases of some lactic acid bacteria, such as Lc. lactis in buttermilk andLb. delbrueckii ssp. bulgaricus in yogurt. Some of the peptides thus produced andpresent in the fermented milk can reduce blood pressure in individuals with hypertension.

MICROBIOLOGY OF THE HUMAN GI TRACT

The GI tract of humans contains more than 1014 microorganisms.It is estimated that the human GI tract harbors 1000 bacterial species, but only 30 to 40 species constitute 95% of the population. Normally, the microbial level in the small intestine (particularly in the jejunum and ilium) is 106–7/g, and in the large intestine (colon) 109–10/g of the content. . Bacterial numbers fall dramatically to <103 colony forming units (cfu)/ mL of gastric contents as they encounter the stomach, which provides a highly effective barrier against invading micro-organisms, both pathogenic and benign.

The most predominant types in the small intestine are several species of Lactobacillus and Enterococcus, and in the large intestine are several genera of Enterobacteriaceae, different species of Bacteroides, Fusobacterium,Clostridium, Eubacterium, Enterococcus, Bifidobacterium, and Lactobacillus.

The intestine of a fetus in the uterus is sterile. At birth, it is inoculated with vaginal and fecal flora from the mother. Subsequently, a large variety of microorganisms enter in the digestive tract of infants from the environment. From these, the normal flora of the GI tract are established. In both breast-fed and formula-fed babies during the first couple of days, Escherichia coli and Enterococcus appear in large numbers in the feces. Then, in the breast-fed babies, large numbers of Bifidobacteriumspecies and a lower level of both E. coli and Enterococcus species appear. In formula-fed babies, in contrast, E. coli and Enterococcus, together withClostridium and Bacteroides, predominate, with Bifidobacterium being almost absent; this situation may lead to diarrhea. As breast-fed babies are introduced to other foods, the levels of E. coli, Enterococcus, Bacteroides, Clostridium, and others increase, but Bifidobacterium still remains high. When breast-feedingis completely stopped, Bacteroides, Bifidobacterium, and Lactobacillus species predominate, along with some E. coli, Enterococcus, Clostridium, and others. By the second year of life, the different microflora establish themselves at their specific ecological niche in the GI tract, and the population resembles that of adult GI tracts. The intestinal microflora are divided into indigenous and transient types. Many indigenous species can adhere to intestinal cells, which helps maintain them in their specific environment. Whereas the indigenous types are permanent inhabitants, the transient types are either passing through or temporarily colonizing a site from where the specific indigenous type has been removed becauseof some inherent or environmental factors (such as antibiotic intake). Among the indigenous microbial flora, several species of Lactobacillus in the jejunum and ilium, and Bifidobacterium in the large intestine, are thought to have beneficial effects on the health of the GI tracts of the hosts. From the intestines and intestinal content of humans, Lactobacillus acidophilus, Lb. fermentum, Lb. rhamonosus,Lb. reuteri, Lb. casei, Lb. lactis, Lb leichmannii, Lb. plantarum, Bifidobacteriumbifidus, Bif. longum, Bif. adolescentis, Bif. infantis, and others have been isolated. However, age, food habits, and health conditions greatly influence the speciesand their levels. There is some belief that a portion of the intestinal Lactobacillusspecies is transient. Initially, it was considered that Lb. acidophilus, Lb. reuteri, and some Bifidobacterium are the main indigenous species. At present, several other Lactobacillus

species, such as Lb. casei and Lb. rhamnosus, have been tested to be beneficial. The presence of high numbers of the indigenous Lactobacillus species in the feces (and content of the large intestine) probably results from their constant removal from the small intestine.

The prevalence of bifidobacteriain breast-fed infants is thought to confer protection by reducing the gut pH, which inducesa concomitant reduction in other potentially harmful species. Moreover, bifidobacteria areable to exert directly antagonistic activities against gut pathogens. Newborns are susceptibleto intestinal infections and atopic diseases as their immune system and GI tract develop.

Mode of delivery and diet therefore have important implications, both at birth and later inlife, as initial colonisation is involved in the development of the GI tract, its microbiota andin maturation of the immune system. During the first few years of life and upon weaning,the infant microbiota normalises. This composition will remain stable throughout most of adult life .

There is a high degree of variability between the stomach, small intestine and colon interms of numbers and bacterial population types. This is due predominantly to differenttransit times, secretions and nutrient availability.

Micro-organisms are also determinants as they interact with and influence their surroundingsto ensure their survival against competitors. This is achieved through several mechanisms,such as increasing anaerobicity or through the production of compoundssuch as acid or antimicrobial substances. These compounds concurrently affect the host andcan thereby have advantageous or devastating implications.

The rapid transit time, low pH and presence of bile associated with the small intestine donot provide an environment that encourages growth of bacterial populations. The duodenumalso has low microbial populations due to its short transit time and the secretion of intestinalfluids, which create a hostile environment . But the large gut is favourable for bacterial growth with a slow transit time, ready availability of nutrients and favourable pH. The small intestineharbours enterococci, enterobacteria, lactobacilli, bacteroides and clostridia. Organic acids produced from fermentation result in a lower pH of 5.5–6.0than the more neutral pH found in the colon. Transit in the colon is slower andnutrient availability is minimised, producing slower-growing populations that tend towardsmore proteolytic fermentations.

Yeasts, including the opportunisticpathogen Candida albicans, are also present in the gut microbiota, although in healthyindividuals counts do not exceed 104 cfu/ g faeces . The vast majority (>90%) of the total cells in the body are present as

Functions of the gastrointestinal microbiota

The GI tract, along with its microbiota, is one of the most metabolically active organs in thehuman body. The intestinal microbiota is involved in the fermentation of endogenous andexogenous microbial growth substrates. The metabolic end-products of carbohydrate fermentationare benign, or even advantageous to human health .

Major substrates available for the colonic fermentation are starches that for various reasonsare resistant to the action of pancreatic amylases, can be degraded by bacterial enzymes, aswell as dietary fibres such as pectins and xylans. Other carbohydrate sources available forfermentation in lower concentrations include oligosaccharides and a variety of sugars andnon-absorbable sugar alcohols. Saccharolysis results in the production of short-chain fattyacids (SCFA) such as butyrate, acetate, propionate and lactate which contribute towardsenergy metabolism of the large intestinal mucosa and colonic cell growth; they can also bemetabolised by host tissues such as the liver, muscle and brain. The production of SCFAresults in a lower pH that can protect against invading micro-organisms andalso reduces the transformation of primary bile acids into secondary pro-carcinogenic bileacids . This is believed to be one of the mechanisms utilisedby the beneficial gut microbiota that results in protection for the host.Proteins and amino acids can be effective growth substrates for colonic bacteria, whilstbacterial secretions, lysis products, epithelial cells and mucins may also make acontribution. However, the diet provides, by far, the predominant source of nutrients, witharound 70–100 g/d of dietary residues being available to the colonic microbiota. Thesematerials are degraded by a wide range of bacterial polysaccharidases, glycosidases, proteasesand amino-peptidases to smaller oligomers and their component sugars and aminoacids.

The indigenous gutmicrobiota is better adapted to compete for nutrients and attachment sites than the incomingmicro-organism, which it may also inhibit through the production of compounds .

Another important function of the gut microbiota is the production of vitamins B and K. Several studies have indicated that beneficial effects of these bacteria areproduced when they are present in relatively high numbers in the intestinal tract(106–7/g intestinal content). Diets rich in foods from plant sources, as opposedto those rich in foods from animal sources, seem to favor their presence in higher numbers. Many other conditions in a host also can reduce bacterial numbers in the GI tract, such as antibiotic intake, mental stress,starvation, improper dietary habits, alcohol abuse, and sickness and surgery of the GI tract. This, in turn, can allow the undesirable indigenous or transient bacteria to grow to high levels and produce enteric disturbances,including diarrhea and infection by enteric pathogen.

By changing the species composition of the GIT bacteria using probioticsand prebiotics it is possible to beneficially modulate enteric immune functions,thereby improving resistance to GIT pathogens and other health challenge.

Microorganisms considered as probiotics

The majority of probiotic microorganisms belong to the genera Lactobacillus and Bifidobacterium . There are also other genera ofBacteriastreptococci,enterococci, lactococci and some yeasts such as Saccharomyces spp. widely used and reported below:

IMPORTANT CHARACTERISTICS OF PROBIOTIC BACTERIA

Respecting the “Guidelines on probiotics characteristics can be summarized as follows:

• Must not lose its properties during storage.

• Must be normally present in the human intestine.

• Must be able to survive, to overcome the gastric barrier, resisting to the action of digestive gastric juice, intestinal enzymes and bile salts and colonize the intestine: for this reason, the minimum effective dose, which is very indicative because it depends on the strain and preparation used, is 107 CFU/day.

• Must be able to adhere to and to colonize the intestinal cells: the bacterial membrane structure is involved in the mechanism of adhesion and direct switch with the mucosa, the surface proteins and possibly also the secreted ones. Beneficial strains differ in adherence ability and specificity . An adherent strain should probably be favored over a nonadherent strain. Also, strains adherent to humans should be preferred over strains adherent to other species. The selected strains should have a strong adherence property.

• Must exert metabolic functions at the enteric level, with beneficial effects for human health, and antagonism against pathogenic microorganisms by producing antimicrobial substances.

• Should not cause immune or otherwise harmful reactions and then be considered as safe (GRAS status: generally recognized as safe).

• Resistance to antibiotics must be intrinsic or due to genetic mutations, whereas if it is caused by a horizontal gene transfer (i.e. transposons, genomic DNA segments that breaks off to join another, conjugative plasmids carrying genes for resistance, virulent or temperate phages) his choice becomes more problematic;

• Must also be administered in adequate doses and have a favourable cost-efficacy ratio.

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