on October 26, 2026
Beyond the gut: the microbiota as a communication system
Microbiota research is progressively changing the way the relationship between the gut and the body is interpreted.
The gut microbiota is not an isolated compartment. Its metabolites can interact with the immune system, metabolism, the nervous system, and epithelial barriers. From this communication network arise concepts that are widely studied today, such as the gut-brain axis, gut-skin axis, and gut-joint axis.
Probiotics represent one of the tools through which these interactions are sought to be modulated. But it is precisely the expansion of their possible applications that makes it even more important to distinguish between biological hypothesis, preclinical result, and clinical efficacy.
A probiotic capable of producing a metabolite in the laboratory is not necessarily able to modify that same pathway in the human body. Similarly, a result observed in a particular strain cannot automatically be attributed to all microorganisms belonging to the same species.
Microbiota medicine is therefore evolving toward a more precise paradigm: not "probiotics against a disease", but specific microorganisms, in specific biological conditions, for specific goals.
Probiotics and gut-brain axes
The bidirectional communication between the gut and the brain involves the enteric nervous system, the vagus nerve, the immune system, microbial metabolism, neurotransmitters, and stress hormones.
This network has led to the development of the concept of psychobiotics, a term used to describe microorganisms or microbiota-directed interventions capable of potentially influencing psychological and neurological functions.
Some gut bacteria can produce or modulate neuroactive molecules, including GABA, serotonin, and other metabolites involved in neuroimmune communication. These mechanisms represent one of the biological bases of interest in probiotics in the context of mood and stress.
However, a plausible mechanism does not equal an antidepressant treatment.
A meta-analysis published in 2025, including 72 randomized studies and over 6,000 participants in total, found statistically significant reductions in depression, anxiety, and sleep disorder scores with interventions including probiotics, prebiotics, and synbiotics. However, the authors highlighted the high heterogeneity of the studies and the not always high methodological quality, emphasizing the need for larger and longer-lasting trials.
Even a meta-analysis specifically focused on strain specificity showed an uneven picture: some studies highlighted a reduction in depressive symptoms, while other clinical scales did not detect significant differences. This result reinforces the idea that the effect cannot be generically attributed to the "probiotics" category.
Therefore, the probiotic should not be considered a substitute for psychological or pharmacological therapies when they are indicated. The possible clinical interest concerns above all its complementary role, which is still under research.
Bifidobacterium and neurotransmitters: from biology to clinic
Among the most interesting mechanisms is the ability of some microorganisms to interact with the production and availability of neuroactive molecules.
Bifidobacterium adolescentis, for example, has been studied for its ability to produce GABA. Experimental studies suggest that the microbial production of this molecule can influence gut-brain communication.
However, this is primarily mechanistic and preclinical evidence. It is not correct to transform the ability of a microorganism to produce GABA in the laboratory into the statement that that probiotic is able to treat anxiety or depression in humans.
More interesting from a clinical point of view are the studies conducted on specific strains of Bifidobacterium longum, in which changes in stress response, cortisol, sleep, or some cognitive functions have been observed.
In this case too, the research is promising but not sufficient to speak of an established therapy.
The microbiota and skin health
The skin represents another complex microbial ecosystem. The skin microbiota interacts with the epidermal barrier and the local immune system, while the gut microbiota can contribute, through metabolites and immune signals, to the systemic regulation of inflammation.
This interaction is often defined as the gut-skin axis.
Atopic dermatitis has been one of the conditions most studied in the context of microbiota modulation through probiotics. However, the most recent evidence calls for caution.
A 2026 meta-analysis including 18 randomized trials and over 10,000 children did not detect a significant reduction in the risk of atopic dermatitis when probiotic supplementation was started exclusively during childhood. The high heterogeneity of the studies suggests that any effects may depend on the timing of the intervention, the population, and the specific formulation.
This result is particularly important because it shows how a biologically plausible theory is not sufficient to justify a universal recommendation.
Probiotics and acne: a field still evolving
Acne is also studied through the microbiota-inflammation paradigm.
The biological hypothesis is interesting: the modulation of the gut and skin microbiota could influence the immune response, barrier function, and some mediators involved in follicular inflammation.
Clinical studies, however, produce results that are still not completely consistent.
A 2026 meta-analysis including 13 studies and 1,453 participants did not show a significant reduction in the overall number of acne lesions, nor in inflammatory or non-inflammatory lesions. Instead, a reduction in the severity of acne measured through the Global Acne Grading System was observed, especially in some subgroups treated for at least 12 weeks.
Another review, more restrictive and including exclusively three double-blind RCTs for a total of 231 participants, detected a possible reduction in inflammatory lesions, but with low-moderate certainty evidence and significant heterogeneity.
A more recent GRADE review, focused on probiotics or postbiotics based on Lactobacillus, did not find a significant reduction in acne lesions compared to controls.
The scientific message is therefore clear: probiotics in acne represent an area of research, not an established standard therapy.
Recurrent urinary tract infections and vaginal microbiota
Another possible field of application concerns recurrent urinary tract infections in women.
The vaginal microbiota, in particular the presence of lactobacilli, represents an important component of the urogenital ecosystem. The production of lactic acid and other antimicrobial molecules can contribute to hindering colonization by some uropathogens.
Several strains of Lactobacillus have been studied, including L. rhamnosus GR-1 and L. reuteri RC-14.
The overall evidence, however, is not uniform. A review of clinical studies detected a possible reduction in recurrences, but also marked variability between strains, routes of administration, and protocols.
A randomized trial conducted in postmenopausal women with recurrent urinary tract infections also showed that L. rhamnosus GR-1 and L. reuteri RC-14 did not achieve non-inferiority compared to antibiotic prophylaxis with trimethoprim-sulfamethoxazole, although they did not determine the increase in antibiotic resistance observed in the pharmacologically treated group.
Urological guidelines also highlight discordant results among the different meta-analyses and emphasize that not all strains of Lactobacillus are equivalent.
A 2026 review including four RCTs and 616 women nevertheless highlighted a favorable signal, especially for the intravaginal administration of specific lactobacilli, indicating a possible preventive role that deserves further confirmation.
Microbiota and joints: the gut-joint axis
The relationship between microbiota and joint inflammation is one of the most interesting frontiers of research.
Alterations of the intestinal ecosystem can influence barrier permeability, exposure of the immune system to microbial components, and the production of immunomodulatory metabolites. From here arises the concept of the gut-joint axis.
In rheumatoid arthritis, some studies have shown that the modulation of the microbiota can change inflammatory markers. An updated meta-analysis including 12 RCTs and 703 patients with autoimmune diseases detected reductions in some markers, including IL-6, TNF-α, and ultrasensitive PCR; in the subgroups with rheumatoid arthritis, improvements in some inflammatory mediators were observed.
A 2025 randomized trial conducted in 100 people with newly diagnosed rheumatoid arthritis evaluated a combination of specific probiotic strains as an addition to conventional therapy, observing improvements in disease activity, inflammatory markers, pain, and disability. This is an interesting result, but it comes from a single study and requires independent confirmation.
The most recent research on knee osteoarthritis is equally interesting.
A 2025 meta-analysis, including five RCTs and 694 participants, detected improvements in the WOMAC score, pain, and ultrasensitive PCR in patients treated with probiotics.
A broader analysis published in 2026, including 18 RCTs and 2,080 participants with osteoarthritis, confirmed favorable signals on pain, function, and some inflammatory markers, but highlighted strong heterogeneity and variable certainty of evidence. Above all, no evidence emerged of cartilage protection or modification of disease progression.
This distinguishes two fundamental concepts: reducing some symptoms does not mean modifying the disease.
Probiotics could therefore have a complementary role in modulating inflammation and joint symptomatology, but they cannot be considered a treatment capable of regenerating cartilage or halting the evolution of osteoarthritis.
The central problem: why do studies produce different results?
One of the most evident characteristics of probiotic research is the variability of results.
The reasons are numerous.
The first element is the strain. Two microorganisms of the same species can possess different metabolic and immunological capabilities.
The second is the host microbiota. A probiotic introduced into an ecosystem already characterized by a certain microbial composition can behave differently compared to the same strain administered to a person with a different microbiota.
Diet, age, medications, immune status, metabolism, duration of intervention, and dose also count.
This complexity explains why the simple count of CFU is not sufficient to predict clinical benefit.
The most advanced research is therefore trying to integrate metagenomics, metabolomics, transcriptomics, and host clinical characteristics to predict which people may respond better to a specific intervention.
Personalized probiotics: toward precision medicine
One of the most interesting perspectives is represented by personalized probiotics.
The idea goes beyond the "one size fits all" model: instead of administering the same strain to very different people, it seeks to identify the host's microbiological and biological profile and select the intervention most compatible with that ecosystem.
New omic technologies could allow the identification of:
· which microorganisms are present;
· which metabolic functions are deficient;
· which metabolites are produced;
· which strains have the greatest probability of taking hold or interacting with the ecosystem;
· which host characteristics can predict the response.
The most recent reviews describe precisely the shift towards stratified and personalized strategies, in which the selection of the strain, dose, and formulation is guided by the individual biological profile. However, important obstacles remain: standardized selection criteria and large clinical trials demonstrating that personalization produces superior results compared to conventional administration are still lacking.
The future could therefore not simply be "more probiotics", but more precise probiotics.
Combination therapies and microbiota
Another area of research concerns the use of probiotics in combination with other interventions.
In some conditions, the probiotic could act as a support to conventional therapy, modifying the intestinal microenvironment, mitigating some adverse effects, or influencing the immune response.
The most established example is the use of specific probiotics as adjuvants to antibiotic therapy for Helicobacter pylori, where some meta-analyses have shown improvements in eradication and a reduction in adverse gastrointestinal events.
In rheumatoid arthritis as well, the most recent studies are evaluating probiotics as an additional, non-substitutive intervention to pharmacological therapy.
This perspective is important because it shifts the focus from the idea of a probiotic capable of "curing" a disease to the possibility of integrating microbiota modulation within multidimensional therapeutic strategies.
Safety: generally good, but not absolute
In healthy people, probiotics are generally well tolerated, and the most common adverse effects are mild gastrointestinal disturbances, such as flatulence and bloating.
However, this does not mean they are without risk in any situation.
Rare cases of bacteremia, fungemia, and other infections associated with microorganisms used as probiotics have been described, especially in severely ill or immunocompromised people. Vulnerable populations therefore require an individual assessment before use.
Another issue concerns long-term safety. Many clinical trials are relatively short and are designed primarily to evaluate efficacy. Longer studies are needed to fully understand the effects of prolonged administration, especially in fragile populations.
How to navigate information and products
The increasing availability of probiotic products makes an evidence-based approach essential.
The first question should not be "how many billions of bacteria does it contain?", but which strain does it contain and for what specific objective was it studied?
The second concerns the dose: an effective amount in one study cannot automatically be transferred to a formulation containing a different strain or a different combination.
The third concerns the quality of evidence: an in vitro study, an animal model, a small clinical trial, and a meta-analysis of RCTs represent different levels of knowledge.
Finally, the person must be considered. Age, clinical conditions, pharmacological therapies, immune status, diet, and microbiota characteristics can modify the response.
The concept of strain specificity therefore remains the starting point for a rational choice.
A new idea of probiotic
Probiotic research is progressively moving away from the idea of a generically "good" microorganism toward a more complex vision.
A probiotic is a biologically active organism placed within an already existing ecosystem. Its effect depends on its interaction with that system.
From this perspective, the microbiota becomes part of a broader network in which diet, metabolism, immunity, the nervous system, the intestinal barrier, and the environment contribute to the body's response.
The future perspective therefore does not simply consist of increasing the number of bacteria considered favorable, but of understanding which biological function is necessary, which microorganism can perform it, and in which individual that function can be effectively modulated.
This is the shift from traditional probiotics to precision microbiota medicine.
Future perspectives
The main lines of research can be traced back to four major directions.
Personalized probiotics. The integration of microbiota profiling, metabolomics, and clinical characteristics could allow for the individualized selection of strains and formulations.
Combination therapies. Probiotics could be used as adjuvants to dietary, pharmacological, or rehabilitative interventions when there is biological plausibility and specific clinical evidence.
Mechanistic studies. It will be necessary to better understand how individual strains interact with the intestinal barrier, immune system, metabolism, and nervous system, and which metabolites are truly responsible for the observed effects.
Long-term safety. Prolonged studies will be essential, especially in elderly, immunocompromised, severely ill people, or those undergoing complex therapies.
The most promising direction is therefore that of a probiotic that is more selective, more personalized, and more strongly based on clinical data, in which enthusiasm for the microbiota is accompanied by necessary scientific caution.
Scientific bibliography
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4. Role of Probiotics in Managing Acne Vulgaris: A Systematic Review and Meta-Analysis of Clinical Trials. 2026.
5. Lactobacillus-Based Microbiome Therapy for Acne Vulgaris: A GRADE Systematic Review and Meta-Analysis of Randomized Controlled Trials. 2026.
6. Use of Lactobacillus spp. to prevent recurrent urinary tract infections in females. 2018.
7. Lactobacilli vs antibiotics to prevent urinary tract infections: a randomized, double-blind, noninferiority trial in postmenopausal women. Archives of Internal Medicine. 2012.
8. Probiotic prophylaxis in adult women with recurrent uncomplicated urinary tract infections: A systematic review of randomized controlled trials. 2026.
9. Effect of Probiotic and Synbiotic Oral Supplementation in Autoimmune Diseases: An Updated Systematic Review and Meta-Analysis of Randomized Controlled Trials. 2026.
10. Therapeutical efficacy of immunobiotics in patients with newly diagnosed rheumatoid arthritis. Journal of Infection in Developing Countries. 2025.
11. Clinical efficacy of probiotic supplementation in the treatment of knee osteoarthritis: a meta-analysis. 2025.
12. Effects of gut microbiota-targeted interventions on pain, physical function, and inflammatory biomarkers in osteoarthritis: a systematic review and meta-analysis of randomized controlled trials. 2026.
13. Effectiveness and safety of probiotics in treating knee osteoarthritis: an updated systematic review and meta-analysis of randomized controlled trials. 2026.
14. Montazeri-Najafabady N. From One-Size-Fits-All to Precision Medicine: The Promise of Personalized Probiotics. Probiotics and Antimicrobial Proteins. 2026.
15. Jiang Y, et al. Precise probiotic therapy: Advances, bottlenecks, and the road to microbiome-informed nutrition. Gut Microbes. 2026.
16. From global to local: rethinking the design of probiotic intervention strategies. Trends in Microbiology. 2026.
17. National Institutes of Health, Office of Dietary Supplements. Probiotics – Health Professional Fact Sheet.
