PI-IBS may change sensitivity in the intestine, increasing signalling from the viscera to the brain, with peripheral sensitisation. Symptom severity will vary depending on the cause (whether bacterial, viral, or parasites), but protozoal enteritis has been associated with the highest risk of PI-IBS.^8,11

Among changes to the microbiome, IBS has been associated with high Bacteoriodes levels and reduced Bifidobacterium levels. Bacterioides fragilis affects glycoproteins that cause diarrhoea and Bifidobacterium infantis moderates the effect of pro-inflammatory cytokines while maintaining anti-inflammatory cytokines.¹¹

Other studies have found other low-level inflammatory states linked to bacteria in the gut and the extent to which the patient’s genes expressed certain cell receptors moderating the immune response.

Metabolites produced by gut microorganisms can also activate intestinal responses, for example butyrate has anti-inflammatory effects. Animal studies have found Cryptosporidium parvum can alter levels of bacteria which produce butyrate.¹¹

C parvum increases gut permeability. Glutamine and glutamate production in the caecum may also be affected, with glutamate deficiency promoting symptoms, while glutamate supplementation may improve intestinal permeability reducing IBS with diarrhoea.