The gut microbiota predispose to the pathophysiology of acute postradiotherapy diarrhea.

Introduction: Severe diarrhea may complicate pelvic radiotherapy and force interruption of treatment. As there is no current clinical or experimental information on the role of the gut microbiota in this pathogenesis, we conducted a pilot observational study on the fecal microbiota in patients receiving pelvic radiotherapy.

Methods: The study involved 10 patients who underwent 5 wk of radiotherapy for abdominal tumors and 5 controls. Four fecal samples were collected from each individual: before, during, at the end, and 2 wk after treatment. Following the amplification of the bacterial 16S rRNA gene from the samples, DNA fingerprinting and cloning-sequencing techniques were used to determine their microbial profile and composition, respectively.

Results: Six patients suffered acute postradiotherapy diarrhea and 4 did not. In patients without diarrhea, as well as in healthy volunteers, microbial diversity was stable over a period of 7 wk. However, patients exhibiting diarrhea showed a progressive modification in their microbial diversity. A radical drop in similarity index was observed at the end (P= 0.026) and still 2 wk after radiotherapy (P= 0.014). Interestingly, cluster analysis of the microbial profile in the first sample (S1) (collected before radiotherapy) displayed a dendogram where patients that presented diarrhea clustered separately from those that did not develop diarrhea after radiotherapy. Moreover, sequence analysis of dominant bacteria in the S1 sample confirmed differences between the diarrhea and nondiarrhea groups.

Discussion: In this set of patients, susceptibility or protection against diarrhea after radiotherapy could be linked to different initial microbial colonization.