*4.1. IBS*

The influence of the gut microbiota and its dysbiosis on the pathophysiology of IBS was investigated in many studies that compared differences in the GI tract microbiome between IBS patients and controls [89–92]. Those studies showed that the intestinal microbiome of IBS patients had reduced amounts of *Bacteroides*, *Prevotella*, and *Parabacteroides* sp. Noor et al. and Maccaferi et al. showed that IBS patients had an increased population of *Bacillus*, *Bifidobacteria*, *Lactobacillus*, *Clostridium*, and *Eubacterium rectale* [93–95]. Those studies led to research about probiotic intervention and its benefits for IBS patients. In a study by Sisson et al., Symprove, a probiotic containing three *Lactobacillus* types and one Enterococcus, was shown to improve symptom severity in IBS [96]. In another study by Guglielmetti et al., *Bifidobacterium bifidum* MIMBb75 alleviated IBS symptomology by decreasing pain, discomfort, digestive upset, or bloating [97]. Further studies on probiotics for IBS presented another bacterial species with a positive influence on the relief of IBS symptoms [13,98]. Butyrate producers such as *Faecalibacterium* sp. have an anti-inflammatory impact on the GI tract, and *F. prausnitzii* is a source of serine protease that was shown to have anti-nociceptive activity by decreasing the excitability of dorsal root ganglia neurons [99–102]. Unknown IBS pathophysiology led to the creation of the term 'psychobiotics', referring to probiotics and bacterial metabolites that signal directly to the brain. In a randomized controlled trial (RCT) of 44 adults with IBS, patients were treated with *Bifidobacterium longum* NCC3001. Patients showed a significant reduction in depression and an increase in quality of life with no change in IBS symptom severity or the fecal microbiota profile. This suggested that there is some direct signaling of *B. longum* metabolites to the central nervous system (CNS) [103].
