Ecological resilience in ulcerative colitis: microbial dynamics of donor and resident species in a longitudinal fecal microbiota transplantation study

Fecal microbiota transplantation (FMT) is a promising treatment for the chronic immune-mediated disease ulcerative colitis (UC). However, the microbial dynamics underlying clinical remission remain poorly understood. To investigate these dynamics, we analysed data from 22 UC patients treated with four rounds of FMT donated by two healthy donors. Microbiota samples from patients were collected at nine timepoints before, during, and after treatment, covering a period of 14 weeks. Additionally, 27 donor samples were analysed. Species in the recipients' gut microbiota were categorised into ecological categories based on their origin and temporal dynamics: species already present in the recipient pre-FMT, species derived from the donor, or novel species, i.e. absent before FMT in both recipient and donor but detected during or after treatment. Overdispersed Poisson regression models were employed to model the number of species within each category over time. Furthermore, we investigated the change in relative abundance for recipient, colonising, and novel species. The results revealed that recipient species with higher relative abundances prior to FMT were more likely to persist following FMT. Notably, patients who achieved combined clinical and endoscopic remission at week 14 retained a higher number of recipient species compared to non-responders. In contrast, non-responders initially exhibited colonisation of more donor species than responders, but colonisation rate decreased over time in non-responders whereas colonisation rate remained stable in responders. These findings suggest that clinical remission following FMT is associated with controlled incorporation of donor species without replacement of resident species, which may reflect a resilient recipient gut community.