The role of the microbiome in the balance between health and disease: permeability of biological barriers, eubiosis, and new frontiers in pharmacomicrobiomics and oncology

The breakdown of biological barriers and the pathway to autoimmunity

The human gastrointestinal tract hosts a complex, dynamic ecosystem that exceeds the total number of cells in the host organism by a factor of at least ten, with an estimated total weight of between 1.5 and 2 kilograms. This microbial network, now defined as the microbiota, operates in close synergy with the host's immune system, serving as a true functional extension of it. While the interaction between the immune system and the bacterial flora ensures the maintenance of homeostasis under physiological conditions, the disruption of this balance can trigger or exacerbate multiple pathological conditions. A central focus of biochemical research is the impact of specific nutrients on the structural integrity of the mucosa. It is well documented that gluten consumption can alter the architecture of the intestinal walls in predisposed individuals, inducing an increase in epithelial barrier permeability, a condition known clinically as leaky gut.

The pathogenic mechanism is expressed through the modulation and subsequent widening of the tight junctions, intercellular protein complexes responsible for selectively regulating paracellular transit. The loss of this filtering function allows the indiscriminate passage of undigested macromolecules, bacterial fragments, viruses, and xenobiotic metabolites into the bloodstream, which under conditions of eubiosis would be confined to the intestinal lumen. The persistent passage of these antigens stimulates the mucosa-associated lymphoid tissues (GALT), causing chronic immune activation mediated by the release of pro-inflammatory cytokines. This continuous inflammatory state, associated with the loss of immunological tolerance and a specific individual genetic predisposition, can over time drive the transition to systemic or organ-specific autoimmune diseases, such as rheumatoid arthritis, Crohn's disease, multiple sclerosis, type 1 diabetes, and celiac disease.

The hypothesis of intestinal permeability, initially met with skepticism by the academic establishment and confined mainly to integrative medicine therapeutic models, has found full scientific validation thanks to advances in genetics and molecular biology. Genomic data have clarified how the expression of barrier-regulating genes is not limited to the digestive tract. Defects in "traffic" and similar permeability dysfunctions can affect the pulmonary mucosal barrier and the blood-brain barrier. Biopsy studies post-mortem performed on subjects with multiple sclerosis have confirmed a structural and functional impairment of these internal protective barriers. Overlapping phenomena of membrane alteration have been found in several neurodegenerative and neurodevelopmental disorders, stimulating the design of rigorous clinical trials focused on restoring barrier integrity as a primary therapeutic target.

Eubiosis, dysbiosis, and the frontiers of pharmacomicrobiomics

The maintenance of a state of eubiosis, understood as the perfect quantitative and qualitative balance between the different microbial populations, is an essential requirement for systemic health. Within this ecosystem, approximately 80% of the resident bacterial species (including the genera Lactobacillus and Bifidobacterium) are responsible for the fermentation processes of the carbohydrate matrix and plant fiber. The remaining 20% (comprising species of the genera Escherichia, Bacteroides, Eubacterium, and Clostridium) manage the putrefaction processes of protein residues. Many of these microorganisms, although harmless or beneficial in the context of adequate microbial biodiversity, can turn into opportunistic pathogens if a condition of dysbiosis occurs, i.e., a breakdown of the correct numerical ratios between bacterial families. The causal factors of dysbiosis include the adoption of inappropriate dietary regimens, the presence of established gastrointestinal diseases, and, preeminently, the abuse of antibiotic therapies.

Exposure to antibiotics rapidly alters the microbiome profile, reducing its biodiversity (the average in the Italian population is estimated to be around 459 species, a figure constantly being updated) and promoting the selection of resistant strains. The human microbiome can thus become a reservoir of resistance genes (resistome), with the concrete risk of horizontal gene transfer between bacterial species, complicating the clinical management of nosocomial and community-acquired infections. Indirect exposure also occurs through the food chain due to the use of antimicrobial agents in intensive farming and agriculture.

Advances in pharmacomicrobiomics have revealed how the composition of the microbiota is able to directly influence the pharmacokinetics and pharmacodynamics of exogenous molecules, activating or inactivating the administered active ingredients. This biochemical interaction takes on crucial importance in oncology. Recent clinical studies indicate that patients who did not respond to modern immunotherapy protocols were often concurrently receiving systemic antibiotic or corticosteroid therapies, treatments capable of eradicating specific bacterial families necessary for the correct modulation of the antitumor immune response. Preliminary investigations in cancer patients show that high indices of systemic inflammation are associated with severe dysbiosis.

For example, in subjects with colon cancer resistant to standard therapies, a severe depletion of the Ruminococcaceae and Lachnospiraceae families has been observed. This deficiency correlates with a reduced intake of dietary fiber, which represents the fundamental substrate for the microbial synthesis of short-chain fatty acids (SCFA), such as butyrate and propionate. From a biochemical perspective, propionate deficiency depresses the cellular production of cyclic adenosine monophosphate (cAMP), altering the efficiency of the Krebs cycle and fueling the patient's asthenic state. At the same time, the butyrate deficit compromises the differentiation and maturation processes of the epithelial cells of the colonic mucosa, precisely at the site of the tumor lesion. Although the empirical reduction of fiber in the diet of the colon cancer patient is sometimes adopted to mitigate immediate symptoms such as flatulence or aerophagia, in the long term, this deprivation damages the symbiotic flora, reducing the production of SCFA and triggering a vicious cycle that supports neoplastic proliferation and local inflammation.

Targeted modulation and therapeutic personalization

The administration of probiotic strains in oncology must abandon standardized logic and move toward precision criteria based on the metagenomic profile of the individual patient, the cytotoxicity of the drugs, and the stage of the disease. Clinical evidence shows, for example, that the administration of Lactobacillus acidophilus in combination with platinum-based chemotherapeutic agents enhances its therapeutic efficacy, down-regulating genes associated with tumor angiogenesis and stimulating the cytotoxic activity of T lymphocytes, thus promoting the apoptosis of neoplastic cells. Conversely, clinical trials conducted on patients with melanoma treated with the monoclonal antibody Nivolumab indicate that a high intestinal density of Bacteroides – a genus that proliferates in conjunction with high-protein diets rich in meat – is associated with an increase in inflammatory indices and a higher incidence of severe side effects. Other evidence documents the synergistic efficacy of the immunotherapy Ipilimumab when combined with high concentrations of bifidobacteria, while the concomitance of high densities of specific bacterial families during the use of taxanes or oxaliplatin can predispose to the development of severe peripheral neuropathies at the edematous and acral levels.

The integration of genomics and metagenomics now allows us to overcome the limits of the traditional clinical approach, providing a synoptic mapping of the interactions between the host's genotype and the microbiome's phenotype. The individual metabolic phenotype can be expressed by the equation:

Phenotype=Genotype+Environment+(Genotype×Environment)

This model highlights how genetic susceptibility to degenerative diseases is modulated by environmental exposure and diet. Xenobiotic detoxification processes occur through three distinct enzymatic phases: Phase I chemically modifies toxins mainly through the cytochrome P450 system (including the CYP1A1, CYP1A2, and CYP1B1 isoenzymes, responsible for the metabolism of polycyclic aromatic hydrocarbons such as benzo[a]pyrene from combustion or cooking, and mycotoxins such as aflatoxin B1); Phase II conjugates intermediate metabolites with chemical groups (through sulfation, acetylation, or conjugation with glutathione mediated by glutathione-S-transferases) to generate water-soluble compounds that can be eliminated (mercaptans); Phase III provides for final excretion via the bile or urine. High enzymatic activity in Phase I, associated with excessive meat consumption and genetic defects in the conjugation phases, increases the risk of gastrointestinal and hepatic carcinogenesis due to the accumulation of reactive intermediates. Microbiota analysis allows for the verification of the molecular remodeling action of the flora on such compounds, enabling targeted lifestyle changes.

The dietary intake of bioactive compounds such as quercetin and polyphenols selectively alters microbial growth profiles. Approximately 95% of the polyphenols consumed in the diet reach the colon unaltered, where the microbiota transforms them into biologically active and assimilable secondary metabolites. Controlled supplementation with resveratrol extracts or cocoa flavanols promotes the expansion of health-promoting strains such as bifidobacteria and lactobacilli. Similarly, moderate red wine consumption or the introduction of food matrices rich in non-absorbable oligosaccharides, such as apples, supports the dynamic balance of the microbiome. Apple pectin derivatives have demonstrated the ability to optimize the barrier function of the liver and intestine, while flavan-3-ols and procyanidins act positively on the urinary metabolome, reducing the inflammatory impact induced by bacterial lipopolysaccharides (LPS, the main promoters of GUT permeability), reducing the Firmicutes/Bacteroidetes ratio, and significantly increasing the presence of Akkermansia muciniphila, a bacterial strain associated with the control of lipid homeostasis and the reduction of insulin resistance.

Fecal sample diagnostics based on bacterial DNA sequencing represent the tool of choice for personalizing nutritional intervention in the healthy patient – ideal at the change of season to monitor the adaptive variations of the flora – and in the oncology patient, for whom performance is recommended at the beginning and end of therapeutic cycles (or every three months in the presence of severe toxicity). If the classic nutritional route is precluded or there are specific contraindications to the use of live strains (probiotics), modern clinical practice makes use of postbiotics, complexes derived from standardized bacterial fermentation processes, such as direct butyrate supplementation, capable of restoring mucosal homeostasis without inducing risks of bacterial translocation in immunosuppressed organisms.

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