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Probiotics are live microorganisms  (e.g., bacteria) that are either the same as or similar to microorganisms found naturally in the human body and may be beneficial to health. If you picture the human body as a “host” for bacteria and other microorganisms, you might have a better understanding of probiotics. The body, especially the lower gastrointestinal tract (the gut), contains a complex and diverse community of bacteria. Although we tend to think of bacteria as harmful “germs,” many bacteria actually help the body function properly.
Areas Of Scientific Research
Several beneficial effects of probiotics [2,3,4,5,6] on the host intestinal mucosal defenses system have been identified. Probiotics have been found to enhance innate immunity and modulate pathogen-induced inflammation via toll-like receptor-regulated signaling pathways .
- The mucosal immune system is very specific in protecting the whole inner surface, involving the oral-pharyngeal cavity, respiratory tract, gastrointestinal (GI) tract, and urogenital tract, as well as the exocrine glands in the human body.
- Results of evidence-based analysis from human studies and animal models suggest that probiotics have the potential for clinical effectiveness on intestinal diseases, including infectious diarrhea, antibiotic-associated diarrhea, atopic diseases, necrotizing enterocolitis, ulcerative colitis, and irritable bowel syndrome, and extraintestinal diseases, such as allergy.
Among the possible mechanisms [8,9] of probiotic therapy is the promotion of a nonimmunologic gut defense barrier, which includes the normalization of increased intestinal permeability and altered gut microecology. Many probiotic effects are mediated through immune regulation, particularly through balance control of proinflammatory and anti-inflammatory cytokines. More recent data show that differences exist in the immunomodulatory effects of candidate probiotic bacteria.
The interaction of the gut microbial flora [10,11,12] with intestinal epithelial cells and immune cells exerts beneficial effects on the upper respiratory tract, skin and urogenital tract. The capacity for probiotics to modulate perturbations in immune function after exercise highlights their potential for use in individuals exposed to high degrees of physical and environmental stress.
- One of the most known health effects  of probiotics is in preventing and ameliorating bowel diseases by improving the immune system.
- Although probiotic treatments improve the severity of the diseases by decreasing inflammation, they did not treat the actual root cause.
Within the past decade, it has become clear that the gut microbiota is a key regulator of the gut-brain axis .
- The role of microbiota composition as a susceptibility factor for various stressful insults, especially at key neurodevelopmental windows, is rapidly emerging , and there is growing evidence that targeted manipulations of the microbiota might confer protection to the brain to ameliorate the negative effects of stress during vulnerable developmental periods.
- Results from subsequent studies have continued to support a connection between gut microbiota and stress responsiveness, including reports that stress exposure early in life or in adulthood can change the organism's microbiota composition, and that microbial populations can shape an organism's stress responsiveness [16,17,18,19,20,21]. Recently, investigators have used fecal microbiota transplantation approaches to demonstrate that stress-related microbiota composition plays a causal role in behavioral changes.
Gruenwald and colleagues’ 6-month study [22,23] evaluating the effects of a probiotic multivitamin in 42 adults suffering from stress and exhaustion found that participants’ general condition improved by 41%, with a mean increase of 17% in positive condition and a mean decrease of 23% in negative condition . Improvement in the general condition of 22% was apparent by month 2 and reached 36% by month 4, with values for activity, elation, fatigue, and agitation contributing to these changes. 73% of participants rated the treatment’s effect as “good” or “very good” .
- Forty healthy young adults consumed either a probiotic supplement or placebo for 4 weeks and found that consumption of the multispecies probiotic formula significantly reduced overall cognitive reactivity to depression, in particular, aggressive and ruminative thoughts .
- As mentioned above, Rao and colleagues found that the consumption of probiotics significantly improved anxiety .
This review demonstrates the importance [28,29] of a healthy microbiome, particularly the gut microbiota, for patients suffering from anxiety and depression, as dysbiosis and inflammation in the CNS have been linked as potential causes of mental illness. Of note, studies have shown that probiotics effectively mitigated anxiety and depressive symptoms similar to conventional prescription medications [30,31,32,33,34].
Probiotic organisms are crucial for the maintenance of the balance of human intestinal microbiota .
Treatment with probiotics [36,37,38,39,40,41,42,43] has been used in clinical practice with L. rhamnosus and S. boulardii being administered. Several studies that have been carried out suggest that probiotic use is associated with a reduced risk of antibiotic-associated diarrhea [44,45].
- Probiotic supplementation of infant formulas has been aimed both at the prevention of rotaviral infections and the treatment of established disease. Well-controlled clinical studies have shown that probiotics such as L. rhamnosus GG, L. reuteri, L. casei Shirota, and B. animalisBb12 can shorten the duration of acute rotavirus diarrhea with the strongest evidence pointing to the effectiveness of L. rhamnosus GG and B. animalis Bb12 [46,47].
- Probiotics are proving to have roles in diarrhea prevention and control after antibiotic treatment. Lactobacillus GG, Lactobacillus reuteri, and S. boulardii, Bifidobacteria spp., are used for curing diarrhea. Also, Probiotics are able to suppress travelers’ diarrhea .
The proposed mechanisms  include competitive blockage of receptor site signals regulating secretory and motility defenses, enhancement of the immune response, and production of substances that directly inactivate the viral particles.
- The main action of probiotics [50,51] can be summarised as a reinforcement of the intestinal mucosal barrier against deleterious agents. Experimental data indicate that some probiotics reduce pathological alterations in paracellular permeability to large molecules or bacteria, stimulate mucosal immunity, display a trophic action on the mucosa, reduce mucus degradation and interact with mediators of inflammation.
- Irritable bowel syndrome (IBS) is characterized by symptoms of abdominal pain and altered bowel habits which occur over at least three months [56,57,58,59]
- With probiotics, patients might experience a global improvement in symptomatology rather than specific improvement in bowel function. Since treatment options for IBS remain limited in both number and efficacy, a therapeutic trial of probiotics is reasonable for patients interested in this approach.
The use of probiotics in IBS has been confirmed by recovery and gradual healing [60,61,62,63]. Additionally, probiotics stabilize immune dysregulation in IBS, thus enhancing cellular integrity to protect the colon [64,65]. Probiotics also modify the intestinal microbiota, altering the fermentation pattern inside the colon and reducing flatulence .
Although probiotic organisms exert beneficial effects to the host , they can act as a double-edged sword with both negative and positive effects. Therefore, precaution is necessary before they are administered. Pain assessment analysis showed that probiotics significantly reduce pain severity after eight and ten weeks of administration [68,69]. However, the reduction rate was rather higher at week eight than week ten, suggesting reduced effectiveness with long-term use.
One of the most popular and thoroughly investigated applications for probiotic therapy for CVD is the reduction of serum cholesterol [70,71,72,73,74]. Elevated low-density lipoprotein cholesterol (LDL-C) is a major risk factor for CVD . The accumulation of LDL-C in the blood is a precursor to hypertension, hyperlipidaemia, and causes the formation and build-up of atherosclerotic plaque in the arteries
- Probiotic consumption significantly lowered LDL-C and total cholesterol levels among all categories, compared to the control . The cholesterol-lowering properties of probiotics are strain and species specific with several postulated mechanisms of action.
- Accumulating evidence shows [77,78,79] probiotics to lower low density lipoproteins (LDL)-cholesterol and improve the LDL/high density lipoproteins (HDL) ratio, as well as lower blood pressure, inflammatory mediators, blood glucose levels and body mass index. Thus, probiotics have the scope to be developed as dietary supplements with potential cardiovascular health benefits.
- The results  of this meta-analysis suggest that probiotic consumption with daily doses from 109 to 1012 CFU for a duration of 3 to 9 weeks may improve BP. The magnitude of improvement is greater among those with elevated BP, when daily dose of probiotic consumption is ≥1011 CFU and when intervention lasts ≥8 weeks. The study also suggests a greater effect from consuming multiple rather than single species of probiotics.
Perhaps most accepted mechanism [81,82,83,84] is the processing of bile acids in the gut by Gram-positive organisms including Lactobacillus and Bifidobacterium [85,86]. Metabolism of cholesterol, a precursor of bile acids, is mediated through gut microbes expressing the enzyme bile salt hydrolase (BSH). Probiotics with high BSH activity promote the deconjugation of bile acids in the gut to secondary amino acid conjugates. When these secondary conjugates are excreted, cholesterol is broken down to replace the processed bile salts. Overall, this process promotes the catabolism of cholesterol, leading to reduced serum levels. There is variability in BSH phenotypes among probiotic species, indicating that the genes which encode it are likely to be horizontally acquired. Many probiotic strains express more than one BSH homolog , potentially helping them survive in the gut when exposed to different types of bile salts.
- However, there is not only ambiguity [88,89] regarding the exact strains and dosages of the probiotics that will bring about positive health effects, but also factors like immunity and genetics of the individual that might influence the efficacy of probiotics. Therefore, further studies are required not only to understand the mechanisms by which probiotics may beneficially affect the cardiovascular system, but also to rule out any of their probable negative effects on health
Now, there is emerging evidence  that probiotics can provide a direct, cardioprotective effect to the heart that results in reduced ischemic injury and improved cardiac function, post-infarction. The protective role of probiotics against apoptotic injury was first investigated in intestinal cells.
Consumption was at least 8 weeks in duration , and the probiotics consisted of multiple strains rather than a single strain. A significant reduction was found in LDL in trials which contained Lactobacillus Acidophilus strain compared to other types of strains.