Synopsis of the course

This CME-accredited session explores the critical role of the gut microbiome in shaping response and resistance to immune checkpoint inhibitors in patients with solid tumors. It examines the impact of antibiotic use on treatment outcomes, highlighting the mechanisms underlying antibiotic-mediated resistance and the importance of antimicrobial stewardship in oncology care. The session also reviews emerging strategies to modulate the gut microbiome, including experimental approaches under investigation in clinical trials. Participants will gain insight into how microbiome-targeted interventions may enhance immunotherapy effectiveness and inform future clinical practice.

Who is Arielle Elkrief?

Dr. Arielle Elkrief is a medical oncologist and clinician-scientist at the Centre hospitalier de l’Université de Montréal and Assistant Professor at the Université de Montréal. She co-directs the CHUM Microbiome Centre, where her research focuses on the role of the gut and tumor microbiome in cancer immunotherapy and the development of microbiome-based therapeutic strategies.

What is Xpeer?

• Xpeer Medical Education is the first accredited medical education app in the market, with video microlearning engaging videos of just 5 minutes.
• With a powerful algorithm to personalize the user experience and the contents as the most popular entertaining streaming platforms, it offers a brand new experience for the continuing education and professional development of the healthcare professionals.
• Accredited by the European Union of Medical Specialists, it delivers high quality scientific medical education pieces. On Xpeer, you will find this curriculum on Microbiota and 500 hours of medical education in 2021 in your specialty, technologies and professional and personal skills.

Information on accreditation

The app Xpeer is accredited by the European Accreditation Council for Continuing Medical Education (EACCME) to provide official ECMEC credits recognized officially in 26 countries.

The credits for the users of the module will be 1 European CME credit (ECMEC®) for every hour (60 minutes of actual e-learning excluding introductions etc.) of use, provided that the users have completed a module and have passed the relevant assessment.

Vaginal birth, presence of siblings, pets, living in rural settings : all early exposures that shape immune system development. Everything appears to be decided very early: the presence in infants’ blood of IgE specific to food sensitivity is an early indicator of impaired immune tolerance and future development of allergic diseases. Could gut microbiota play a key role?

Infant-type ALDH+ Bifidobacterium strains

Researchers examined changes in the gut flora of 56 full-term Swedish infants from the ALADDIN cohort ¹ followed until age 5. While they were breastfed, their microbiota were dominated largely by Bifidobacterium species carrying the aromatic lactate dehydrogenase (ALDH) gene, including B. bifidum and B. longum, as well as B. infantis and B. breve. The ALDH enzyme catalyzes the final step in converting aromatic amino acids into aromatic lactates (including 4-hydroxyphenyl lactate, or 4-OH-PLA). These aromatic lactates modulate host immunity, although it remains unknown whether they regulate IgE.

A link between bifidobacteria and IgE

This leads to a question from the team: is this early abundance of ALDH+ bifidobacteria associated with allergic sensitization and circulating IgE? Data from 146 children in the same cohort show that ALDH+ bifidobacteria are more abundant in children without food allergen-specific IgE. Conversely, ALDH- bifidobacteria are more abundant in children with such IgE. This association is confirmed in the German PAPS cohort ².
The levels of aromatic lactates produced by intestinal ALDH+ bifidobacteria during the breastfeeding phase are therefore associated inversely with circulating allergen-specific IgE levels in early childhood.And also with atopic dermatitis at age 2.

Protective effect of a bifidobacterial metabolite

The microbial metabolite 4-OH-PLA, which specifically inhibits IgE production (without affecting IgG), could explain 40% of the inverse association between the presence of ALDH+ bifidobacteria and circulating IgE levels.
This protective association seems limited to an early immune window (before 5 months of age), when 4-OH-PLA is most abundant in the infant intestine. As solid foods are introduced, its levels drop sharply.

Therefore, an early interaction among gut microbiota, bacterial metabolites and immunity seems to influence allergic sensitivity, reinforcing the importance of the first 1,000 days. In this regard, vaginal delivery, the presence of siblings, and exclusive breastfeeding were each associated with higher 4-OH-PLA levels in stools at 2 months of age, while early introduction of infant formula reduced them. This is yet another reason to support breastfeeding when possible.

Gut–mind–pelvic axis: New insights from microbiome science

Beyond surgery, hormones and tumor staging, women with endometrial cancer often experience persistent symptoms affecting mental Beyond surgery, hormones and tumor staging, women with endometrial cancer often experience persistent symptoms
affecting mental health, gastrointestinal comfort and sexual well-being. A recent study from the University of Oklahoma suggests that the gut and vaginal microbiota may contribute to these quality-oflife outcomes.

Researchers followed 140 women scheduled for hysterectomy, including patients with endometrial cancer and women with benign gynecological conditions. Before surgery, participants completed validated questionnaires assessing physical and mental health, stress, gastrointestinal symptoms and sexual function. Vaginal and rectal samples were collected for microbiome analysis, enabling correlations between microbial profiles and patient-reported outcomes.

Women with endometrial cancer showed higher vaginal microbial diversity, a pattern usually considered unfavorable in other clinical contexts. Greater diversity was associated with increased vaginal dryness and irritation. Several bacterial species, including Lactobacillus iners, Lactobacillus gasseri and Streptococcus agalactiae, were more frequent in women reporting worse vaginal symptoms, suggesting that oncological conditions may alter vaginal ecosystem dynamics.

The gut microbiota also displayed meaningful associations. In endometrial cancer patients, certain bacterial taxa correlated with better mental well-being, lower stress levels and fewer gastrointestinal complaints, while others were linked to bloating, discomfort or reduced
sexual interest.

These findings reinforce the concept of a gut–mind–pelvic axis connecting microbial ecosystems with psychological and pelvic health.
From a clinical perspective, this work opens new opportunities for precision microbiome interventions, from targeted probiotics to dietary strategies in supportive cancer care. Microbiome profiling could help identify patients at risk of persistent symptoms and guide personalized interventions, including dietary strategies, lifestyle interventions or targeted microbiota modulation, with the aim of improving quality of life alongside standard oncological treatments.

Gautam NJ, Jimenez NR, Łaniewski P, et al. Microbiome impacts quality of life in patients with endometrial cancer and benign gynecological conditions. Qual Life Res 2025 ; 34: 2935-48.

Exercise-induced microbiota metabolite enhances CD8 T cell antitumor immunity promoting immunotherapy efficacy

Sedentary lifestyle increases cancer risk, and exercise is known to enhance immune checkpoint inhibitor (ICI) efficacy. However, the mechanisms have remained largely unknown. Gut microbiota promotes antitumor immunity and exercise modulates gut microbiota, but
whether these factors are linked has not been studied. Phelps et al. used preclinical cancer models to explore possible connections.

They found that prolonged exercise limited melanoma tumor growth without affecting body weight. Exercise also boosted CD4 and CD8 T cells in tumor-draining lymph nodes. Importantly, this effect required gut microbiota, which was involved in exercise-induced antitumor activity. To establish causality, the authors performed fecal microbial transplantations (FMTs) into antibiotic-treated mice using feces from exercised and sedentary donors. FMT from exercised mice suppressed tumor growth, prolonged survival, and enhanced tumor immunity. While bacterial cell wall components are known to increase immune responses, the exercise- FMT effect appeared dependent on microbiota-derived metabolites.

Indeed, oral administration of microbial metabolites from exercised mice’s feces restrained melanoma tumor growth. To understand the role of metabolites, the authors used targeted metabolomics of 1-carbon (1C) metabolites and found that mainly precursors of the folate-dependent 1C pathway were diminished in exercised mice. Further experiments revealed that elevated formate levels promoted
antitumor immunity and restrained tumor growth, and that exercise specifically increased formate. In addition to melanoma, these effects were observed in adenocarcinoma and lymphoma models. Formate also dramatically reduced lung metastases. Formate’s effect on antitumor immunity was mediated via nuclear factor erythroid 2-related factor-2. Ultimately, they provide some translational evidence
that high-formate-producing human microbiota is associated with enhanced tumor suppression and immunity.

Phelps CM, Willis NB, Duan T, et al. Exercise-induced microbiota metabolite enhances CD8 T cell antitumor immunity promoting immunotherapy efficacy. Cell 2025; 188: 5680-700.

Temporal dynamics and microbial interactions shaping the gut resistome in early infancy

Antibiotic resistance stems from antibiotic resistance genes (ARGs), which enable bacteria to withstand antibiotics. ARGs existed before human antibiotic use, but modern overuse has amplified their prevalence globally. When resistance reaches pathogenic microbes, it threatens public health by undermining antibiotics. However, more studies, especially in infancy are needed to understand gut resistome’s role in spreading antimicrobial resistance (AMR).

This study investigated infant gut resistome dynamics in a birth cohort with longitudinally collected fecal samples 8 times from birth up to five years of age. Early in infancy (3–6 days to 2 months), ARG richness showed a bimodal pattern, which disappeared by 6 months as most infants exhibited high ARG counts. At 12 months, bimodality reappeared, followed by a decline of ARGs at 60 months. ARG abundance relative to total genes was highest in the first 6 months and dropped after 12 months. Absolute ARG abundance varied largely between infants during the first 2 months of life, peaked at 6 months, and then dropped at 12 months.

It was further found that ARGs conferring resistance against tetracyclines, fluoroquinolones, penams, and cephalosporins were the most common and the most abundant until 6 months of age. ARGs against tetracyclines and fluoroquinolones remained the most common across all ages. ARG relative and absolute abundance did not differ between antibiotic-naïve infants and those exposed to antibiotics before the 1st sample at 3–6 days of life. Interestingly, microbial composition and birth mode seem to influence ARG diversity, while only few bacterial taxa have high number of ARGs. To conclude, this study revealed key temporal patterns and microbial interactions that shape the early-infant gut resistome, suggesting opportunities for targeted strategies to limit AMR during this critical developmental stage.

Chatzigiannidou I, Johansen PL, Dehli RK, et al. Temporal dynamics and microbial interactions shaping the gut resistome in early infancy. Nat Commun 2025; 16: 8139

Quantifying the varying harvest of fermentation products from the human gut microbiota

The gut microbiota influences the host largely through the exchange of fermentation products, mainly short-chain fatty acids produced by microbes in the large intestine. The microbes metabolize complex carbohydrates from plant-based foods, as well as dietary proteins that escape digestion in the small intestine. While metabolomics can identify a wide variety of compounds, it gives only snapshots and provides little insight into the overall flux of fermentation products that microbes produce and that host’s body takes up. To overcome this limit, this study established orthogonal approaches to quantify this flux, integrating data on bacterial metabolism, digestive physiology, and metagenomics.

This framework allowed creating many important findings. For instance, the bulk of carbon in microbiota-available carbohydrates, 90%, ended up in fermentation products, which were mostly taken up by the host. Variation in diet largely determined the total yield of fermentation products. Low yields may occur when diets are rich in highly processed foods lacking complex carbohydrates or when they include carbohydrates that resist digestion and pass through the gut unchanged. Somewhat surprisingly, the microbes themselves had less impact on the total daily fermentation product harvest, excluding some specific fermentation products, such as butyrate and lactate.

While not being the main aim of the study, it was found that mice harvest far more fermentation products per body weight than humans (≈400 mmol/kg/day vs. 7 mmol/kg/day), contributing over 21% of their daily energy needs compared to 1.7–12.1% in humans. Combined with differences in microbiome composition and gut anatomy, this disparity must be considered when extrapolating mouse data to human systemic effects. The authors conclude that due to dynamic, flowing environment of the intestine, developing this type of framework is critical to move away from assumptions based on point measurements of metabolites toward an integrated model of host-microbiome functions in health and disease.

Arnoldini M, Sharma R, Moresi C, et al. Quantifying the varying harvest of fermentation products from the human gut microbiota. Cell 2025 ; 188 : 5332-42.

Are the claims made on social media justified?

In the laboratory, under very specific experimental conditions, certain bacteria in the gut microbiota ¹ or lactic acid bacteria ² (often used as probiotics) can bioaccumulate PFAS or “sequester” them on their surface, suggesting a possible “detoxification effect.” In practice, however, ingested PFAS are almost entirely absorbed in the small intestine and rapidly enter the bloodstream. The gut microbiota is mainly
located in the colon, a compartment that is only minimally exposed to PFAS, although a small fraction of the microbiota, located in the small intestine, is directly exposed. As a result, direct contact between PFAS and intestinal bacteria remains marginal compared with the total amount of PFAS already accumulated in the body. Based on current knowledge, it is therefore difficult to conclude that the microbiota and/or probiotic intake play a major role, contrary to what is suggested in the media, in the microbial detoxification of total PFAS from the body.

What are the impacts on the microbiota?

Furthermore, this limited contact may nevertheless contribute to the increasingly documented effects of PFAS on the gut microbiota. Studies in rodents show that PFAS exposure can alter the composition of the microbiota and disrupt some of its functions, including metabolites production and interactions with the immune system ^3,4. These alterations have been shown to disrupt the balance of the digestive ecosystem and impair intestinal health.

Are there any important limitations to keep in mind?

Most of the available scientific data comes from animal models exposed to doses generally higher than those representative of human exposure, and concerns a very limited number of PFAS, often studied individually. In humans, studies remain rare, mainly observational, and influenced by many confounding factors (e.g. diet, age, sex). Research on PFAS-microbiota interactions is therefore still emerging.

What are the implications for healthcare professionals and how should they respond to patients?

Given the media coverage of the topic and patients’ concerns, it is essential to communicate on the basis of scientific evidence, without excessive extrapolation. It should be explained that there is no “miracle” solution for PFAS detoxification, particularly through probiotics.

In practice, healthcare professionals can advise limiting known sources of PFAS exposure (e.g. food packaging, non-stick cookware) and encouraging a varied, minimally processed diet rich in fruits, vegetables, and whole grains to support the protective function of the microbiota.

Acute infectious diarrhea

Current guidelines recommend certain strains of Saccharomyces and Lacticaseibacillus as the best-documented strains for acute gastroenteritis and prevention of antibiotic-associated diarrhea in children ^1-4. Randomized controlled trials and meta-analyses consistently demonstrate a clinically relevant reduction in illness duration and stool frequency when these probiotics are used together with oral
rehydration therapy. For antibiotic-associated diarrhea, both strains maintain moderate-quality evidence and favorable safety profiles, making them the most reliable options in routine pediatric care. Some Limosilactobacillus may also be considered in acute diarrhea and infantile colic, reflecting the increasing understanding that early-life microbial modulation supports immune tolerance and intestinal barrier maturation ⁵. Beyond probiotics alone, emerging studies presented at LASPGHAN explored how combinations with prebiotic substrates can accelerate the restoration of microbiota diversity after infection or antibiotic exposure. This synergistic “biotic” approach may represent a new frontier for preventing recurrence and enhancing gut recovery in children.

Functional gastrointestinal disorders and colic

Beyond acute settings, the role of probiotics in functional gastrointestinal disorders is being more cautiously defined. Some strains of Lacticaseibacillus have shown potential to reduce the frequency of abdominal pain and improve quality of life in functional abdominal pain and irritable bowel syndrome when combined with dietary and behavioral measures ^{1, 3}. The overall strength of evidence remains
modest, but emerging data suggest a role for specific strains as adjuvants within a multimodal management approach. In functional constipation, Lacticaseibacillus may also be used as an adjunct to standard therapy, while in infantile colic it remains the most consistently recommended strain. Preventive use of Lacticaseibacillus from birth to four months has been associated with lower incidence of colic in high-risk infants ². Strain selection should always consider patient phenotype, symptom pattern, and concomitant interventions such as fiber intake or behavioral therapy. Integrating probiotics or synbiotics into a holistic management strategy—rather than using them as isolated supplements—emerges as a key principle for optimizing outcomes in functional gastrointestinal disorders.

Food allergy and mucosal immunity

Increasing attention has been directed to the interaction between microbiota and immune development. Lacticaseibacillus administered for at least three months may help promote tolerance and clinical improvement in cow’s-milk protein allergy ^{1 ,2}. These findings are biologically plausible given the immunomodulatory properties of the strain and its capacity to influence epithelial and cytokine responses. Nevertheless,
further studies are needed to validate the magnitude and durability of these effects in long-term allergic outcomes.

Necrotizing enterocolitis prevention

Prophylactic use of certain strains of Lacticaseibacillus greater than or equal to 30 days remains supported for reducing necrotizing enterocolitis and mortality, provided that product quality, strain identity and clinical surveillance are ensured in preterm infants ¹. The benefit appears to be strain-dependent, underlining the need for standardized formulations and microbiological traceability in neonatal care. Recent evidence highlights how early microbial exposure—through vaginal delivery, breastfeeding, and dietary diversification— modulates mucosal immunity and oral tolerance. The concept of the “first 1,000 days” remains a crucial window for interventions aiming to prevent allergic and inflammatory diseases. Growing data on postbiotics, non-viable microbial products with signaling and anti-inflammatory potential, are opening new therapeutic perspectives in allergy prevention and immune education.

What do we already know about this subject?

Attention deficit hyperactivity disorder (ADHD) is common in paediatrics, with a 3-5% prevalence. Various environmental factors play a role in the development of ADHD, including perinatal, social-emotional and nutritional factors. These risk factors also influence the gut microbiota (GM). Early alterations can disrupt neurological development. These effects could be mediated by the microbiota-gut-brain axis through three pathways (immune, neuronal and endocrine/systemic).

Figure 1 - Specific differences in bacterial taxa between each of the ADHD groups.

What are the main insights from this study?

One hundred and eighty-eight (188) children were included in a single-centre Chinese study, including 94 children with ADHD who were divided into three subgroups, 56 with predominant inattention symptoms (group IA), 9 with predominant hyperactivity-impulsivity symptoms (group HA) and 29 with combined symptoms (group C), and 94 controls (group TD).

The GM analysis found no difference between these three groups in terms of bacterial richness and diversity, the relative abundance of the 10 main genera and the Firmicutes/Bacteroidetes ratio. However, specific taxa were associated with ADHD subtypes (figure 1).

The GM analysis found no difference between these three groups in terms of bacterial richness and diversity, the relative abundance of the 10 main genera and the Firmicutes/Bacteroidetes ratio. However, specific taxa were associated with ADHD subtypes (figure 1).
The family Lactobacillaceae and species Lactobacillus sanfranciscencis were enriched in the TD group. The TD group had more Bifidobacteriales than group C. In groups IA and C (inattention symptoms), the genus Streptococcus and species Streptococcus salivarius were identified as harmful bacteria, while family Flavobacteriaceae, genus Paraprevotella, genus Fructilactobacillus, species Paraprevotella xylaniphila, and species Phocaeicola salanitronis were beneficial. In groups HA and C (hyperactivity-impulsivity symptoms), the family Lactobacillaceae and species Lactobacillus sanfranciscencis were beneficial (figure 1).

The metabolomic profiles of ADHD children revealed disturbances in fatty-acid synthesis, with a significant reduction in the synthesis of unsaturated fatty acids and linoleic acid, as well as amino-acid metabolism (figure 2). The various symptoms, inattention, hyperactivity and impulsivity, were negatively correlated with different metabolites, imidazoleacetic acid and inattention (p<0.001). Thus, Lactobacillus
sanfranciscencis had a direct effect and an effect mediated by imidazoleacetic acid on inattention.

An experiment with three cycles of faecal microbiota transfer (FMT) from children with ADHD and a low abundance of beneficial Lactobacillus sanfranciscencis bacteria (FMT-A group) and controls (FMT-H group) was conducted in four-week-old male C57BL/6J mice. The mice then received either Lactobacillus sanfranciscencis (1 x108 organisms/mouse/day, group FMT-A-R1), or 150 mmol/L sodium acetate (group FMT-A-R2) or PBS for the controls (group FMT-A-C). Hyperactivity symptoms improved significantly in the FMT-A-R1 group but not in the FMT-A-R2 group, while inattention symptoms improved in both groups. Finally, fatty-acid and unsaturated
fatty-acid synthesis was improved in the FMT-A-R1 and FMT-A-R2 groups versus FMT-A-C (figure 3).

Figure 2 - Alteration of the different metabolic pathways, size of the circles corresponding to the number of genes involved.

Figure 3 - Faecal transplantation with stools from ADHD children reduces fatty-acid synthesis, which is corrected by treatment with Lactobacillus sanfranciscencis or sodium acetate.

What are the consequences in practice?

This study showed that GM alterations in ADHD children were symptom-dependent. The sub-group analysis showed that family Lactobacillaceae and species Lactobacillus sanfranciscencis were enriched in the TD group. In addition, after faecal transplantation with ADHD stools, treatment with Lactobacillus sanfranciscencis improved symptoms.

Fatty-acid and unsaturated fatty-acid synthesis was reduced in ADHD children. Similarly, after faecal transplantation with ADHD stools, treatment with short-chain fatty acids (imidazoleacetic acid) led to an improvement in symptoms.

What do we already know about this subject?

CRC is the third most frequent and the second most lethal tumour type worldwide ². CRC originates in the epithelial layer of the proximal colon (right colon) or distal colon plus rectum (left colon). Progression from benign precancerous lesion (adenoma) to a malignant tumour (carcinoma) may take several years and is characterised by an accumulation of tumour-cell mutations, alteration in the gut mucosal barrier and intestinal inflammation.

The gut microbiome is proposed as one of the important hallmarks of cancer. Certain microbes have been put forward as major contributors to carcinogenesis, particularly Escherichia coli pks+ and Fusobacterium nucleatum ³. Several studies have observed distinct microbiome signatures in CRC patients when compared with patients with adenomas or healthy controls ⁴. A few metagenomic studies also investigated
microbiome changes along the adenoma-carcinoma sequence and based on the primary neoplasia location, and have suggested links between CRC and oral species. Further evidence points toward the enrichment of oral-typical microbes and of oral biofilm-forming species
in the gut metagenomes of patients with proximal CRC. However, no metagenomic studies have gone beyond characterising already well-known strain-specific factors that influence CRC risk, and no untargeted searches for subspecies and strain-level genomic associations with CRC phenotypes are available.

What are the main insights from this study?

Using 3,741 samples from 18 cohorts and applying new strain-level computational methodologies, the authors investigated the links between faecal microbiota and CRC. They improved CRC prediction accuracy based solely on gut metagenomics, with an average area under the curve (AUC) = 0.85. The five SGBs (Species- level Genome Bins) assigned to the species F. nucleatum were more abundant
in CRC than in the controls: F. nucleatum subsp. animalis, vincentii, nucleatum, polymorphum. This was in addition to other well-characterised CRC-associated microbes such as Parvimonas micra and Bacteroides fragilis. The authors also identified 19 additional uncharacterised SGBs with neither cultivated strains nor taxonomically defined species, which highlighted a more complex CRC-associated
microbial signature than previously thought.

Although interstage microbiome variations during CRC progression are not as strong as those observed between CRC and controls, the authors found several biomarkers for advanced and metastatic CRC, as well as several microbial species consistently and monotonically increasing (or decreasing) from control to cancer or advanced disease. In particular, latestage CRC was enriched in oral-derived species, such as P. micra, already involved in the stimulation of tissue invasion pathways and Hungatella hathewayi, which was shown to promote intestinal cell proliferation in in-vitro experiments (figure 1). Compared with the other stages, metastatic CRC had a higher abundance of Methanobrevibacter smithii, supporting previous findings linking methane producers with stage IV CRC. Stool samples from patients with CRC originating in the right or transverse colon were also enriched in oral species.

These findings strengthen the notion that the number and cumulative abundance of orally derived species are significantly higher in CRC samples than controls and adenomas, but also show that later stages of CRC were particularly enriched in oral species. However, many non-oral bacteria were also associated with CRC, including species previously associated with high cardiometabolic risk. Interestingly, adenoma and later cancer stages were enriched in species linked with poor cardiometabolic health and immune-mediated diseases.

What are the consequences in practice?

This is the largest and most accurate study ever carried out on faecal microbiota associated with CRC. A signature based solely on faecal microbiota offers a relatively good predictive value. In addition, differences between early and late stages have been identified. However, it should be noted that the study did not identify sufficiently accurate markers for the diagnosis of pre-cancerous lesions (adenoma). This work paves the way for microbiota-based tests for diagnosing CRC, but their use in clinical practice still requires validation and, above all, the detection of pre-cancerous lesions needs to be improved.

Figure 1 : Microbial signature according to CRC stage and location of primary tumour.

From nociperception to chronic pelvic pain

Pain is defined as an unpleasant sensory and emotional experience, which may or may not be associated with tissue damage. It is referred to as chronic when the pain persists for over three months, resists treatment and leads to changes in functional abilities and social relationships. Chronic visceral pelvic pain is characterised by deep, throbbing and diffuse pain, making its diagnosis complex and imprecise. This type of pain is particularly common in women and may require input from several internal-medicine specialists, i.e. gastroenterology, gynaecology and urology.

Pelvic pain can be caused by several types of organ lesions (e.g. endometriosis) or functional syndromes characterised by visceral hypersensitivity (e.g. irritable bowel syndrome (IBS) or interstitial cystitis). A combination of several painful pelvix condition is often observed in a single patient. Other patients may experience debilitating pain for which no specific cause has been identified. Internal-medicine specialists usually analyse pain as the expression of a single lesion. While it is certainly necessary to treat the injured organ, this may sometimes not be enough to provide relief to patients with chronic pelvic pain. Some patients present with an extensive set of symptoms, combining sensitivity disorders with issues affecting several pelvic organ functions at the same time. Such phenomena are linked to sensitisation mechanisms that appear several months or years after the onset of pain ¹. This type of sensitisation is characterised by a lowering of sensitivity thresholds leading to increased pain or pain provoked by stimuli of normally non-nociceptive intensity (e.g. intolerance to the sensation of rectal filling). Pain spreading over time has also been described. In fact, the painful sensation can persist despite the lack
of stimulation (e.g. post-defecation pain). Finally, pain spreading beyond the stimulated area is also observed (e.g. pain during rectal filling causing bladder pain).

On a pathophysiological level, a distinction is made between psychological, adaptive and chronic pain. Acute pain occurs following an initial painful stimulus (heat, pressure, pH variation or algogenic substances) which activates receptors at the endings of the peripheral nociceptive fibres. In the case of the viscera, sensory innervation occurs in the spinal nerves whose endings are located in the muscles and/or mucosa of organs with cell bodies in the dorsal root ganglia ². Under physiological conditions, activation of Aδ or C afferent fibre receptors generates an action potential (AP) that is translated into nociceptive information in the dorsal root ganglia. This information is then transmitted to the second-order spinal neurons in the dorsal horn. Second-order neurons transmit information to the thalamus via the spinothalamic and spinoreticulothalamic pathways. In the thalamus, the third neuron transmits the message to various areas in the cortex (prefrontal, cingulate,
somatosensory or insular cortex). This third neuron is responsible for transforming nociception into pain, with its affective-emotional, sensitive, cognitive and behavioural dimensions.

This hypersensitivity may be partly due to gut permeability alterations, thereby increasing the passage of food or bacterial antigens, leading to inflammatory mechanisms with mast-cell recruitment and release of pro-inflammatory mediators such as histamine or proteases. This inflammatory environment could contribute to hyperexcitability in the nociceptive fibres, which in turn contribute to maintaining the inflammatory microenvironment by releasing neuropeptides (Substance P: SP and Calcitonin-Gene Related Peptide: CGRP). These repeated stimuli trigger phenotypic and excitability alterations to the nociceptive neurons in the Dorsal Root Ganglion (DRG), known as peripheral sensitisation. In the spine, repeated APs from DRG neurons increase the release of excitatory neurotransmitters such as glutamate ⁴. In the long term, this leads to synaptic reinforcement by increasing the number of glutamatergic receptors, combined with dysfunction of the inhibitory systems, resulting in sensitisation of the spinal neurons ⁴. Thus, central sensitisation is a pathological state of nociception function linked to its dysfunction, with a strengthening of facilitatory systems and lowering of pain-inhibitory systems.

The microbiota, a potential pain modulator

The factors responsible for visceral fibre hypersensitivity are still poorly understood, but the gut microbiota could play a role (figure 1).

Figure 1 : Mechanisms of abdominal pain

The number of bacteria colonising the human body (3.8∙1013) is roughly the same as the number of human host cells in adulthood (3.0∙1013) ⁵. The gut microbiota plays a key role in the two-way communication between the gut and various organs, including the brain. For several years this has been referred to as the microbiota-gut-brain axis. In the past, studies have typically focused on the role of the microbiota in gastrointestinal disorders (irritable bowel syndrome, inflammatory bowel disease). More recently, it has been recognised that dysfunction of this axis is involved in the pathophysiology of many other conditions, including metabolic diseases (obesity, diabetes) and neurological diseases (autism, Parkinson’s disease, depression).

The gut microbiota and the brain communicate with each other using several pathways such as the vagal nervous system, the immune system and the humoral pathways, after modulation of enteroendocrine functions. Nociceptive afferent fibres can also be directly modulated by various bacterial metabolites ⁶. The main mediators identified are bacterial metabolites (e.g. short-chain fatty acids, secondary bile acids), neurotransmitters and neuromodulators (e.g.: GABA) and bacterial products (e.g.: PAMPs, tryptophan derivatives). Some molecules increase neuronal excitability by activating nociceptors, producing Nerve Growth Factor (NGF) and increasing local inflammation. Others,
however, have the opposite effect (e.g.: GABA) and can inhibit the transmission of the nociceptive message by producing endogenous opioids or anti-inflammatory mediators.

Clinical examples: irritable bowel syndrome and endometriosis

Irritable bowel syndrome

IBS is characterised by chronic functional intestinal disorders, mainly combining abdominal pain and bowel disorders (diarrhoea, constipation or both in alternation). This condition affects around 5-10% of the population, primarily young adult females. The pathophysiology of IBS is not yet fully understood but it has been extensively documented that gut-brain-communication changes lie at the root of digestive motility disorders and visceral hypersensitivity. On a central level, IBS patients experience impaired information processing,
hypervigilance and increased anxiety. In recent years, the gut microbiota has been proposed as one of the causative factors in IBS ⁷ and several studies have highlighted changes in the composition and diversity of the microbiota in IBS.

A reduction in the abundance of the genus Bifidobacterium was also found in stool and mucosal samples from IBS patients, along with an increase in the genus Bacteroides. An increase in pathogens (i.e. Escherichia coli and Enterobacterium families) was also observed. An enrichment of certain bacterial taxa such as Enterobacteriaceae, Streptococcus, Fusobacteria, Gemella and Rothia, and a depletion of bacterial genera recognised as beneficial to health, such as Roseburia and Faecalibacterium, were also observed in IBS patients. In IBS, the causal role of these kinds of microbiota changes has been strongly suggested in view of the ability to induce some symptoms in
preclinical models after the transfer of stools from IBS patients ⁹. In addition, the role of several mediators produced by the intestinal microbiota, such as LPS, short-chain fatty acids and secondary bile acids, has been suggested in chronic abdominal pain, visceral hypersensitivity and gut inflammation ¹⁰. The gut microbiota could therefore be a major cofactor in chronic abdominal pain and associated
inflammation.

Endometriosis

Endometriosis is defined as the transplantation of endometrial cells outside the uterine cavity, which may be facilitated by chronic pelvic inflammation. The main symptoms of endometriosis are chronic pelvic pain, gastrointestinal problems and infertility. A systematic review analysed studies of the gut microbiota in women with endometriosis and chronic pelvic pain ¹¹. A total of 28 clinical studies and six animal studies were included in the review. In these human and animal studies, increased gut microbiota diversity was observed in the endometriosis groups. However, there was no clear consensus on the composition of the microbiota associated with endometriosis. None of the studies analysed composition or diversity based on the characteristics of pain.

In the study by Yuan, no early-stage differences were observed after endometriosis induction versus a control group. Differences appeared 21 days after the start of the experiment and increased thereafter, with decreased diversity and richness of the microbiota in the endometriosis group, increased genera Bifidobacterium, Proteobacteria and Verrucomicrobia, decreased Bacteroidetes and Firmicutes phyla and an increase in the Firmicutes/Bacteroidetes ratio, reported ¹². Conversely, treatments targeting the microbiota, such as broad-spectrum antibiotics, reduced the volume and weight of endometriosis lesions, induced a decrease in endometrial
lesion cell proliferation and inflammatory markers (cytokines, macrophages) ¹³. Apart from inflammation, the gut microbiota could contribute to the pathophysiology of endometriosis through its role in regulating oestrogen metabolism. A specific microbiota called the estrobolome actually plays a central role in hormone regulation, especially oestrogen ¹¹. Estrobolome contains bacteria that produce beta-glucuronidase,
which is an enzyme able to modify the active form of oestrogen. Gut microbiota disturbances could therefore lead to increased circulating oestrogen levels and favour the development of endometriosis. However, these studies did not investigate pain modulation.

Therapeutic prospects via the microbiota

The development of therapeutic approaches modulating the composition or function of the gut microbiota are becoming increasingly recognised as playing a complementary role in the current therapies used for the management of chronic functional digestive or pelvic conditions. The basic aim of these approaches is to restore a balanced and functional microbiota (via dietary interventions or prebiotics), or provide bacteria with beneficial effects for the host (probiotics). Specific combinations of probiotics, or specific species and strains ¹⁴, appear to have beneficial effects on overall IBS symptoms and abdominal pain [15]. Symbiotic approaches combining pre- and probiotics are also used. Approaches based on the transplantation of faecal microbiota also suggest a therapeutic potential in functional diseases such as IBS ¹⁶. In addition, an increasing number of studies are suggesting that the effectiveness of the response to faecal-microbiota transplantation in IBS or to probiotic treatments is influenced by the composition of the microbiota of the recipient.

In general, the effectiveness of these approaches in the management of pelvic functional disorders, although demonstrated in some cases, remains limited by the small population sizes in the studies and, above all, by the wide diversity and variability of symptoms observed in these disorders, as shown in a recent literature review ¹⁵.