SOS microbiota: from too many antibiotics to the risk of inflammatory diseases

EDITORIAL NOTE (origin and update): This article was previously published and has been updated according to scientific and divulgative criteria to reflect recent evidence and available systematic reviews. The text is for informational purposes only and does not replace professional medical advice.

IN BRIEF

• Antibiotics rapidly change the composition of the gut microbiota; in many cases, reconstitution is incomplete and individual.
• Repeated or broad-spectrum use is associated, in observational studies, with a higher risk of inflammatory bowel diseases and, in some series, with a slight increase in the risk of colon neoplasms.
• In experimental models, various antibiotics pre-condition the microbiota, making interventions such as fecal transplantation or certain oncological therapies less effective.
• Evidence includes experimental studies, observational studies, and meta-analyses; correlation does not always equate to causality, and results depend on dose, duration, antibiotic class, and clinical context.

Abstract: what does science say?

The gut microbiota is the collection of microorganisms that live in the intestine and participate in digestion, metabolism, and the immune system. Exposure to antibiotics — especially if repeated, broad-spectrum, or early in life — alters microbial composition and the metabolites produced, with possible consequences for intestinal immune balance. Experimental studies show that different antibiotic regimens can modify the microbiota's ability to control inflammation and the response to microbiota transplants; observational research and meta-analyses suggest an association between antibiotic use and an increased risk of inflammatory bowel diseases (IBD) and, in some data, colon cancers. However, the methodological limitations of available studies (confounding, individual variability, exposure measurement) require caution in interpretation: much evidence is compatible with an important role of the microbiota as a risk mediator, but does not always establish a direct causal relationship.

Why antibiotics change the microbiota and immunity

Antibiotics are drugs intended to remove or reduce the growth of pathogenic bacteria; however, in practice, they often affect benign microbial communities that coexist with the host. Experimental and review literature shows that antibiotic administration leads to reductions in bacterial diversity, the loss of key species, and changes in the production of microbial metabolites that modulate mucosal immune cells [2]. Studies with long-term monitoring document variable individual responses: in many subjects, the microbial composition does not return identical to the pre-treatment state, and some species disappear or remain in low abundance for months [5].

From an immunological perspective, microbial products (e.g., short-chain fatty acids) and contact with commensal species are fundamental signals for the maturation and regulation of mucosal lymphocytes and for maintaining the epithelial barrier. Prolonged perturbation can alter the balance between inflammatory and regulatory responses, increasing susceptibility to abnormal responses or opportunistic infections [7]. These relationships are supported by animal models, observational clinical studies, and conceptual reviews linking the intestinal ecosystem with immune function and tissue homeostasis [7][5].

What experiments and clinical models show

Experimental research conducted on colitis models and in ex vivo experiments with human tissue has shown that different antibiotic regimens alter the microbiota's ability to control intestinal inflammation in different ways. A study published in Microbiome compared pre-treatments with vancomycin, streptomycin, and metronidazole and found that the use of certain antibiotics predisposed to a loss of protective species and an increase in genera associated with worse inflammatory outcomes; subsequently, fecal transplantation from differently conditioned donors showed variable efficacy in mitigating experimental colitis [1].

In animal models, microbiota transplantation from "responder" subjects in terms of immunity can transfer protection against inflammation, while microbiota preconditioned with certain antibiotics lose this ability. These results suggest that not only the loss of biodiversity, but the specific redefinition of the microbial community and metabolic profile are central to the observed biological effect [1][8].

Antibiotics, oncological therapies, and immunotherapy

In recent years, evidence has accumulated that the composition of the microbiota influences the response to oncological therapies based on immune system activation, particularly checkpoint inhibitors. Prospective and translational studies have shown that patients who received antibiotics around the time of anti-PD-1 treatment presented, in some clinical contexts, reduced therapeutic responses and worse survival compared to unexposed patients [6].

These observations are supported by experiments in which the transfer of microbiota from responder patients to germ-free mice improved the efficacy of immunotherapy in tumor models, while altered or antibiotic-treated microbiota reduced the anti-tumor response. Such evidence does not imply that antibiotics should always be avoided in oncology, but emphasizes the need for judicious use, considering the timing and class of the drug [6][2].

Relationship with inflammatory bowel diseases (IBD)

Numerous observational studies and meta-analyses indicate an association between antibiotic exposure (especially repeated or early) and an increased risk of developing IBD, particularly Crohn's disease. Recent updated systematic reviews show a consistent signal of association, albeit with heterogeneity between studies and different exposure measures [3]. It is important to emphasize that most available data are observational: this means that the association can be influenced by confounders (for example, pre-existing infections, use of other drugs, socioeconomic factors) and does not directly prove causality.

Relationship with the risk of colon cancer

Meta-analyses on large populations have shown a modest but repeated increase in the risk of colorectal neoplasms associated with cumulative antibiotic exposures, particularly for antibiotics with anti-anaerobic activity. The effect appears to vary by anatomical site (proximal colon vs. rectum) and by antibiotic class; however, estimates are heterogeneous and depend on latency period and control of confounding factors [5]. These results are consistent with the hypothesis that microbial and metabolic alterations can contribute to long-term promotional processes, but do not establish a unique cause-effect relationship.

What it means in practice

For the public, the practical interpretation of the evidence requires balance. Antibiotics remain fundamental tools for treating serious or potentially life-threatening bacterial infections; the goal is not to create alarm or call for indiscriminate restrictions, but rather to promote appropriate and informed use. Several operational recommendations emerge from the literature: limit the use of broad-spectrum antibiotics when targeted therapy is sufficient, avoid unnecessary prolonged treatments, consider the timing relative to other therapies (e.g., oncological immunotherapy) when possible, and evaluate strategies to promote microbial recovery after therapy (fiber-rich nutrition, experimental approaches such as fecal transplantation in specific clinical contexts) [5][1][3].

For patients: always ask your doctor the reason for the antibiotic, the expected duration, and alternatives; do not interrupt or modify therapy without consulting a professional; report any persistent gastrointestinal symptoms after antibiotic therapy. For healthcare professionals: integrate antibiotic stewardship and consider the long-term impact on the microbiota in clinical decisions, especially when immunomodulatory therapy or interventions that may depend on microbial integrity are anticipated [6][7].

Limitations of the evidence

The available conclusions derive from a heterogeneous set of studies: experiments on animal models, ex vivo studies, observational cohorts, case-control studies, and meta-analyses. Each design has strengths and limitations. Observational studies highlight associations but are vulnerable to confounding and selection bias; antibiotic exposure is often measured imprecisely (prescriptions vs. actual intake) and may be linked to clinical events that themselves increase risk (for example, prodromal gastrointestinal infections) [3][5].

In animal models, controlling all human variables is impossible, so biological observations must be interpreted as evidence of plausibility and potential mechanisms, not as automatic demonstration of clinical effects in humans. Finally, individual variability of the microbiota, age, diet, concomitant use of other drugs, and environmental factors make generalization difficult: many questions remain open about which class of antibiotics, which duration of exposure, and which time window represent the greatest risks [2][5][1].

KEY POINTS TO REMEMBER

• Antibiotics can profoundly alter the gut microbiota; reconstitution is not always complete and varies among individuals. [5]
• Antibiotic exposure is associated, in observational studies and meta-analyses, with an increased risk of IBD and colorectal neoplasms in some analyses, but causality is not universally proven. [3][5]
• In experimental models, some antibiotics predispose to the ineffectiveness of microbial interventions (e.g., FMT) or reduced response to oncological immunotherapy. [1][6]
• The clinical decision on antibiotic use must always balance immediate benefit and potential long-term effects on the microbiota; stewardship is fundamental. [7]

Editorial conclusion

Accumulated research shows that the microbiota is a key element in the balance between immunity, inflammation, and gut health. The prudent and motivated use of antibiotics, along with strategies for microbial preservation and restoration, now represents a reasonable recommendation based on available evidence. Further controlled and interventional studies are needed to define direct causes, critical time windows, and effective practical solutions to protect long-term intestinal health.

Editorial note (bottom)

This update has been prepared according to institutional editorial standards. The statements are based on peer-reviewed literature and meta-analyses available up to the date of the last update. The article is for informational purposes only and does not constitute therapeutic indication: for clinical choices, always consult qualified healthcare professionals.

SCIENTIFIC RESEARCH

1. Strati F, Pujolassos M, Burrello C, Giuffrè MR, Lattanzi G, Caprioli F, Troisi J, Facciotti F. Antibiotic-associated dysbiosis affects the ability of the gut microbiota to control intestinal inflammation upon fecal microbiota transplantation in experimental colitis models. Microbiome. 2021;9:39. https://doi.org/10.1186/s40168-020-00991-x
2. Dethlefsen L, Relman DA. Incomplete recovery and individualized responses of the human distal gut microbiota to repeated antibiotic perturbation. Proc Natl Acad Sci U S A. 2011;108(Suppl 1):4554–4561. https://doi.org/10.1073/pnas.1000087107
3. Palleja A, Mikkelsen KH, Forslund SK, et al. Recovery of gut microbiota of healthy adults following antibiotic exposure. Nat Microbiol. 2018;3:1255–1265. https://doi.org/10.1038/s41564-018-0257-9
4. Scharf E, Schlattmann P, Stallhofer J, Stallmach A. Do Antibiotics Cause Inflammatory Bowel Disease? A Systematic Review and Meta-Analysis. Visceral Medicine. 2024;41(1):32–47. https://doi.org/10.1159/000541601
5. Yang B, et al. Antibiotic use and risk of colorectal cancer: a systematic review and dose–response meta-analysis. Br J Cancer. 2020;123:1825–1832. https://doi.org/10.1038/s41416-020-01082-2
6. Routy B, Le Chatelier E, Derosa L, et al. Gut microbiome influences efficacy of PD-1–based immunotherapy against epithelial tumors. Science. 2018;359(6371):91–97. https://doi.org/10.1126/science.aan3706
7. Belkaid Y, Hand TW. Role of the microbiota in immunity and inflammation. Cell. 2014;157(1):121–141. https://doi.org/10.1016/j.cell.2014.03.011
8. Systematic review and meta-analysis: Fecal Microbiota Transplantation for Treatment of Active Ulcerative Colitis. (Moayyedi P et al. and subsequent meta-analyses). MIB/Various. 2017–2022. https://doi.org/10.1097/MIB.0000000000001228

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