Probiotics: A Comprehensive Review of Their Classification, Mode of Action and Role in Human Nutrition

Probiotics are live microorganisms that live in gastrointestinal (GI) tract and are beneficial for their hosts and prevent certain diseases. In this chapter, after a complete introduction to probiotics, definition, mechanism of action, and their classification, currently used organisms will be discussed in detail. Moreover, different kinds of nutritional synthetic products of probiotics along with their safety and drug interac‐ tion will be noticed. This chapter mentions all clinical trial studies that have been done to evaluate probiotic efficacy with a focus on gastrointestinal diseases. In the end, findings of our pilot study regarding the effect of probiotic on Small Intestinal Bacterial Overgrowth (SIBO) will be presented. The nutritional effects of Probiotics on a host’s health will be collected and their usage criteria will be discussed. Some suggestions for the Probiotics daily consumption will be presented and the follow-up for their new adverse reaction will be emphasized, if any.


Introduction to probiotics
The term probiotic is derived from Greek and literally means "for life."It was first coined in 1965 by Lilley and Stillwell to describe substances secreted by one microorganism that stimulate the growth of another [1,2].In 1974, Parker modified this definition to "…organisms and substances which contribute to intestinal microbial balance" [1,3].The current definition of probiotics by Food and Agriculture Organization of the United Nations (FAO) and the World Health Organization (WHO) is "live microorganisms which when administered in adequate amounts confer a health benefit to the host" [4][5][6].Probiotic organisms require certain characteristics to enable them to exert maximum therapeutic effects.Of these characteristics, there are some that are considered almost essential for a probiotic to have therapeutic effects, including gastric acid and bile salt stability, ability to adhere to the intestinal mucosa, and ability to colonize the intestinal tract [1,7].

Mechanism of action
The exact mechanisms by which probiotics accomplish their beneficial actions have not been well documented.However, there are several postulated mechanisms that explain many of their favorable effects [8] (Figure 1).
One of such mechanisms is a competition for adhesion sites, which means probiotics fight for cellular attachments.Many pathogenic organisms must associate with the GI tract epithelium to colonize effectively [9].However, some strains of bifidobacteria and lactobacilli can adhere to the epithelium and act as "colonization barriers" by preventing pathogens from adhering to the mucosa [1,10].This effect was demonstrated with the Lactobacillus rhamnosus strain GG and Lactobacillus plantarum 299v.Both of these organisms showed the ability to inhibit attachment of Escherichia coli to human colon cells [1,11].
Another possible mechanism of action is the modification of the microbial flora through the synthesis of antimicrobial compounds [12].Many types of lactobacilli and bifidobacteria produce bacteriocinsor and other antimicrobial compounds.Bacteriocins are defined as "compounds produced by bacteria that have a biologically active protein moiety and a bactericidal action" [1,13].Other biologically active compounds produced by lactic acid bacteria include hydrogen peroxide, diacetyl, and short-chain fatty acids.The release of these compounds by probiotic organisms results in a beneficial modification of the microflora [1,14].However, not all strains of lactobacilli or bifidobacteria produce antimicrobial compounds, and some produce compounds that are fairly nonspecific in their activity, so that beneficial bacteria, as well as pathogenic organisms, may be negatively affected [1].
It has also been observed that probiotics can stimulate the immune response [15].This immune response may take the form of increased secretion of immunoglobulin-A (IgA) [1,16], elevated numbers of natural killer cells, or enhanced phagocytic activity of macrophages [1,17].Increased secretion of IgA may decrease numbers of pathogenic organisms in the gut, thus improving the composition of the microflora [1,10].Due to these immunomodulating effects, some researchers think probiotics might not only fight intestinal and urogenital pathogens, but might also be helpful for conditions, such as inflammatory bowel disease (IBD), pouchitis, food allergy, and for use as an adjuvant to vaccination [18][19][20][21][22]. Probiotics may also compete for nutrients that would otherwise be utilized by pathogens [1,23].This situation occurs with Clostridium difficile, a potentially pathogenic organism that is dependent upon monosacchar-Probiotics and Prebiotics in Human Nutrition and Health ides for its growth.Probiotic organisms in sufficient numbers can utilize most of the available monosaccharides, which results in the inhibition of C. difficile [1,24].

Classification
There are many different microorganisms currently used as probiotics [1,20,25] (Table 1).To better understand how bacteria are named and classified, the following discussion may be helpful.Genus is the first name of a bacterium (e.g., Lactobacillus).It is somewhat general and refers to a grouping of organisms based on similarity of qualities, such as physical characteristics, metabolic needs, and metabolic end products.
Species is a bacterium's second name (e.g., acidophilus).It is a much more narrow classification based on shared common characteristics that distinguish them from other species.Strain is an even more specific classification that divides members of the same species into subgroups based on several properties that these bacteria have in common that are distinct from other members of the species (e.g., strain LA5) [1,26].
The name lactobacillus refers to the bacterium's ability to produce lactic acid, not to the ability to digest lactose [28].Lactobacilli are used therapeutically as probiotics, the opposite of antibiotics.They are considered "friendly" bacteria and are taken for the purpose of recolonizing areas of the body to provide nutritional benefits including inducing growth factors and increasing the bioavailability of minerals [32].Lactobacilli also stabilize the mucosal barrier and decrease intestinal permeability [33].
Altering the normal flora allows for potential colonization by pathogenic organisms [34], which can result in side effects, such as diarrhea, cramping, and less commonly pseudomembranous colitis (PMC), caused by C. difficile.The theory is that taking lactobacillus probiotics during antibiotic treatment can prevent or minimize normal flora depletion and pathogenic bacteria colonization.There is some evidence to support this theory [35,36].Hydrogen peroxide-producing lactobacilli are bactericidal to the vaginal pathogen Gardnerella vaginalis, and their presence in the vagina has been associated with decreased frequencies of bacterial vaginosis and trichomoniasis [37].In the vagina, lactic acid from lactobacilli lowers vaginal pH, which can prevent pathogen growth.
There is some preliminary evidence that lactobacilli and other probiotics might help protect against cancer.In animal models, lactobacillus has been shown to bind dietary carcinogens [38] and decrease development of tumors in the colon after carcinogen challenge [39,40].Preliminary research also suggests that lactobacilli, especially L. plantarum, can reduce the severity of chemotherapy-induced enterocolitis [41].According to other research studies, Lactobacillus bulgaricus and Lactobacillus sporogenes might have hypolipidemic and antiatherosclerotic effects.Limited clinical evidence suggests that it can reduce total and low-density lipoprotein (LDL) cholesterol with no effect on high-density lipoprotein (HDL) [42,43].
Fermented dairy products, such as yogurt and acidophilus milk, also seem to have a beneficial effect on cholesterol.Lactobacilli and other probiotic bacteria seem to bind bile acids to cholesterol.They also seem to increase fatty acid production in the intestine, which decreases circulatory fatty acid concentrations either by inhibiting hepatic cholesterol synthesis or redistributing cholesterol from the plasma to the liver.
Most researchers agree that the effectiveness of lactobacilli and other probiotics for all indications depends on their ability to colonize an area of tissue.To do this, lactobacillus preparations must contain live and viable organisms.Products stored for long periods of time or stored improperly may contain few live and active organisms.For oral preparations, bacteria must also remain viable after passing through the gut, and then they must be able to latch on to the intestinal epithelium.Lactobacilli strains might vary in their effectiveness due to differences in their ability to adhere to the epithelial cells by host factors such as hormone levels [30,44,45].This ability can change during a woman's menstrual cycle in response to changing hormone levels.In postmenopausal women, correcting low estrogen levels can help restore lactobacillus colonization without supplementation [29,30].

Bifidobacterium species
Bifidobacterium is an anaerobic, Gram-positive, nonspore-forming, pleomorphic rod.Bacteria in the Bifidobacterium genus produce lactic and acetic acids as by-products of glucose utilization.BB536 is a type of probiotic bacteria, which, according to secondary sources, was first isolated from the intestinal tract of healthy infants.Bifidobacteria, in combination with Lactobacillus species and the probiotic yeast Saccharomyces boulardii, seem to reduce the adverse effects of Helicobacter therapy, but do not seem to improve compliance [46].In addition, Bifidobacterium infantis in combination with Lactobacillus acidophilus seems to reduce the incidence of NEC and NEC-associated mortality in critically ill neonates [47].

Bacillus species
Bacillus coagulans is a Gram-positive rod, which produces lactic acid, and therefore is often misclassified as lactic acid bacteria, such as lactobacillus.In fact, some commercial products containing B. coagulans are marketed as Lactobacillus sporogenes or "spore-forming lactic acid bacterium."It forms spores, which is an important factor in differentiating these species.B. coagulans is used therapeutically in a similar manner as other probiotics such as lactobacillus and bifidobacterium; however, B. coagulans is not a component of the normal human flora.In order to be effective for restoring normal flora and prevent pathogenic colonization, probiotics must have the ability to persist and colonize in the intestinal mucosa.When the Bacillus spore is ingested by humans, it is unknown what happens to the spore.It is unknown if the Bacillus spore is capable of germinating in the intestinal tract or if colonization occurs [48].
B. coagulans might reduce pathogenic bacteria colonization through several mechanisms.B. coagulans produces coagulin and lactic acid, which have antibacterial activity and might reduce pathogenic bacteria growth through this mechanism [29,49,50].Animal model research also suggests that ingesting bacillus spores increases immune response [48].Proponents of B. coagulans suggest that this species of probiotics offers advantages over others such as lactobacillus because Bacillus species can be stored indefinitely in desiccated forms [48].Bacillus spores are also resistant to high temperatures and to acid.

Saccharomyces spp.
S. boulardii, also known as Saccharomyces cerevisiae, is a nonpathogenic yeast strain that has been used for the treatment and prevention of diarrhea resulting from multiple etiologies.S. boulardii has been isolated from the skins of tropical fruits found in Indochina.The indigenous population of Indochina has long used these fruit skins to prevent and treat diarrhea [51].
S. boulardii is prepared by lyophylization (freeze drying) of live yeast organisms and encapsulation using lactose in the preparation.S. boulardii cannot be distinguished from other S. cerevisiae strains by phenotypic criteria, so identification of these infections requires molecular typing.Comparative molecular studies show that S. boulardii is genetically very close or nearly identical to S. cerevisiae [52].Results suggest that microsatellite polymorphism analysis of the YKL139w and YLR177w genes and the analysis by Ty917 hybridization are the most useful tools for the correct identification of S. boulardii strains [53].However, metabolically and physiologically, S. boulardii shows a very different behavior than S. cerevisiae, particularly in relation to growth yield and resistance to temperature and acidic stresses, which are important characteristics for a microorganism to be used as a probiotic.The German Commission E monograph lists S. boulardii as S. cerevisiae Hansen CBS 5926.
Probiotics and Prebiotics in Human Nutrition and Health

Commercial forms
There are two main forms in which probiotic organisms can be ingested-fermented foods and supplements.Fermented foods can be of both dairy and vegetable origin, with the most commonly known of each being yogurt and sauerkraut, respectively.Probiotic supplements consist of freeze-dried (lyophilized) bacteria in powder, capsule, or tablet form.Regardless of the form in which the microorganisms are consumed, for clinical efficacy, products containing probiotic organisms must provide live organisms in sufficient numbers to exert therapeutic effects.Both types of fermented foods and supplements are able to do this.Pros (advantage) and cons (disadvantage) of common probiotic delivery systems are compared [1] (Table 2).

Using the right strain
To achieve successful and reproducible clinical outcomes, it is imperative to use the exact probiotic strain that has been proven to have the specific therapeutic action that is desired.For example, L. rhamnosus GG was found to prevent viral gastroenteritis [1,54] and maintain ulcerative colitis in remission [1,55].Other strains of L. rhamnosus cannot be assumed to act in a similar manner.The clinician who chooses to use the exact strain that had the effects in clinical trials can be confident of similar results.Using another closely related strain may or may not have any effect.Whenever possible, use the exact strain used in research, as other strains, even closely related ones, may not have the same effects [1].

Dosage
The dosage of probiotic foods and supplements is based solely upon the number of live organisms present in the product.Successful results have been attained in clinical trials using between 10 7 and 10 11 viable bacteria per day [1,56,57].Interestingly, it appears that 100 times fewer viable bacteria need to be given in a dairy medium than in a freeze-dried supplement to achieve similar numbers of live bacteria in the lower bowel [1,58].Dairy appears to work as an ideal transport medium for the bacteria, enhancing their survival through the upper GI tract [1,59].

Safety and adverse reactions
While probiotics are used widely and adverse effects are uncommon, there is no systematic reporting system for probiotics.Most studies did not report a statistically significant increase in adverse events compared with controls, but it has been questioned if probiotics are safe in immunosuppressed individuals [60].There are isolated case reports of bacteremia with Lactobacillus and fungemia with S. boulardii.A case-review study found sepsis, liver abscess, and endocarditis from Lactobacillus GG to occur mostly in patients with severe illness [61].The same paper reviewed S. boulardii fungemia and found numerous cases, some related to ingestion of S. boulardii, but others resulting from suspected contamination of central lines when the product capsules were opened, and the lyophilized yeast was allowed to become airborne.Again, most, but not all, cases were in immunosuppressed individuals [60,62].Two systematic reviews and an Agency for Healthcare Research and Quality study have evaluated the safety of probiotics and concluded that adverse effects are uncommon, but serious infections with Lactobacilli or S. boulardii can occur [63,64].Given this conclusion, it is prudent to avoid probiotics in individuals who are immunosuppressed or severely ill.

Drug interaction
Lactobacilli and bifidobacteria are negatively affected by alcohol and antibiotics [1,65].
Although there is no evidence that the organism interferes with the activity of most antibiotics, the metabolism of sulfasalazine, chloramphenicol palmitate, and phthalylsulfathiazole may be affected by some strains of L. acidophilus [1,66].

Clinical studies of probiotics
Table 3 lists conditions for which probiotics have been studied in more than 800 randomized, controlled clinical trials (RCT) [4].It is notable that there has been at least one clinical trial in a variety of clinical conditions.GI tract conditions, such as inflammatory illnesses (e.g., inflammatory bowel diseases or necrotizing enterocolitis in neonates) or enteric infections, have been studied most often [4,67].

Efficacy in GI diseases
An ambitious meta-analysis of 11 species of probiotics evaluated their efficacy in the prevention and/or treatment of eight major GI tract diseases and concluded that there was efficacy in treatment of infectious diarrhea, antibiotic-associated diarrhea (AAD), C. difficile infection (CDI), Helicobacter pylori eradication, IBS, and pouchitis; there was a lack of efficacy for traveler's diarrhea (TD) and necrotizing enterocolitis (NEC) [60,68].Some of these results conflict with meta-analyses of individual diseases, and results of all studies should be interpreted with caution.Rigorous blinded RCTs of specific probiotics are needed to provide robust data on efficacy, adverse events, and cost benefit before widespread use can be recommended for many products on an evidence-based approach (Table 4);despite the lack of data, these products are widely used [60].

A pilot study to evaluate the efficacy of probiotic on treatment in patients with small intestinal bacterial overgrowth (SIBO)
Generally, small intestinal bacterial overgrowth (SIBO) can be the result of a change in the clinical condition which has altered the pH and the bowel movements.In addition, immune deficiency and malnutrition are the other risk factors accompanying it [69,70].SIBO can leads to steatorrhea, vitamin B12-absorptive impairment, injury to the small intestinal microvilli, which itself causes malabsorption, coma, neurological deficit, and acidosis-induced shock [70].SIBO has also been proposed to be a common causative factor in the pathogenesis of irritable bowel syndrome (IBS) [71].The diagnosis of this syndrome is made by hydrogen breath test (HBT) [71,72].
This study was performed on the patients with chronic stomach pain and discomfort or changes in their defecation, who were referred to the Infectious and Internal Diseases Clinics of Quaem Hospital, Mashhad, Iran, from May 2010 to October 2011 [73].The study protocol was approved by the Research Council Ethics Committee of Mashhad University of Medical Sciences.Accordingly, the diagnosis was confirmed by hydrogen breath test (HBT) after obtaining informed written consent.Thirty consecutive cases with a positive test result were included in the study and were randomized in a double-blind manner into two groups: probiotic drug user and control group.After an initial 3-week aggressive therapy with broadspectrum antibiotics, a 15-day maintenance antibiotic therapy with minocycline, 100 mg twice a day, and 15 days with a probiotic (named Lactol), including Bacillus coagulan spores and fructo-oligosaccharides (Bioplus Life Sciences Pvt. Ltd., India), twice a day after meals, were administered for the study group, and the same regimen without probiotic for the control group.After 6 months, the HBT result and the GI symptoms were analyzed and compared between the two groups.
As presented in the following tables, the number of patients with complaints of bloating, belching and diarrhea was remarkably less in the patients receiving a probiotic in comparison to controls (Tables 5 and 6 ).In spite of the aggressive and maintenance treatments adminis-tered for all the cases, 93.3% patients showed negative result of the HBT at the end of treatment in the study group compared to 66.7% in the control group, showing the effectiveness of the probiotic treatment.As an additional finding, 33.3% patients of the study group and 53.3% of the controls had a Bachelor degree or higher education, with no significant difference [75][76][77][78][79].In conclusion, the results of this pilot study showed that addition of a probiotic to the maintenance regimen may improve the GI tract symptoms and prevent the probable complications in patients with SIBO.Therefore, based on low side effects of the probiotics, it seems that their long-term prescription in SIBO, considering the recurrence favor of this syndrome, is desirable (e.g., probiotic containing dairy products or supplement daily drugs).Probiotics and Prebiotics in Human Nutrition and Health

Figure 1 .
Figure 1.Schematic diagram illustrating potential or known mechanisms whereby probiotic bacteria might impact on the microbiota.These mechanisms include (1) competition for dietary ingredients as growth substrates, (2) bioconversion of, for example, sugars into fermentation products with inhibitory properties, (3) production of growth substrates, for example, EPS or vitamins, for other bacteria, (4)direct antagonism by bacteriocins, (5) competitive exclusion for binding sites, (6) improved barrier function, (7) reduction of inflammation, thus altering intestinal properties for colonization and persistence within, and (8) stimulation of innate immune response (by unknown mechanisms).IEC : intra epithelial cells, DC: dendritic cells, T:T-cells.

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Affordability and easy Availability -Ease of incorporation into daily patterns -Additional nutritional benefits -Enhanced bacterial survival through upper GI tract (100× less bacteria can be given per dose) -Effective in the upper GI tract -Contains dairy proteins and lactose -Taste can be issue -Not suitable when travelling the upper GI tract -Dosages can be easily adjusted -Can be incorporated into foods or drinks -Contain no binders small bowel injury Prevention and treatment of pediatric cow's milk allergy Prevention and treatment of diarrheal diseases (infectious and noninfectious) Prevention of antibiotic-associated diarrhea (AAD) Prevention of necrotizing enterocolitis (NEC) Primary sclerosing cholangitis (PSC) in patients with IBD Small intestinal bacterial overgrowth (SIBO) Tolerance of enteral feeds in ICU patients Viral shedding Oral and respiratory tract conditions Gingivitis Probiotics: A Comprehensive Review of Their Classification, Mode of Action and Role in Human Nutrition http://dx.doi.org/10.5772/63646Dental caries Halitosis Prevention of upper respiratory tract infections (URTI) Pulmonary exacerbations in cystic fibrosis (CF) mortality in preterm infants Effect on CD4 count in patients with HIV Estrogen metabolism Fasting glucose, insulin sensitivity, and glucose control in diabetic patients Hyperlipidemia Hypertension Infant blood pressure and metabolic profile from mothers treated with probiotics Inhibition of nasal, oral, or fecal colonization with pathogenic bacteria Markers of metabolic syndrome and cardiovascular disease Mastitis Pediatric otitis media Probiotics and Prebiotics in Human Nutrition and Health Pregnancy after in vitro fertilization (IVF) Prevention and treatment of gestational diabetes Prevention of type-1 diabetes mellitus Prevention of infections in preterm infants, infants, and young children Prevention of nosocomial infections in ICUs Prevention of infections in the postoperative setting Prevention of skeletal muscle damage under oxidative stress Psychological distress, mood, and cognition Reduction of biologically active aflatoxin Rheumatoid arthritis (RA) Spondyloarthropathy Urinary oxalate excretion (risk factor for nephrolithiasis) Vaccine-specific antibody development Waist circumference and obesity

Table 6 .
Clinical characteristics data of subjects after 6 months of treatment (values in parentheses are percentages).

Table 2 .
The pros and cons of different probiotic delivery systems.

Table 3 .
Clinical conditions or settings studied in randomized, controlled clinical trials to evaluate probiotic efficacy.

Table 4 .
Indications, efficacy, and quality of evidence for probiotics in GI diseases.Probiotics: A Comprehensive Review of Their Classification, Mode of Action and Role in Human Nutrition http://dx.doi.org/10.5772/63646

Table 5 .
Clinical characteristics of subjects at baseline (values in parentheses are percentages).Probiotics: A Comprehensive Review of Their Classification, Mode of Action and Role in Human Nutrition http://dx.doi.org/10.5772/63646