microbiota
true or false: probiotics are generally good and able to produce mild beneficial effects to virtually everyone with any disease
*FALSE* selected probiotic preparations are effective in selected conditions among specific populations
microbiota therapeutics
*food* - *prebiotics*: complex carbs/polysacchardies; present in food or extracted and added to foods - *probiotics*: naturally present in some foods, or added to foods *medicinal products/drugs* - *fecal microbial transplantation (FMT)*: transferring complex communities
healthy gut microbiota vs. unhealthy gut
*healthy intestine (left side of image)* - level of inflammation is very low - commensal bacteria can be sensed by dendritic cells or epithelial receptors - this generates an expansion and activation of regulatory T cells that produce anti-inflammatory cytokines such as IL-10 and TGF-b - those cytokines inhibit the activity of pathogenic CD4-T cells Th1 and macrophages and promote the release, into the lumen, of anti-inflammatory products such as defensives or IgA, which prevent pathogen bacteria entry - additional help for the maintenance of the gut's immune balance can be obtained through the introduction of probiotics - probiotics are known to mimic commensal bacteria and stimulate the gut's immune system, although their exact role is still under investigation *less healthy gut (right side of image)* - a diet rich in fats and sugars, antibiotic treatments, or other environmental factors, can strongly reduce microbiota diversity and therefore compromise gut health and encourage inflammation
does everyone who ingests c. diff spores get an infection?
*no* - *people with undisturbed gut microbiota are far more likely to not become infected* - resistant microbes may compete with C. difficile and/or outright kill it (using *bacteriocins*), to the extent that it cannot survive - or, c. diff may continue to live in the colon, but be kept in check by resident microbiota and only exist in small populations --> but these people can be *carriers* and can spread c. diff endospores, which will be shed in their feces
limitations to probiotics: PRODUCT RISKS
*rare complications* - bacteremia - endocarditis - sepsis-- requires probiotic to translocate from the gut to the systemic circulation (would only occur with a disrupted gut epithelial or mucosal layer)
probiotics
- "live micro-organisms which, when consumed in adequate amounts, confer a health benefit on the host" - most common strains of bacteria used are: *bifidobacterium*, *lactobacillus*
main SCFAs produced by saccharolytic prebiotics in the proximal colon
- *butyrate*-- 15%; used by colonic epithelium as main energy source - *acetate*-- 60%; bypasses liver for peripheral use - *propionate*-- 25%; to liver for ATP production
most commonly used prebiotic additives
- *inulin (from chicory root) and fructooligosaccharides* - they are fibers present in plants as storage carbohydrates - wheat, chicory, bananas, onions, leeks, Jerusalem artichokes, asparagus, and garlic contain prebiotics that can be extracted from them
phylotype
- *microbial group defined by 16S rRNA sequence similarity* - 16S rRNA is a component of the 30S small subunit of prokaryotic ribosomes - portions of the sequence from distally related organisms are remarkably similar and stable over time - sequences from distally related organisms can be precisely aligned and compared, making the true differences easy to measure
fermentation of prebiotics by gut microbiota
- *saccharolytic*: proximal colon (produces SCFAs, lactate, H2, CH4, CO2) - *proteolytic*: distal colon
commensal bacteria
- *term that is used to indicate normal/expected microbiota in different environments of the body* - although friendly bacteria inhabiting our bodies are often referred to as commensal, the relationship between that and our gut microbiota and us is more complicated than that - relationship between humans and microbiota is often interdependent and mutualistic (both species benefit) - but mutualistic "friendly" bacteria can become pathogenic when its growth becomes uncontrolled or occurs in the wrong anatomical place (ex. streptococcus and staphylococcus from the skin can enter the body and cause harm)
metagenome
- *the genetic information of a complex population — typically from microbes in an environmental or host niche sample — that is made up of the genomes of many individual organisms* - the human microbial metagenome (all of our microbes' genes) can be considered a counterpart to the human genome (all of our human genes) - the genes in our microbial metagenome outnumber the genes in our genome by at least 100 to 1 - human microbiome project (similar to human genome project but for all of the microbes in the gut)
microbiota
- *the microbial organisms that make up a specified microbiome* - composition of the microbiota in a community can vary substantially between environmental sites, among host niches and between health and disease - not just bacteria but often focused on bacteria
how many bacteria make up our total body microbiome?
- 100 trillion? - ratio of bacterial cells to human cells 10:1
brain-gut communication
- 2-way communication between the ENS (enteric nervous system) and CNS (central nervous system) - disturbances in the microbiome can lead to disturbances in the CNS and vice versa - it is very complicated!
hypervirulent strain of C. diff
- NAP1/BI/027 appeared in early 2000s - appears to be associated with emergence of a newer class of antibiotics, the fluoriquinolones (but not definitively proven) - produces a third toxin, CDT (C.diff transferase), which modifies host cell cytoskeleton --> cell collapse and death - also produces larger quantities of Toxin A and Toxin B than other strains - associated with lower infection cure rates and higher rates of recurrence - associated with more severe disease, worse outcomes, including death (esp. in elderly)
rationale for microbiota-directed therapy: *other (GI and systemic)*
- NEC prevention (VLBW infants) - immune system support - reduce respiratory tract - infection - inflammatory bowel disease - inflammatory bowel syndrome - vaginal yeast infection; candidiasis - atopic disease - autism spectrum disorders
prebiotics
- a food ingredient that... is resistant to digestion; is *fermented* by the intestinal microbiota; resists gastric acidity, hydrolysis by mammalian enzymes and absorption in the upper GI tract - has selective effect on the microbiota: composition, growth, activity - all prebiotics are "fibers".. but not all dietary fibers meet the definition of a prebiotic - some prebiotic additives - all about long-term dietary patterns: fruits, veggies, grains --> increase richness/diversity of bacteria and their genes --> health associations - agrarian vs. western diet
microbe
- a living thing that is too small to be seen with the naked eye - requires a microscope to be seen - includes many different life forms: bacteria. archaea (often live in extreme environments), fungi, protists (paramecium, amoebas, others), viruses, microscopic animals (mites are an example)
FDA announced that it intends to exercise enforcement discretion regarding FMT, provided...
- adequate concept is obtained from the patient. the consent should include, at minimum, a statement that the use of FMT products to treat C. difficile is investigational and a discussion of its reasonably foreseeable risks - the FMT product is not obtained from a stool ban - the stool donor and stool are qualified by screening and testing performed under the direction of the licensed healthcare provider for the purpose of providing the FMNT product for Tx of the patient
rationale for microbiota-directed therapy: *diarrhea*
- antibiotic-associated (inc. CDI (c. diff)) - traveler's - infectious/rotavirus - radiation-induced - enteral nutrition-associated - HIV associated
s/s of c. diff
- at least 3 watery, unformed bowel movements per day (although many experience more) - *diarrhea is the cardinal symptom*-- may result in dehydration and can be very debilitating and disruptive - abdominal pain, cramping (colon is inflamed and painful = *colitis*) - nausea, but rarely vomiting - fever - leukocytosis (increased WBCs) - confusion may be seen in older adults - severe infection
how does the hypervirulent C.diff strain enter the host cell
- bacterial toxins usually exert their full deadly effect inside the host cell - the CDT toxin overcomes the host cell membrane by binding to a surface receptor, which moves the toxin past the cell membrane, to the cytoplasm - once inside the cell, the CDT toxin modifies the cell's cytoskeleton, causing cellular collapse and death - CDT fits very well onto a real human receptor and uses that existing receptor to enter the cell
c. diff and antibiotic use
- c. diff is usually preceded by antibiotic use - spores either ingested or small amounts of c. diff may already be present in the bowel, but under control - antibiotics disturb the balance of the microbiota in the gut, killing bacteria that would normally be able to keep c. diff uncheck, reducing bacterial diversity - c.diff bacteria able to reproduce rapidly without interference from the now-decimated gut microbiota
ways that gut microbiota can influence health and disease
- carbohydrate fermentation, including carbs that are otherwise indigestible by the human GI system (ex. fiber) --> produces important by-products such as SCFAs - digest proteins that reach the gut, including host-derived proteins such as epithelial cells - metabolism of bile acids that escape enterohepatic circulation; removal of amino acid side chain - conversion of bilirubin to products excreted in feces and urine - role in cholesterol metabolism - role in vitamin synthesis-- vitamin K, B12, and other B vitamins - prevent colonization by pathogens by competing for nutrients and sites of attachment - produce bacteriocins which kill or antagonize non-endogenous species (ie. pathogens) --> more than 1.2 of the antibiotics used today were from compounds produces by the bacteria of the Streptomyces genus-- although the bacteria are typically found in soil, not the human gut - play a role in the development and maintenance of the protective mucous layer in the gut - important role in training the immune system - 2 way communication with the immune system - role in controlling inflammation or promoting it (dysbiosis)
treatment for c.diff
- causative antibiotic stopped, if at all possible... and another antibiotic is started, to kill the vegetative c.diff - can be very difficult to eradicate and can recur (there has been a recent increase in multiple recurrent c.diff infections) - FMT may be necessary
what is a "functional GI disorder"?
- comprises symptoms arising in the mid or lower GI tract that are not attributable to anatomic or biochemical defects - Sx include abdominal pain, early satiety, nausea, bloating, distention of the abdomen, and various symptoms of disordered defecation
gut bacteria are implicated in:
- depression - anxiety - autism - obesity - response to cancer chemotherapy - atopy/allergy - autoimmune diseases - type 2 diabetes - IBS - IBD - and many more!
rationale for microbiota-directed therapy
- diarrhea - other (GI and systemic)
role of gut microbiota in training the immune system
- distinguishing self from non-self (when this goes wrong can lead to an autoimmune disease) - distinguishing friendly bacteria from pathogenic bacteria - promoting anti-inflammatory activity of Tregulatory cells, a subset of CD4-expressing T cells that prevent the overactive or inappropriately-activated function of TH1 cells - promotes production of anti-inflammatory products such as defenses or IgA, which prevent pathogen bacteria entry into tissue
dysbiosis
- disturbed homeostasis of the microbiota composition - antibiotics (and other drugs) are a major cause - relationship is bi-directional: meds and other factors, including disease, can cause dysbiosis, but dysbiosis can cause disease - cause and effect is not always clear (i.e. observed differences in microbiota in disease states may or may not be causing the disease)
gut microbiome changes dramatically in early life
- due in part to diet changes - but keep in mind that the child undergoes many and complex environmental exposures
probiotic major mechanisms of action
- enhancement and repair of the epithelial barrier - increased adhesion to intestinal mucosa --> inhibition of pathogen adhesion (pathogens get crowded out by the probiotics) - competitive exclusion of pathogenic microorganisms - production of anti-microbial substances - promote digestion and uptake of dietary nutrients - modulation of the immune system
factors associated with the development of the gut microbiome
- factors such as vaginal birth vs. c-section and breastfeeding vs. formula fed result in different microbiota but insufficient evidence to state whether one is more efficient than the other and unclear how far in life these effects will last - early childhood has a huge effect on the microbiome
2017 Cochrane Review
- focused on "the use of probiotics to prevent C. diff diarrhea associated with antibiotic use" - reviewed 39 randomized trials - "our results suggest that when probiotics are given with antibiotics, the risk of developing CDAD is reduced by 60% on average. Among trials enrolling participants at high risk of developing DAD (>5%), the potential benefit of probiotics is more pronounced with a 70% risk reduction on average... *the short-term use of probiotics appears to be safe and effective when used along with antibiotics in patients who are not immunocompromised or severely debilitated*"
limitations to probiotics: PRODUCT REGULATION
- food additive, dietary supplement, drug or biologic - claims may be vague and unsupported - little incentive for the industry to maintain standards of quality, or even to test for specific advertised strains (because there is not much punishment for not doing it)
prebiotic
- food ingredients resistant to digestion (human body is unable to completely break them down) - ex. fiber - fermented by gut microbiota, with a selective effect on the microbiota and consequent beneficial effect on the host's health - stimulate growth or activity of certain types of bacteria
C.difficile
- gram + - spore-forming - anaerobic bacillus that infects the colon - may be present in small numbers in the colon, but spores usually ingested via fecal oral route
prebiotics presence in agrarian (vs western) diet
- greater microbial richness - increased SCFA production - appears to protect integrity of gut mucous layer and epithelial tight junctions
microbiota metabolites
- gut microbiota can carry out a suite of biochemical activities that can convert luminal compounds to secondary metabolites - these conversion reactions can, in some cases, detoxify ingested toxins, but in other cases can result in the production of compounds that can be deleterious - the specific composition of the gut microbiota can thus determine the balance between beneficial and harmful chemical conversion reactions in the gut lumen
SCFAs role in metabolism and inflammation
- important role! NOTE: obesity can shift the balance of microbiota species in the gut, leading to an imbalance of the SCFAs that play the metabolic roles in the image; when there are not sufficient SCFAs, it can lead to decreased insulin sensitivity, increased lipogenesis, adipose tissue, increased inflammation
SCFA roles in the colonic epithelium and immune system
- important roles! - SCFA (derived from plant polysaccharides) regulate inflammation through GPR43 - SCFA are produced by the gut microbiota as a byproduct of fermentation of dietary fiber and have several beneficial effects - in the colonic epithelium, butyrate is the main energy source of colonic epithelial cells and is transported into cells via monocarboxylate transporters (such as MCT1 and SLC5A8) - SCFA are important for maintaining epithelial barrier function, regulating proliferation and tumor suppression - SCFA also diminish oxidative DNA damage and regulate cytokine production - effects of SCFA on epithelial cells relate mainly to their role as an energy source and their inhibition of histone deacetylases - SCFA could also operate through GPR41, GPR43, and GPR109A - in immune system, SCFA have several anti-inflammatory effects but are also important for stimulating immune function, and their role seems to be important for the regulation of timely immune responses and in resolution of inflammation - acetate enhances production of ROS and phagocytosis but also induces apoptosis and modulates neutrophil recruitment; many of these anti-inflammatory effects are mediated through GPR43 (a c-protein-couples receptor that has been implicated in the regulation of fatty-acid and glucose homeostasis in adipose tissue and the intestines)
theoretical model for treating sequelae of obesity with prebiotics
- in animal studies, has been shown that dietary carbs with prebiotic properties change the gut microbiota composition by favoring bacteria involved in the control of gut barrier function and host immunity - in gut, prebiotics help reinforcing the gut barrier and promote gut hormones that control appetite, glucose homeostasis, and systemic inflammation - the prebiotic approach also counteracts hepatic steatosis, hepatic insulin resistance and adiposity by modifying gene expression at the tissue level - inulin are a group of natural occurring polysaccharides produced by many types of plants. the inulin belong to a class of dietary fiber known as fructans.
composition of gut microbiota based on location in the gut
- in upper gut: primary gram-positive bacteria - in lower gut: gram-negative bacteria and anaerobes - also differs between the lumen and outer mucin layer - also varies by tissue layer of the gut
concentration of gut microbiota
- increases distally down the gut (from stomach and proximal GI to colon and distal GI) - From 10^1 to 10^12 cells per gram of content NOTE: the phyla of bacteria also change as you go down the GI tract from proximal to distal --> *from proximal to distal there are much more gut bacteria and there is much more variety*
how do probiotics modulate the immune system?
- interact with epithelial and dendritic cells (DCs) and with monocytes/macrophages and lymphocytes - help DCs and naive helper T-cells mature, influence development of anti-inflammatory Tregulatory cells
3 most common functional bowel disorders
- irritable bowel syndrome (IBS) - constipation - functional dyspepsia
prevention of c.diff infection in the hospital
- judicious antibiotic prescribing! - contact precautions: wear *gloves* and gown in patient's room - hand washing still very important especially after removing gloves - immediate contact precautions if c. diff infection is even suspected, prior to labs coming back; also applies to known carriers of c.diff - private room or cohosting with another c.diff patient - dedicated BP cuff, stethoscope, etc. in patient's room - scrupulous attention to disinfection of all surfaces with a germicide effective against c.diff. chlorine bleach is effective. -- give environmental services workers time to do their job! - proper training of all hospital personnel - patient and family education is important
probiotic
- live micro-organisms which, when consumed in adequate amounts, confer a health benefit on the host - must meat identification criteria, safety assessment, and efficacy assessment to qualify
summary of major SCFA benefits
- main energy source of colonic epithelium - maintains barrier function of colonic epithelium - regulates cytokine production - protects DNA from damage; tumor suppression - anti-inflammatory effects, but also promotes robust immune response when needed - promotes glucose control, improved insulin sensitivity - other roles in metabolism, including inhibition of inappropriate lipogenesis - a role in the signaling pathway that generates gut hormones such as GLP-1 (which increases insulin secretion and decreases glucagon secretion)
FGID: IBS
- most common functional GI disorder (approx. 10% prevalence) - chronic or recurrent symptoms of lower abdominal pain related to bowel movements, change in bowel habit (*diarrhea, constipation, or alternating*), a sense of incomplete rectal evacuation, passage of mucous with stool, and abdominal bloating and distention - known to sometimes occur as post-infectious problem (connection with Abx?) - emotional stress is one trigger for an IBS exacerbation - good evidence that microbiota pertubance (ie. dysbiosis) is implicated in the etiology of IBS
are probiotic supplements regulated by the FDA?
- no - but... they are considered OTC supplements
adult diet influences types of bacterial species in the gut
- people on different diets exhibited different gut microbiota - relative proportions of each type of bacteria varied consistently to the diets
limitations to probiotics: PRODUCT INFORMATION
- practical - strain(s), dose, route, frequency, duration - clinical - trial methodology, endpoints, outcomes, safety
prebiotics effect on high-fat-diet obesity and metabolic consequences
- prebiotics may mediate high-fat-diet obesity and its metabolic consequences - when you take sufficient prebiotics, there is inhibition of inflammatory signaling and promotion of GLP, which increases insulin sensitivity, inhibition of certain kinds of bacteria, but promotion of other kinds of bacteria, and promotion of the barrier function of the mucous layer and goblet cells - prebiotics have important roles in maintaining a healthy gut environment
risk factors for C. difficile infection
- recent antibiotic use - hospitalization or nursing home residency (longer stay = greater risk) - age 65+ (most infections and deaths occur in this age group) - gastrointestinal surgery - immunocompromised - serious illnesses - use of gastric acid-reducing medications - hospital bed previously occupied by someone with a C. difficile infection
culture-independent methods of analysis of microbiota
- sequencing target genes (including I6S rRNA analysis) - metabolomics - "shotgun metagenomic sequencing" - proteomics
pseudomembranous colitis
- severe inflammation of the colonic mucosa - "pseudomembranes" occur following c. diff toxin-induced ulcer formation on the mucosal surface of the intestine --> release of serum proteins, mucous, and inflammatory cells - pseudomembranes manifest as raised yellow or off-white plaques scattered over the colonic mucosa, replacing the normal healthy, pink mucosa - pseudomembranes do not function as a normal mucosal layer
SCFAs
- short-chain fatty acids - have < 6 carbons - very important in the body!
where are microbiota found?
- skin and hair - conjunctiva - nares - airways - oral cavity (teeth, tongue, lips/cheeks/palate, gingival crevices) - entire GI tract - urogenital tract (all of these sites have different compositions)
c. diff spores
- spores transferred to patients on the hands of healthcare workers. healthcare workers' hands are the *major route* of infection in hospitals. *wash your hands! alcohol-based sanitizers are ineffective!* - remain viable in the environment for weeks to months - resistant to killing by heat, acid, sanitizers, and antibiotics (would need to autoclave for 15 minutes) - NOTE: antibiotics can kill c.diff in it's vegetative state but not the spores
C. difficile pathophysiology
- spread via fecal-oral route - once ingested, spores germinate to their vegetative (bacteria) state in the small intestine --> travel to colon --> attach to the colonic epithelium --> reproduce - can severely injure the colonic mucosal layer - injury to the colon caused by release of exotoxins by the bacteria; attach to the colonic mucosa - toxin A and toxin B - toxins cause colonic epithelial cell necrosis, apopotosis, and disruption of cellular tight junctions
limitations to probiotics: PRODUCT EFFICACY
- survival in GI transit; pH of stomach - present in sufficient numbers? - refrigerated? non-refrigerated?
fecal microbiota transplantation (FMT)
- the administration of a solution of fecal matter from a donor into the intestinal tract of a recipient in order to directly change the recipient's microbial composition and confer a health benefit - first described in the literature in 1958 to treat pseudomembranous colitis - a donor (sometimes a family member) without a family history of autoimmune, metabolic, and malignant diseases is selected and screened for any potential pathogens in blood or stool - some protocols keep the patient on vancomycin (an antibiotic used to treat c. diff) until 2 days before the procedure, to minimize the population of c. diff in the bowel - feces mixed with water or 0.9% NaCl solution, followed by a filtration step or multiple filtration and centrifuge steps - mixture (50-250 ml) can be administered through a nasogastric tube, nasojejunal tube, esophagogastroduodenoscopy, colonoscopy, or retention enema. may need > 1 transplantation to be effective. - more recently, frozen samples used. roughly same efficacy as fresh. - small studies done with capsules swallowed orally - FDA: FMT is both drug and biological without approval yet - proof of concept established via effective treatment of refractory c.difficile infection. cure rates of refractory infection ~ 85-90% - small trials in metabolic syndrome, IBD, and IBS - requires an IND (investigational new drug application) and participant consent, but... - recently, FDA announced that it intends to exercise enforcement discretion regarding FMT
hygiene hypothesis
- the idea that being exposed to animals, microbes, etc. early in life "trains" the immune system to respond better and more appropriately to pathogens and allergens later - possibly implicated in autoimmune disease and possible connection to allergy/atopy
microbiome
- the totality of microbes, their genetic information and the milieu in which they interact - typically consist of environmental or biological niches containing complex communities of microbes (ex. gut microbiome; tongue microbiome)
severe infections that could result from c.diff
- toxic megacolon (colon severely dilated, inflamed; image) - pseudomembranous colitis - perforation of the colon - can lead to death (approx. 29,000 deaths within 30 days of Dx in 2011)
gut microbiota
- usually referring to gut bacteria, but includes other types of microbes as well
gut microbiota composition
- ~ 700 genera, many more species - estimated 14 core genera common to all humans - *phyla* are dominated by firmicutes (2/3) and bacteroidetes (1/4) --> the relative percentages of these can change in disease states, inflammatory states, etc... (specific bacteria do specific things and if you flip the normal percentages, you will have a change in what the gut microbiota is doing) - strict (obligate) anaerobes outnumber facultative anaerobes by about 100:1 - considerable diversity from one individual to another
C. diff toxin B
10x > potent than Toxin A, causes much more damage to the colonic mucosa
what is the most common healthcare associated infection, and an example of dysbiosis
C.difficile
ways that the gut microbiome can influence health and disease (image)
DON'T NEED TO MEMORIZE just understand BUT NOTE: the study of human microbiota has found many more *associations* than *direct causations*! plenty of caution is called-for. *correlation is not the same as causation!*
how to identify different microbiota
NOTE: much still remains unclear about different types of bacteria and how to identify bacteria in a particular sample
true or false: the benefits of probiotics are oversold
TRUE
commensalism
a type of symbiotic relationship between 2 different organisms in which one species benefits and the other is unaffected
"shotgun metagenomic sequencing"
all DNA is sequenced, yielding data on taxonomic properties, as well data on function(s) of bacteria
what is the number one risk factor for c. diff?
antibiotic use
facultative anaerobes
can grow either in the present of oxygen or in anaerobic conditions, they have the ability to switch between aerobic and anaerobic respiration (or fermentation)
strict/obligate anaerobes
cannot grow in the presence of oxygen
C. diff toxin A
causes inflammation leading to intestinal fluid secretion and mucosal injury
metabolomics
characterizing small molecule metabolites that result from biological and biochemical processes in which microbiota are involved
proteomics
characterizing structure/function of proteins produced by bacterial genes
therapeutic use of probiotics
don't need to memorize
which drugs should not be administered to patients with c.diff?
drugs that slow bowel motility, including drugs like loperamide (Imodium) due to increased toxicity as the colon is exposed to the c. diff toxins for a longer period of time
how does a fiber free diet effect the gut?
fiber free diet --> encourages the growth of microbiota that erode gut mucus layer --> disruption of tight junctions
why is it important to prevent a pathogen from getting into endothelial tissue
if they get deeper into the endothelial tissue, they can make their way into the bloodstream
is gut flora = gut microbiota
no... it is often used to refer to bacteria in the gut but it is not technically correct
is the fetal environment sterile? are babies born sterile?
nope (watch videos for more notes!)
gut microbiota variation by tissue layer
several microenvironment exist within the large intestine in which microorganisms can reside... 1) epithelial surface and inner mucin layer-- minimal colonization in the healthy state 2) diffuse mucin layer-- specialist colonizers (ex. Akkermansia muciniphila) 3) gut lumen-liquid phase-- diverse microbial community 4) gut lumen-substrate particles--specialized primary colonizers (ex. Ruminococcus spp.)
probiotics and IBS- 2016 meta-analysis of multiple studies
take-home message: *some* probiotics help, *some* patients with *some* symptoms of IBS. degree of "help" varies