Introduction
Probiotics
Large number of micro-organisms live on various sections of human body (skin, mouth and gastrointestinal tracts) that are exposed to the outside environment. These are known as commensal microbes have associated with humans. They are greatly interdependent with them.
The highest concentration of commensal organisms believed to be in the gastrointestinal (GI) tract that has above 400m2 surface area (this makes up the second largest surface area of the body).
The GI tract can harbour high extent of flora (>500 various bacterial species) among which some are beneficial, e.g. stimulating and supporting the host from invading bacteria and viruses and assisting digestion.
Tannock (1998) states that germ free animals are more sensitive to infection in comparison with their conventional counterparts. This can be in relation to a low immune system and probably the lack of competitive colonization as Van dev Waaij et al. (1972) suggested.
Competitive colonization refers to interference of virulence by attacking pathogens by commensal microbes.
A research about germ free animals has shown that microbial colonization has essential (health) roles for humans. However microbes may occasionally develop a pathogenic relationship with a host that can result in occurring disease or even death of the host.
Microbial metabolites may have mutagenic or carcinogenic activity. As a result cancer can develop during the long period of exposure. Therefore understanding the influence of colonizing microbes in addition to learning desired factors to encourage the positive and inhibit the negative activities of commensal attacking microbes.
The probiotic abstraction has been an important issue. They are known as live micro-organisms involving a health influence on the host when consumed in adequate amount (Guarner and Schaafsma 1998).
Prebiotics are the foods for the beneficial bacteria. They can be added to the diet to help the beneficial bacteria grow and survive the digestive mechanism.
Influences involved in human health
Lactose tolerance
Many people are not capable to digest and absorb lactose in the body. The lactose are metabolised by bacteria and the consequence by-products may cause abdominal pain, bloating, diarrhoea and nausea.
The influence of lactose maldigestion determined by measuring breath hydrogen excretion ( Levitt and Donaldson 1970) which is related to colonic fermentation and lactose maldigestion.
The involvement of lactase by bacterial cultures used to produce the yoghurt is believed to improve the lactose digestion. This can be confirmed by pasteurised yoghurt or yoghurt with low cell count which is not able to lower breath hydrogen excretion (although pasteurised yoghurt is able to develop gastrointestinal symptoms as Savaiano et al. (1984) reported).
The general outcomes have implied that yoghurt starter culture (Streptococcus thermophilus and lactobacillus delbrueckiisubsp. Bulgaricus) present in yoghurt (at the extent of >108 / g) can enhance the digestion of lactose in lactose maldigesers.
Cholestrol
Certain amount of blood lipids is considered to be hazardous in terms of cardiovascular disease. Eyssen (1923) reported that conventional animals excrete higher levels of cholesterol in faeces than germ-free animals which indicates that colonizing microbes may affect the extent of serum cholesterol.
There have been numerous studies regarding the analysis of the blood lipids in human consuming fermented milk products. Statistically considerable lowering of total cholesterol occurred ranging from 5.4 to 23.2 % and LDL cholesterol shown to reduce from 9.0 to 9.8 %.
One of the possible mechanism (of probiotic bacteria) affecting the serum cholesterol reduction is said to be on the basis of the capability of certain probiotic lactobacillus and bifidobacteria to deconjugate bile acids enzymatically, resulting in increase of their excretion rates (Desmet et al. 1994).
Cholesterol precedes bile salt; it can facilitate the reduction in serum cholesterol as cholesterol molecules are converted to bile acids to replace those lost.
Action of this particular mechanism to control the serum cholesterol rates may result in deconjugated bile acids to secondary bile acids by colonic microbes.
The secondary bile acids are said to develop cancer. A possible increased destruction of colon cancer can be greater than the effect of reducing serum cholesterol levels. Therefore, it is crucial to select those probiotic strains that are bile salt hydrolase negative.
Another mechanism specified by Mann (1977), was that 3-hydroxy-3-methyl glutaric acid (HMG), occuring in fermented milk can discourage hydroxyl methyl glutaryl CoA reductase; the rate-limiting enzyme, in cholesterol biosynthesis.
Other
Probiotics may offer further advantages by affecting against Helicobacter pylori infections in the stomach (Coconnier et al. 1998, Kabir et al. 1997, Midolo et al. 1995, alcoholic liver disease (Nanji et al. 1994), small bowel bacteria over growth (Simenhoff et al. 1996, Stotzer et al. 1996), ulcerative colitis (Kruis et al. 1997), allergy to milk protein (Pelto et al. 2996), Juvenile chronic arthritis (Malin et al. 1996), antioxidative effects (Ahotupa et al. 1996), asthma (Wheeler et al. 1997), hepatic encephalopathy (Read et al. 1966) and their applications as vaccine delivery vehicles (Mercenier 1999).
The functions of probiotic cultures in the control of gastrointestinal health
Probiotics have shown to be functional in treatment / prevention of some certain gastrointestinal disorders among which the following can be included:
Antibiotic-included diarrhoeal disease
There have been studies in relation to evaluating the effectiveness of probiotics in antibiotic-included diarrhoea. It is impossible to predict which patient will face diarrhoea.
Various probiotics have been determined including Saccharomyces, Lctobacillus, Bifidobacterium and Streptococcus. Only S. boulardii, E. Faecium and Lactobacillus have shown to be clinically useful in prevention of antibiotic-contained diarrhoea.
In general, the patients who are prone to develop antibiotic-involved diarrhoea can advantage from prophylactic probiotic treatment.
Clustridium dificile-associated intestinal disease
This is one of the opportunistic build-up of an intestinal disease after decomposing of colonization resistance because of antibiotic application.
When antibiotic is consumed by animal or humans C. difficile colonizes the intestine and discharges two protein exotoxins; toxin A and toxin B that can involve the diarrhoea and colitis occurred by this microbes.
Recognised treatment of C. difficile-related intestinal illness includes vancomycin or metronidazole. It is costly and difficult and also approximately 25% of patients are liable to degenerate with disease when the therapy is stopped. There is a desirable alternative to antibiotic treatment by using probiotics to recover intestinal homeostasis.
Hepatic encephalopathy
This is a neurologic disease as a result of increased blood extent of ammonia. The ammonia occurs in the intestine by the bacterial ureases.
The absorbed ammonia in healthy body can be detoxified by the action of liver. But patients with unhealthy liver can be in danger of blood concentration of ammonia which may lead to toxic stage.
Carcinogenesis
The normal intestinal flora may affect carcinogenesis by creating enzymes which convert precarcinogens to the active carcinogens.
The involved enzymes include glycosidase, beta-glucuronidase, azoreduclase and nitroreductase (Goldin et al. 1980, Goldin 1990, Ling et al. 1994, Marteau et al. 1990, Pedrosa et al. 1995).
Some certain Micro-organisms proved to be protective for the host against this carcinogenic reaction.
Three proposed mechanisms involved in protecting the host are as follows:
i. probiotics are able to discourage the bacteria that contribute to conversion of precarcinogens to carcinogens.
ii. Animal studies have implied that some probiotics are able to suppress tumor-cell formation.
iii. Some bacteria have been reported to attach and /or inactivate carcinogens (Orrhage et al. 1994, Rowland and Grassa 1975).
Mechanism of action
Generally the process in which bacteria can discourage colonization by other strains is known as colonization resistance.
In some cases the same probiotic may discourage pathogens by different mechanisms of action.
In summary the mechanisms involved to support the host from intestinal disease (by bacteria) can be as follows:
Production of discouraging materials
Probiotics bacteria create different substances that are inhibitory to gram-positive and gram-negative bacteria. These inhibitory materials consist of organic acid, hydrogen peroxide and bacteriocins which are able to decrease the number of living cells in addition to influencing metabolism of bacteria or toxin yields.
Blocking of adhesion sites
Another mechanism of action for probiotics is competitive prevention of bacterial attachment parts on intestinal epithelial surface (Conway et al. 1987, Goldin et al. 1992, Kleeman and Klaenhammer 1982). Therefore certain probiotics strains have been considered according to their capability to epithelial cells.
Nutrients competition
This is another example of mechanism for probiotics in which probiotics utilise nutrients otherwise they can be used by harmful micro-organisms.
Stimulation of imunity
Immunity can also be involved in mechanism to protect the host from intestinal disease (Fukushima et al. 1998, Kaila et al. 1992, Link-Amster et al. 1994, Malin et al. 1996, Perdigon et al. 1986, Pollwels et al. 1996, Saavedra et al. 1994).
For example, administration of lactobacillus GG during intense rota virus diarrhoea can be in relation with an enhanced immune response to rota virus (Kaila et al. 1992).
The concept of mechanism of immune stimulation seems to be complex. However the structure and compounds of the cell wall are believed to increase the immune responses.
Technical methods for testing Probiotics and prebiotics
There are numerous
in-vitro and
in-vivo techniques to examine the capability of probitics and prebiotics.
For
probiotics “Challenge tests” to evaluate survivability may be carried out. In this method, the test strain is added to a fermenter or is directly tested on experimental animal or human.
The essential objective is that the probiotic strain must be identified in the research.
For
prebiotics, substrate integrity in the upper gastrointestinal tract is essential. It can be determined by means of in-vitro approaches which can stimulate this environment. A common method for evaluating bacterial fermentability is to apply selective agars (for gut micro-organisms) and examining the response of the main colonic genera within the prebiotic fermentation.
In-vitro fermenters are normally static batch culture. A certain concentration of substrate is added in a container with a faecal suspension or specific cultures and incubated anaerobically at 37C for a short time (~ 24 – 48 hrs).
It takes a nearly short time to evaluate and only small amount is required. However they are closed systems (batch fermenters) and the substrate is limited, so they are usually used for short-time tests.
In-vivo is an appropriate method to test the influence of substrate on the faecal micro-flora which can be used for both probiotics and prebiotics.
In this technique animals associated with their normal flora can be used for specific purposes such as gas production, weight changes and toxicology.
Prebiotics
Prebiotics are known as non-digestible food ingredients that can beneficially influence the host by selectively stimulating the growth or /and activity of a certain number of bacteria in the colon.
In fact, they are the food for the friendly bacteria. They may be added to the diet to provide the situation for effective bacteria to grow and survive in the digestive mechanism.
The list bellow explains different categories of oligosaccharides as prebiotics:
Fructo-oligosaccharides
Fructo-oligosaccharides or FOS are in association with short-chain oligosaccharides comprising D-fructose and D-glucose with 3 or 5 monosaccharides.
FOS are also known as neosugar. Short-chain FOS are created on a commercial scale from sucrose utilising a fungal fructosyltransferase enzyme.
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Furthermore, They are digestion resistant in the upper gastrointestinal area. Their activity can stimulate the growth of Bifidobacterium species in the large intestine. In United States the FOS are mixed with probiotic bacteria and also in some functional food products.
Inulins
These are known as a classification of naturaaly-occuring oligosaccharides which contain fructose. Inulins refer to a group of carbohydrates as fructans. They are originated from the roots of chicory (cichorium intybus) and Jerusalem artichokes.
Inulins are basically made up of fructose units and normally have a terminal glucose. The link between fructose units in inulins is a beta-(2-1) glycosidic linkage. Inulins can stimulate the growth of Bifidobacterium species in the large intestine.
Isomalto-oligosaccharides
Isomalto-oligosaccharides are composed of a combination of alpha-D- linked glucose oligomers, consisting of isomaltose, panose, isomaltotetraose, isomaltopentanose, nigrose, kojibiose, isopanose and higher branched oligo-saccharides. Isomalto-oligosaccharides develop by various enzymatic reactions.
They can stimulate the growth of Bifidobacterium spp. and lactobacillus spp. in the large intestine. Isomalto-oligosacharides are sold in Japan as dietary supplements and also in functional foods. They are produced in the United States for same purposes.
Lactilol
This is a disaccharide analogue of lactulose. Pharmaceutically it is used as medication for constipation and hepatic encephalopathy.
It is impervious to digestion in the upper gastrointestinal tract and it can be fermented by a certain level of colonic bacteria, which can intensify the biomass of bifidobacteria and lactobacilli in the colon.
Lactilol is defined as 4-0-(beta-D-galactopyranosyl)-D-glucitol. It is not established for the treatment of hepatic encephalopathy or constipation in the United State and its consumption as prebiotic is at the stage of experiment. In Europe it is used as a food sweetener.
Lactosucrose
Lactosucrose is a trisaccharide made up of D-galactose, D-glucose and D-fructose. Lactosucrose occurs enzymatically in which enzymatic transfer of the galactosyl residue in lactose to sucrose occurs.
Lactosucrose is unaffected by digestion in the stomach and small intestine. It is selectively used by intestinal Bifidobacterium spp. stimulating distinctive growth of these bacteria in the colon. So lactosucrose works on the intestinal microflora as a growth element for Bifidobacterium species.
Lactulose
Lactulose is a semisynthetic disaccharide composed of sugars D-Lactose and D-fructose. The sugars are connected by a beta-glycosidic bond which makes it impervious to hydrolysis by the enzymes of human digestive.
It is fermented by a certain amount of colonic bacteria, which can result in changes in the colonic ecosystem in favour of bacteria, e.g. bactobacilli and bifidobacteria that may have health advantages.
In U.S. Lactulose is a medication for constipation relief and treatment of hepatic encephalopathy. It is consumed in Japan as a dietary supplement and in functional foods. However Its consumption in the U.S. as a prebiotic is still at the stage of experiment.
Pyrodextrins
A combination of glucose-containing oligosaccharides which is extracted from the hydrolysis of starch constitute pyrodextrins.
They are able to enhance the proliferation of Bifidobacterium species in the large intestine. unaffected to digestion in the upper gastrointestinal section. Pyrodextrins are produced for the purpose of nutritional supplement.
Soy oligosaccharides
Soy oligosaccharides are attributed to oligosaccharides present in soybeans in addition to other beans and peas.
There are two main soy oligosaccharides including the trisaccharide raffinose and the tetrassaccharide stachyose.
Raffinose is made up of one molecule each of D-galactose, D-glucose and D-fructose. The operation of soy oligosaccharides can stimulate the growth of Bifidobacterium spp. in the large intestine. They are consumed in Japan and United States as dietary supplements and in functional foods.
Transgalacto-oligosaccharides
Transgalacto-oligosaccharides (TOS) consist of a combination of oligosaccharides including D-glucose and D-galactose.
TOS are produced from D-lactose by the enzyme activity beta-galactosidase created from Aspergillus oryzae. They are impervious to digestion in the upper gastrointestinal zone. TOS are used in Japan and Europe as dietary supplements and are utilised in functional foods.
Xylo-oligosaccharides
Xylo-oligosaccharides include oligosaccharides with beta (1-4) linked xylose residues. They are produced by enzymatic hydrolysis of the polysaccharide xylan.
In Japan they are consumed as prebiotics and developed in the U.S. for the same purpose.
ACTIONS AND PHARMACOLOGY
Action
The action of prebiotics may be anti-carcinogenic, antimicrobial, hypolipidemic and glucose-modulatory. They can contribute to mineral absorption and balance and have the anti-osteporotic function.
Mechanism of Action
Anti-carcinogenic action of prebiotics is not specifically clear. It is probably due to potential action of butyrate.
Butyrate accompanied by other short-chain fatty acids are created by bacterial fermentation of different prebiotic oligosaccharides in the colon.
The prebiotics oligosaccharides may help increasing the concentration of calcium and magnesium in the colon. The increased concentrations of these cations in the colon said to aid controlling the extent of cell turnover.
The encreased concentration of calcium in the colon may assist to control the formation of insoluble bile or salts of fatty acids. As a result the possible destructive influences of bile or fatty acids on colonocytes can be reduced.
Prebiotics may encourage the growth of bifidobacteria and lactobacilli in the large intestines.
There are in vitro and animal-based studies indicating that these bacteria can attach to and inactivate some carcinogens.
In addition they are said to be capable of inhibiting the growth of tumors and suppress the bacteria that are likely to convert precarcinogens into carcinogens.
The potential antimicrobial action of the prebiotics can be explained by their influence of growth-increasing on bifidobacteria and lactobacilli.
These bacteria are able to strengthen the barrier part of the intestinal mucosa, assisting in suppressing the connection of pathogenic bacteria mainly by crowding them out.
They can also yield antimicrobial materials and encourage antigen specific and non-specific immune responses.
The prebiotics is believed to be able to reduce triglyceride extent. The actual system of this effect is not understood. Probably is due to the reduced hepatocyte de novo synthesis of triglycerides.
Further, they can reduce total cholesterol and LDL-cholesterol rates to some extent.
Propionate which is a product of oligosaccharide fermentation in the colon have the ability of inhibiting HMG-CoA reductase; the rate limiting stage in cholesterol synthesis.
The possible influence of the prebiotics on blood glucose can be explained in different ways; the oligosaccharides may postpone gastric emptying and /or decrease the transit time in small intestinal area.
This might be by the short-chain fatty acids created from the oligosaccharides in the colon.
Short-chain fatty acids can affect on the so-called “jleocolonic brake” that is attributed to the prevention of gastric emptying by nutrients reaching the jleo-colonicjunction.
Propionate can discourage gluconeogenesis by its metabolic conversion to methylmalonyl-CoA and succinyl-CoA. These metabolites are able to discourage pyruvate carboxylase.
In addition, propinate can decrease plasma contents of free fatty acids. Great extent of free fatty acids can reduce glucose usage and allow for insulin resistance.
It is also said that propionate may increase glycolysis by depletion of citrate in hepatocytes. Citrate is basically an allosteric inhibitor of phosphofructokinase.
In general the system of the potential effects of prebiotics on glucose tolerance is unclear.
The oligosaccharides are able to attach minerals such as calcium and magnesium in the small intestine.
The short-chain fatty acids produced by the bacterial fermentation of the oligosaccharides can help the colonic absorption of calcium and also magnesium ions. This can be useful in inhibiting osteoporosis and osteopenia.
As a result of ingestion, the prebiotic oligosaccharides reach the colon and small number of them becomes digested in the upper gastrointestinal region.
The oligosaccharides are fermented by bifidobacteria, lactobacilli and some certain bacteria in the colon to generate the short-chain fatty acids acetate, propionate and butyrate; the hydrogen gases, hydrogen sulphide, carbon dioxide and methane; and lactate, pyruvate, succinate and formate.
Acetate and propionate and butyrate which are not metabolised in colonocytes are digested from the colon and carried by a great circulation to the liver. Those which are not metabolised in hepatocytes are transported by the circulation to different tissues where they face another metabolism. (butyrate is an essential respiratory fuel for colonycytes).
Conclusion
Probiotics;
Probiotics aim to generate a useful effect on the host by administration of live micro-organisms such as those in traditional yoghurt and other fermented foods or in powders, tablets, liquid suspensions and lyophilised in capsules.
They have considerable therapeutic advantages. They have the ability to have beneficial influences on prevention and treatment of a variety of intestinal disorders.
They also have the capability to inhibit intestinal bacterial enzymes involved in the synthesis of colonic carcinogens.
It has been reported that probiotics are effective on modulation of immune function, humoral, cellular and non-specific immunity.
Some advantages of probiotics over conventional therapy include virtually low cost in addition to the fact that probiotics are not expected to increase the incidence of antibiotic resistance or the mechanisms in which probiotics may suppress pathogens (resulting in reduction the extent of resistance against the probiotic).
Prebiotics;
prebiotics may be more efficient than probiotics in obtaining colonic bacterial adaptation and also affecting lactose intolerance. However, both probiotics and prebiotics may finally have beneficial effects on colonic disease.
Basically prebiotic foods have certain absorption, fibre contribution, gut integrity, immune function and cholesterol control.
Some prebiotics are consumed for the treatment of constipation and hepatic encephalopathy.
They can protect against some intestinal pathogens and may be helpful in some inflammatory bowel disease. They can have some anticarcinogenic influences.
In some cases, prebiotics are able to facilitate mineral absorption and help protect against osteoporosis. In additin, certain prebiotics are believed to be effective on diabetes mellitus.
Simultaneous intake of probiotics and prebiotics may develop the potential effectiveness of both the probiotics and the prebiotics.
The growing demands of natural alternatives over conventional medicine are expected to improve the prebiotic market.
However some precautions should be taken into account:
In the U.S. lactulose is a prescription medicine. Its consumption requires medical supervision. (Its use as a dietary supplement is at the stage of experiment).
People with lactose intolerance should consider caution before consumption of lactulose, lactilol and transgalacto-oligosaccharides.
Those people who are liable to develop gastrointestinal symptoms (flatus, bloating, diarrhea) as a result of consuming dietary fibre, should consider caution when using prebiotics.
Those facing whole body radiation or radiation to the gastrointestinal tract should not use prebiotic supplements.
Pregnant women should only use prebiotic supplements if prescribed by their physicians.
Finally, overdose of prebiotic oligosaccharides can lead to adverse reactions. Doses of prebiotic oligosaccharides up to 10 grams daily are well tolerated. More than this level consumption may cause gastrointestinal symptoms, such as flatulence, bloating and diarrhoea. [3928]
