A distinct gut microbiome signature has been identified in people with IBS who are responsive to low FODMAP dietary therapy.

And if confirmed in further studies, the unique gut microbiome subtypes found in IBS may be useful as a biomarker to enable clinicians to select patients who are likely to be suitable or unsuitable for FODMAP therapy, according Australian expert Professor Peter Gibson.

In the study, published in Gut, UK researchers analysed the taxonomic and functional profiles of the stool microbiota from 56 people with IBS, and compared these with control subjects from the same household.

At baseline, before adoption of the low FODMAP diet, analysis of the stool samples of those with IBS revealed two distinct microbial ‘signatures’, which the investigators labelled IBS Pathogenic-like and IBS Healthy-like subtypes.

IBS Pathogenic microbiomes were enriched in Firmicutes and genes for amino acid and carbohydrate metabolism, but depleted in Bacteroidetes species. The IBS Healthy microbiomes were similar to controls.

Clinical response and microbiota changes were the assessed in 41 patients and controls after four weeks on a low FODMAP diet. In the IBS Healthy group the microbial profile stayed the same as the control group, whereas the microbiome of IBS patients with the Pathogenic profile became healthier, with an increase in Bacteroidetes, and a fall in Firmicutes species. And the bacterial genes involved in the metabolism of amino acids and carbs were no longer overexpressed.

Overall, symptomatic improvement occurred in 75% of patients with IBS after the FODMAP diet (50-point reduction in IBS symptom severity score (IBS-SSS).

However, the clinical response to the diet was greater in those with IBS and a pathogenic microbial signature than it was in those a healthy microbial signature: the difference in degree of response (ΔIBS-SSS) was 194 in IBSP =194 vs 114 in IBSH; Wilcoxon p=0.02)

And notably, the symptomatic benefits and the healthy microbiota profiles were maintained in those who responded for several months despite a return to pre-diet FODMAP intake.

The researchers said their findings could pave the way for the development of a microbiota signature as a biomarker to manage IBS cases with a low FODMAP diet recommendation.

“If the bacteria represented in the [pathogenic] subtype are shown to play a pathogenic role in IBS, perhaps through their metabolic activity, this provides a target for new therapies and an intermediate [marker] by which to assess them,” they wrote.

In a linked editorial, Professor Peter Gibson and Dr Emma Halmos of Monash University said the findings might explain the wide variation seen between patient in responses to FODMAP diet. Some of these variations could be attributed to factors such as dietary non-adherence, poor food selection or overall lack of FODMAPs in the habitual diet, they noted.

The new study may help clarify the role of other factors in lack of symptomatic response related to functional characteristics of the gut microbiota, such as underrepresentation of saccharolytic bacteria with less vigorous gas production in response to exposure to FODMAPs, they suggested.

Another intriguing finding was related to activation of trehalose metabolic pathways in IBS patients with dysbiosis, they added.

It may be that trehalose is yet another unrecognised FODMAP in some people with IBS, they suggested and “the colonic microbial population may be exposed to trehalose— both dietary and fungal in origin—in more people than conventional teaching tells us, and its subsequent bacterial fermentation may play a role in symptom genesis.”

“The beauty of [the study] is not in its definitive nature, but that it enables the creation of feasible innovative hypotheses that can be examined by focused studies. Perhaps the FODMAP diet is not just a symptomatic therapy,” they concluded.