Introduction

Microplastic particles (MPs), defined as plastic fragments smaller than 5 mm, are pervasive pollutants that accumulate in ecosystems and the human food chain (1). Literature points at various health risks, including the induction of carcinogenesis (2). Emerging evidence from animal models (3) suggests that MPs may influence the gut microbiome, but the impact on the human gut microbiome remains poorly understood.

Aims & Methods

This study aimed to evaluate how various MP types influence gut microbial composition and metabolism using an ex vivo bioreactor model, with a focus on exploring potential carcinogenic effects arising from MP–microbiome interactions.
Stool samples from healthy donors were used to inoculate bioreactor cultures, which were maintained under anaerobic conditions for five days with daily nutrient feeding. These cultures were exposed to five common MP types—polystyrene (PS), polypropylene (PP), low-density polyethylene (LDPE), poly(methyl methacrylate) (PMMA), and polyethylene terephthalate (PET)—at concentrations mimicking both estimated human exposure (4) and higher doses to assess dose-dependence. Cell viability and total bacterial counts, as well as culture pH, were measured throughout the experiment. To further explore potential microbiome and metabolic alterations, 16S rRNA gene sequencing and targeted metabolomics were performed.

Results

MP exposure did not lead to significant changes in total or viable bacterial cell counts. However, MP-treated cultures exhibited a consistent and significant decrease in pH compared to controls, suggesting changes in microbial metabolic activity. Microbiome sequencing revealed plastic-type-dependent alterations in microbial composition, with certain bacterial taxa increasing or decreasing in abundance depending on the MP type. These shifts occurred across multiple genera within diverse families, including Lachnospiraceae, Oscillospiraceae, Enterobacteriaceae, and Ruminococcaceae, with the majority of changes occurring withing the phylum Bacillota. These compositional changes were accompanied by shifts in the metabolomic profiles, some of which correlated with the observed pH changes. Several plastic types induced changes in valeric acid levels, while individual MP types were associated with alterations in distinct metabolites such as uracil, lactic acid, and acetic acid.

Conclusion

This study demonstrates that MPs can affect gut microbial activity and metabolism without significantly altering overall bacterial abundance in the short term. The variation in microbiome response across plastic types highlights the complexity of MP-microbiome interactions. Changes in faecal pH are known to be associated with various gastrointestinal diseases (5). Interestingly, some of the MP-induced changes in microbial composition resembled patterns linked to diseases such as depression and colorectal cancer. In contrast, other changes showed patterns that differed from those typically associated with conditions like Parkinson’s disease and irritable bowel syndrome, highlighting the complexity of these associations and the dependency on the context. The bioreactor model provided a controlled environment for identifying direct MP-microbiome interactions; however, in humans, additional factors like diet, immune response, and individual microbiome variations play a role in determining long-term effects. Exposure duration may also play a critical role in shaping the extent and nature of microbiome alterations (6), underscoring the need for further research into the effects of chronic MP exposure and associated health risks.

References

(1) Mamun, A. A., Prasetya, T. A. E., Dewi, I. R., & Ahmad, M. (2023). Microplastics in human food chains: Food becoming a threat to health safety. The Science of the total environment, 858(Pt 1), 159834. https://doi.org/10.1016/j.scitotenv.2022.159834
(2) Kumar, R., Manna, C., Padha, S., Verma, A., Sharma, P., Dhar, A., Ghosh, A., & Bhattacharya, P. (2022). Micro(nano)plastics pollution and human health: How plastics can induce carcinogenesis to humans?. Chemosphere, 298, 134267. https://doi.org/10.1016/j.chemosphere.2022.134267
(3) Hirt, N., & Body-Malapel, M. (2020). Immunotoxicity and intestinal effects of nano- and microplastics: a review of the literature. Particle and fibre toxicology, 17(1), 57. https://doi.org/10.1186/s12989-020-00387-7
(4) Senathirajah, K., Attwood, S., Bhagwat, G., Carbery, M., Wilson, S., & Palanisami, T. (2021). Estimation of the mass of microplastics ingested - A pivotal first step towards human health risk assessment. Journal of hazardous materials, 404(Pt B), 124004. https://doi.org/10.1016/j.jhazmat.2020.124004
(5) Yamamura, R., Inoue, K. Y., Nishino, K., & Yamasaki, S. (2023). Intestinal and fecal pH in human health. Frontiers in Microbiomes, 2, 1192316.
(6) Yin, L., Yang, M., Teng, A., Ni, C., Wang, P., & Tang, S. (2025). Unraveling Microplastic Effects on Gut Microbiota across Various Animals Using Machine Learning. ACS nano, 19(1), 369–380. https://doi.org/10.1021/acsnano.4c07885

Introduction

Complex perianal fistulas represent one of the most challenging manifestations of Crohn’s disease. Combined surgical and medical therapy with biologic drugs represents today the first-line treatment option, but its efficacy does not exceed 60% in terms of healing. Recently, new therapeutic approaches, such as autologous microfragmented adipose tissue (M-Fat) injection, have shown promising results. Indeed, a recent non-profit prospective pilot study on 15 patients has demonstrated that the local injection of M-Fat is a safe and promising “rescue therapy” for patients with multiresistant complex fistulizing Crohn’s disease (PCD), with a 66.7% rate of combined remission in such difficult patients. It has to be highlighted that the M-Fat injection is minimally invasive, with less risk of sphincteric damage and can be carried out in day-surgery setting.

Aims & Methods

The primary aim of this multicentre double-blind randomized controlled trial (ATTIC) is to evaluate the efficacy of the infiltration of M-Fat in the healing of perianal complex fistulas not-responding to treatment with biologics. Consecutive patients referring to the 6 investigation Centers for surgical treatment of persistent complex fistulizing PCD refractory to biosurgical approach were assessed for eligibility. After baseline pelvic magnetic resonance imaging, those included were submitted to surgical drainage with “cone-like” fistulectomy, and then randomized to receive a 15 to 20 cc local injection, at the internal orifice level and along fistula tracts, of M-Fat obtained by a lipoaspiration and processed with an enzyme-free technology (treatment group), or a suture placement and injection of 15 cc of saline solution at the internal orifice (control group). Clinical follow-up were conducted at 2, 4, 8, 12, and 24 weeks; at 24 weeks after surgery, pelvic magnetic resonance was obtained. Success was defined as “combined remission”, combination of clinical (absence of drainage upon gentle finger compression) and radiological healing (absence of residual collections > 3 mm). Clinical remission was defined as clinical healing of all external openings not confirmed at radiological assessment.

Results

An overall 92 patients were screened, and 66 (33 per group) completed the 24-week follow-up. A statistically significant difference in favour of the treatment group in terms of combined remission emerged at the primary endpoint at 24-week follow-up (control group, 18.18%; treatment group: 57.58%; p = 0.0020). No significant differences in clinical outcome measures were encountered at any of the other follow-up points. No significant differences emerged between the groups in terms of recurrence rate, complication rate, and number of adverse events. Clinical remission was 63.64% in the treatment group compared to 51.52% in the control group with no statistically significant difference (p > 0.05).

Conclusion

This multicentre double-blind randomized controlled trial demonstrates that the intraoperative injection of M-Fat, associated with surgical complete drainage of perianal sepsis, is safe, effective, and reproducible in healing complex multirefractory perianal Crohn's disease, while improving the quality of life and significantly reducing the risk of definitive oostomy.