The detailed mechanisms underlying IBS are poorly understood. Gut microbiota appears to play an important role in IBS pathogenesis, but little is known about its interaction with specific dietary components. We have previously shown that some patients with IBS improve symptoms when on low FODMAPs diet, which was associated with low urinary histamine. To investigate mechanisms underlying IBS symptom generation due to the interaction of a high fermentable diet and the intestinal microbiota. Germ-free NIH Swiss mice were colonized with fecal microbiota from two IBS patients (non-constipation), with either a high (HH) or low (LH) urinary histamine level (n=24 mice/patient), and one healthy control (HV) (n=8). Specific-pathogen free (SPF) mice (n=8) were included as additional controls. Mice were assigned to a custom-made low or high fermentable carbohydrate diet (LF and HF, respectively). GI transit (beads study), cecal volume (CT scan), intestinal permeability (FITC-Dextran) and gut microbiota composition (Illumina sequencing) were assessed. Neuronal excitability by patch clamp recordings of DRG neurons (action potential rheobase) exposed to colonic supernatants and changes in mechanosensitivity of single unit afferent recordings in mouse distal colon were measured. Diet significantly altered gut microbiota composition and diversity only in HH colonized mice. All mice on HF diet (HV, HH, LH, and SPF) had increased cecal volume (Diet effect: p<0.001, F(1,55)=14.097; microbiota effect: p=0.019 F(3,55)=3.6) compared to mice on LF diet, more pronounced in mice with HH and LH microbiota. Compared to LF diet, the HF diet induced slower GI transit (p=0.009) and increased permeability (FITC-Dextran: p=0.03) only in HH colonized mice. HF diet increased neuronal excitability in mice with HH and, to some degree, in LH microbiota, but not in HV and SPF microbiota colonized mice. This increased neuronal excitability was inhibited by specific protease and H1 antagonists. In vitro experiments demonstrated that cecal microbiota from HH mice on HF diet produced several fold higher levels of histamine, compared to the microbiota from other groups of mice. High fermentable diet induces changes in gut function in gnotobiotic mice colonized with microbiota from a patient with high urinary histamine. This specific microbiota has the capacity to produce high levels of endogenous histamine, which may contribute to the observed gut dysfunction. CAG, CIHR