## 5 _C. jejuni's Role in Shaping Microbiota and Contributing to PI-IBS  
Our understanding of the human gut microbiome's influence on health and well-being is rapidly expanding. Such microbial communities are crucial in adaptive host metabolism, physiology, nutrition and immune function [50]. The diverse population of gut resident microbes, comprising bacteria, fungi, protozoa, viruses and their metabolic by-products, inhabit both the surface and interior of the host [51]. These gut microbes play crucial roles in food digestion, fermentation and providing energy to the intestinal tract [52]. Metabolic components, including short-chain fatty acids, bile acids, vitamins and amino acids, are generated throughout physiological processes. The microbes, alongside their metabolites, significantly contribute to maintaining gastrointestinal immune homeostasis and influencing resistance to pathogen invasion [53]. Conversely, disturbances in the gut microbiota, known as dysbiosis, have been associated with various pathological and systemic diseases. Although the precise definition of dysbiosis remains controversial, it is clear that disruptions to the gut microbiota can have significant negative consequences [54; 55]. The epithelial lining within the human digestive tract serves as a formidable barrier, defending the host from the invasion of microorganisms, including the resident microbiota [56]. Numerous gastrointestinal microbial pathogens, such as _Campylobacter_, invade cells and compromise the integrity of the epithelial barrier. This invasive behavior is succeeded by transcellular or paracellular migration through the epithelial layer. A recent investigation revealed that patients with PI-IBS exhibit specific patterns of dysbiosis, distinguishing their microbial profiles from those typically observed in individuals with IBS [27]. Using qPCR assays to analyze microbiota changes in patients with PI-IBS and those with IBS-Drevealed comparable microbial compositions between these groups [57]. This similarity in microbiota findings hints at a potentially analogous pathophysiological mechanism underlying both conditions, suggesting that disturbances in gut microbiota associated with PI-IBS might also play a critical role in developing and manifesting IBS-D symptoms [57]. Long alterations in the small bowel microbiota can occur following a _C. jejuni_ infection, including a reduction in the levels of _Methanobrevisbacter smithii_, the predominant archaeal methanogen in humans [58]. This decrease in the _M. smithii_ population is a biomarker for gastrointestinal (GI) diseases such as IBS. Notably, the load of _M. smithii_ is typically higher in healthy adults than in those suffering from chronic gastrointestinal conditions [58]. A recent analysis uncovered a connection between specific microbial markers in PI-IBS and the host's amino acids. Hydrogen sulfide, a by-product of the interaction between digestive microbiota and protein metabolism, is a detrimental compound [21]. This gas can damage the intestinal epithelium by acting as a signaling molecule and leads to the formation of tetrathionate, which can facilitate the growth and proliferation of _Salmonella_, including other pathogens [21; 33]. Changes in the intestinal bacterial microbiota are linked with several chronic gastrointestinal disorders, encompassing PI-IBS, idiopathic chronic diarrhea and microscopic colitis in humans, underscoring the critical role of gut microbiome composition in the pathogenesis and symptomatology of these conditions [29]. The gut microbiome demonstrates a robust resilience to various external stressors, including gastrointestinal infections, exposure to antibiotics, xenobiotics and shifts in diet. For instance, _Bacteroidetes_, _Firmicutes_ and _Actinobacteria_ typically represent the predominant microbial taxa within the gut [10]. Nonetheless, during infectious enteritis and its sequelae, as seen in PI-IBS, there is a notable perturbation in the core microbiome composition.