The impact of the gut microbiota on the metabolism of some drugs has already been reported: it could lead to the activation, deactivation, or increased toxicity of some compounds (sidenote: Some compounds Among others, activation of sulfasalazine, deactivation of digoxin, and increased toxicity of irinotecan ) *. However, the scope and mechanisms of this impact remain largely unstudied. An American team opened the way by studying the disposition of 271 drugs administered orally–except antibiotics–under the effect of 76 species or strains mainly from the human gut microbiota.

Selective bacteria

Primary trials have been conducted in vitro by incubating drugs and bacteria during 12 hours. Two thirds of drugs were metabolized at more than 20% and by at least one bacterial strain (each strain metabolizes between 11 and 95 drugs). Omeprazole, sulfasalazine, risperidone, or even lovastatin, were among the most targeted drugs, thus confirming previous results. Moreover, some chemical components seem to be the preferred targets of this bacterial metabolism: for instance, ester or amide groups of therapeutic compounds are the favorite target of Bacteroidetes. In the specific case of dexamethasone (a glucocorticosteroid), trying to associate a bacterial species to the drug was not very relevant: it is necessary to identity the genes directly associated to the enzymatic conversion. According to the authors, the experiment suggests that it would probably be the case for other glucocorticosteroids (prednisolone, prednisone…).

Combined biotransformations

The next step was performed in vivo and consisted in identifying the genetic markers of the observed bacterial biotransformations. This approach was first validated with the bacterium Bacteroides thetaiotaomicron. 16 other enzymes derived from this bacterium were identified, metabolizing 18 drugs into 41 metabolites. Extending this method to the 76 bacteria selected in this study showed that the transformation of a drug could require the combined effect of enzymes from several species; for example, tinidazole is metabolized by three different species. In total, the team identified 30 enzymes derived from the gut microbiota which, together, transform 20 drugs into 59 different metabolites. This confirms that pharmacokinetics is related to gut bacteria. One more step towards a tailored therapeutic approach capable of anticipating the response of an individual to a given treatment based on his/her microbiotic profile.

The main FGID in infants is colic. This disorder, whose pathophysiology is poorly understood, could originate in the microbiota and is thought to warrant new therapeutic approaches, as the efficacy of standard treatments has proven variable from one individual to another.

A DISEASE WITH IMPRECISE BOUNDARIES

Baby colic has an estimated prevalence of 5% to 28% depending on the study, and is a benign syndrome characterized by recurring bouts of crying, often accompanied by physical symptoms: clenched fists, straightened legs, facial redness. Appearing classically at around two weeks old, it reaches peak severity between 5 to 8 weeks and resolves spontaneously at around the age of 4 months. Its pathogenesis is still unclear, and diagnosis is currently based on Rome IV criteria. Organic causes are thought to represent only a small proportion of causes involved (5%). Additional factors such as an allergy to cow’s milk protein, family tensions and anxiety, etc. are likely to play a part.

CURRENT TREATMENTS

The diversity of causes makes patient care complex and encourages the diversification of treatment options, rendering treatment non-specific. What are the main current approaches? Drugs (mucosal protective agents, antispasmodics…), diet (modified diets, especially formulas based on casein hydrolysate, whey or soy milk…), behavioral techniques (chiropractic, reduced stimulation of the child…) and some probiotics.

INNOVATIVE ETIOLOGICAL HYPOTHESES INVOLVING THE MICROBIOTA

An international team has proposed three etiological hypotheses which could lead to new therapeutic approaches: first of all, immaturity of the enterohepatic circulation and of the action of bile acids leading to malabsorption of fats and other nutrients, as well as possible side effects on the intestinal microbiota.⁸ Secondly, intestinal dysbiosis, triggering an increase in nutrient fermentation and reduced levels of dehydroxylated bile acids in the colon. Finally, immaturity of the enteric nervous system resulting in abnormal sensorimotor function in the intestines and colon. The future characterization of these three mechanisms, which display numerous potential interactions, could lead to a more specific diagnosis and personalized management based on targeted biomarkers.⁸

GUT-BRAIN: BIDIRECTIONAL COMMUNICATION

Psychological and psychosocial factors are key to understanding the pathophysiology of FGIDs. Psychic disorders (anxiety, depression, neurosis…) are frequent comorbidities in patients with FGIDs. It is however difficult to determine whether the former generate the latter, or if it is the opposite.^4,5,6 Recent studies clearly concluded that bidirectionality is at play, i.e. a reciprocal influence. At a visceral level, exchanges are based on the enteric nervous system and substances produced by intestinal bacteria (SCFA, metabolites…). At a central level, the involved structures are those of the emotional motor system (anterior cingulate cortex, hippocampus, hypothalamus…).^4,5,6

ROLE OF THE GUT-BRAIN AXIS IN IBS

Animal models have revealed that bidirectional communication was disrupted in patients with IBS, although mechanisms ensuring communication between microbiota and brain have not been elucidated. However, several elements that appear to contribute to this mechanism have been identified: the microbiota sends signals to the CNS through enteroendocrine cells (release of serotonin), dendritic cells and B-cells (release of cytokines), products of bacterial metabolism (SCFA, GABA…) and stimulation of vagal afferent fibers. In the other direction, stress and feelings affect the composition of the microbiota through stress hormones and sympathetic nervous system.^4,5,6

CHANGE IN STATUS

Viewed as shameful in many cultures, functional GI disorders were long regarded as a private matter and related to stress and feelings rather than an easily identifiable organic trouble. The perception changed in the years 1960’s thanks to technical and scientific advances: works on etiology and pathophysiology provided a foundation for an organic understanding of FGIDs. In the following decades, the “all physiological” vision was gradually abandoned and the related psychosocial processes were deepened, before the standard modern biopsychological model was ultimately reached.

ROME, CRADLE OF CLASSIFICATION

In the fourth edition of its work of reference (Rome IV), the Rome Foundation proposes a definition of FGIDs collaboratively designed by a panel of experts: they include disorders of the gut-brain axis, i.e. a “group of disorders classified based on gastrointestinal symptoms, related to a combination of the following: motility disturbance, visceral hypersensitivity, deterioration of the mucosa and immune functions, change in intestinal microbiota and alteration of central nervous system functions”. The result of that review is a precise categorization of FGIDs as well as a rationale for their study and treatment.

53 DIFFERENT TYPES OF FGID

The Rome IV classification is based on symptomatic criteria grouped by anatomic region (esophageal, gastroduodenal, intestinal, biliary and anorectal). However, symptom location by itself is not enough, especially regarding irritable bowel syndrome (IBS), functional dyspepsia, or abdominal pain syndrome (hard to place and influenced by global effects resulting from deregulation of signaling pathways between central and enteric nervous systems) mediated by the central nervous system. Its 33 items for adults and 20 for newborns, children and teenagers ensure a precise diagnosis and facilitate the implementation of targeted patient care. In this regard, the Rome Foundation insisted on the importance of not limiting therapeutic approaches to medication alone and recommends a biopsychosocial approach to face the variability of cases and individual expectations of patients suffering from FGIDs.

MICROBIOTA: A MAJOR INTESTINAL PLAYER

The intestinal microbiota has a complex influence on the metabolism, nutrition and immune functions of the host. Its alteration plays a central role in FGIDs.^3,4 Dysbiosis has been specifically studied regarding IBS and studies in animals have shown that this imbalance might be involved in the observed visceral hypersensitivity (via endoluminal bile acids), as well as in gastrointestinal dysmotility through the expression of enzymes involved in the synthesis of neuromodulators (gamma-aminobutyric acid [GABA], for instance) and products of colonic fermentation (gas or short-chain fatty acids, SCFA). Finally, dysbiosis seems to promote the disruption of the intestinal barrier: increased intestinal permeability would thus improve the crossing of bacterial antigens responsible for low-grade inflammation leading to sensitization of sensory afferent fibers of the enteric nervous system.^3,4

PROMISING APPROACH

Heterogeneity of FGIDs and contradictory results in terms of bacterial composition depending on studies and methods do not allow microbiota and its metabolites to be used as relevant markers for diagnosis, monitoring of the progression of the disease, or treatment response. Literature confirms however the importance of diversity and composition of the intestinal microbiota in the pathophysiology of FGIDs, and consequently the potential impact of approaches related to modulations of intestinal bacterial populations.

Although alcohol abuse is detrimental to health, the discussion regarding the impact of moderate consumption is still ongoing, as well as the negative impact of alcohol on the oral flora, according to recent works. In a new study, Mexican researchers tested the effects of beer (with and without alcohol) on the gut microbiota.

A beer at lunch to keep the doctor away?

For a month, around thirty volunteers were told to drink one 335-ml non-alcoholic beer (0.5º) at lunch, without changing any other dietary habit. Five months later, they had to follow the same protocol but with a classic beer (4.9º). On Days 1, 15 and 30 of the experiment, samples of blood and stool were collected and a series of body measurements (hip circumference, body mass index, blood pressure...).

Non-alcoholic beer provides more benefits

Bad news for hop lovers, but maybe also some good news... On the plus side, both types of beer led to a strong increase in the proportion of Bacteroidetes and decrease in species from the Firmicutes phylum; and such a ratio is also found in healthy people. However, the match between non-alcoholic beer and alcoholic beer turned in favor of the former: no weight gain, no increase in hip or waist circumference, no change in liver enzymes or blood lipids, and even decrease in blood sugar levels coupled with a better insulin resistance. But let’s be fair play: all these parameters remained within the normal range for drinkers of “true” beer.

A new superfood?

At the intestinal level, non-alcoholic beer also has the upper hand: the bacterial flora is more diversified and with a higher content in beneficial bacteria such as lactobacilli (barrier against obesity and insulin resistance in mice), Streptococcus (promoting the regulation of immune reaction), and other types of bacteria associated with weight loss in humans. One month after the beginning of the experiment, non-alcoholic beer also tripled the amount of bacteria capable of producing polyphenols–already present in beer (with or without alcohol)–and phenolic acids such as resveratrol, which are potentially beneficial components against cancer, diabetes or even neurodegenerative diseases. A convincing argument in support of happy sobriety.

Through which mechanisms does the microbiota influence the efficacy of chemotherapy or immunotherapy?

We have shown that chemotherapy leads to intestinal permeability, which facilitates the spread of bacteria into the immune system. Although this downside is the source of many adverse events (nausea, diarrhea, vomiting), it is also paradoxically very useful since it stimulates the immune system and boosts the effect of the antitumor drug.

As for immunotherapy, its aim is to mobilize the immune system against tumors, and its success also seems to depend on the intestinal microbiota which acts on three main mechanisms:

• Its composition impacts the distribution of lymphocytes (white blood cells) in the digestive tube, and thus impacts the endogenous defense system, as was recently shown in several articles.
• Some of its metabolites could activate the immune system, although it has yet to be demonstrated.
• It activates mechanisms of intestinal barrier repair which contribute to treatment efficacy.

How are these findings going to change the management of cancers treated by chemotherapy or immunotherapy?

Probiotics used as complement to the existing therapeutic arsenal (surgery, radiotherapy, chemotherapy, hormone therapy, immunotherapy) could become a sixth therapeutic modality against cancer. When there is no infection, we will avoid prescribing prophylactic antibiotics (preventive treatment), while in case of infection, we will postpone immunotherapy. Our objective is to identify patients with an intestinal dysbiosis and to restore their microbiota before starting immunotherapy or chemotherapy.

What are the research avenues currently considered to modulate the composition of the intestinal microbiota?

Probiotics administration and fecal transplant (transplant of the microbiota from a healthy individual into the patient’s digestive tract) are the main avenues under investigation to restore a microbiota which is likely to hinder treatment. These bacteria, called “oncomicrobiotics” are not intended to increase the efficacy of chemotherapy or immunotherapy on their own. Their aim is to prepare the organism of a patient with dysbiosis to respond positively to these immunomodulating treatments. More and more biotechnology companies dedicate part of their research efforts to the development of “anticancer” probiotics. However, only multidisciplinary research followed by large-scale clinical trials will be able to identify which are the “friendly” bacteria and to assess their efficacy within the context of chemotherapy or immunotherapy. We can expect progress in the foreseeable future.

Before menopause, estrogens are synthesized by ovaries and later, other tissues take over (fatty tissue, brain, hypothalamus). A portion of estrogens produced are subject to chemical reactions of detoxification by the liver (which makes molecules harmless to the body) and then excreted in the bile. They are then transferred to the intestines where they are deconjugated by the microbiota before being reabsorbed by the tissues or released into the blood flow. Depending on the composition of the microbiota, this reabsorption means that hormonal metabolites with differentiated estrogenic activity are released back into the blood flow. It seems that breast cancer risk is contingent, as least partly, on the nature and ratio between metabolites and estrogens.

This “deconjugation” is led by bacterial genes, mainly an enzyme which is involved in the degradation of complex sugars and whose activity can be modulated by diet and by the intestinal microbiota. Blocking the actions of this enzyme could thus decrease the level of active estrogens released back into the blood flow and reduce breast cancer risk. This is precisely the hypothesis that a team of American scientists is currently testing in mice.

Breast microbiota

Some researchers also discovered a microbiota in the breast tissue. Its composition, and more specifically the abundance or lack of specific bacterial families, seems to be different whether the host is suffering from breast cancer or not. Other researchers made a similar finding in the intestinal microbiota, whose composition seems to vary depending on the stage of cancer. The alteration of the intestinal microbiota (dysbiosis) as a starting point for breast cancer is an avenue seriously considered by researchers

Is there a link between the different microbiotas?

At this time, all these assumptions are leads worthwhile to be investigated. Further research should focus on determining if there are links between the different microbiotas, and whether these links lead them to act together to generate an environment favoring the development of breast cancer.

KEY FIGURES - BREAST CANCER:

Many women complain about dry mouth during menopause. According to an international team of researchers¹², this lack of saliva, associated to the drop in estrogen levels, could have harmful effects such as changes in composition of the oral microbiota, dysbiosis (imbalance of the microbial flora) and onset of inflammatory disorders (e.g. gingivitis or periodontitis) which create a loss of tooth support that potentially leads to tooth loss.

Drop in estrogen levels and diseases

Since female sexual hormones affect the composition of the different body floras, especially the intestine microbiota, a drop in estrogen levels impairs the microbial balance and promotes the onset of autoimmune diseases. This could explain why some autoimmune diseases affect women more than men (lupus, Sjögren syndrome,rheumatoid arthritis) or why they occur at specific moments of their hormonal lives, i.e. after their menstrual period or during their reproductive period (asthma). Moreover, microbiota alteration due to estrogen deficiency leads to metabolic changes. The one most feared by women is abdominal weight gain, which is a proven risk factor for type 2 diabetes. Finally, intestinal microbiota could affect breast cancer risk through its effects on estrogens produced by fatty tissue in menopausal women.

Composition of oral and/or intestinal microbiota and estrogen deficiency have been successfully linked to all these disorders that are likely to occur during menopause. That is why researchers are encouraged to continue investigating and study the effect of prebiotics and probiotics and their potential use as a monotherapy or in combination with HRT.

To prevent the risk of osteoporosis and fracture, menopausal women can decide between HRT or a treatment combining calcium and vitamin D. But in the US, where the types of hormones and their doses are different from those used in France, THM remains controversial since it is suspected to increase the risk of some female hormone-dependent cancers, including breast cancer. What are the alternatives?

According to a Chinese study¹⁰, probiotics associated with isoflavones (naturally occurring plant compound) could be a low-risk and effective alternative treatment for osteoporosis.

Efficacy of probiotics has been demonstrated in mice

Non-clinical tests have shown that the intestinal microbiota plays a role in bone metabolism regulation. First possible mode of action: through an interaction with the immune and/or endocrine (i.e. hormones) systems, both involved in bone metabolism. Second possible mode of action: by aiding calcium absorption, a mineral essential to bone formation and strength. The microbiota could thus promote bone formation as well as limit bone loss, although to a lesser extent.

All these results have led researchers to assess the benefits of probiotics to prevent osteoporosis and evaluate their efficacy in an animal model. Probiotics play a role at two levels: on the one hand, they increase microbiota diversity by restoring the intestinal barrier and modulating the immune response; and on the other hand, they stimulate calcium absorption and production of estrogen-like compounds. These promising results still have to be confirmed in women.

Combination with isoflavones

Isoflavones are compounds naturally found in some plants, especially soy and red clover. They have anti-osteoporotic effects which are boosted by the concomitant administration of probiotics. They mimic certain modes of action of estrogens while counteracting others, i.e. they limit disorders associated with menopause while still protecting against breast cancer. A Danish study¹¹ showed that the combination of isoflavones and probiotics associated with calcium and vitamin D supplementation was more effective than calcium and vitamin D supplementation alone to reduce osteopenia (loss of bone density). As a monotherapy or as a combination therapy with an anti-osteoporosis drug, probiotics could offer an alternative treatment to women who would rather use natural remedies to limit the progression of osteoporosis.

Key figures - Osteoporosis: