Moftah H Alhagamhmad, Andrew S Day, Daniel A Lemberg, Steven T Leach

Crohn disease (CD) is a chronic inflammatory condition primarily affecting the gastro-intestinal tract, and is characterised by reduced bacterial diversity. The exact cause of disease is unknown; however, evidence suggests several components, including microbiota, may contribute to the underlying pathology and disease development. Perturbation of the host-microbe commensal relationship is considered the main driving force of tissue destruction and pathological changes seen in CD. Several putative bacterial pathogens including species from Mycobacteria, Campylobacter and Helicobacter, are postulated in the aetiology of CD. However, to-date, no strong evidence supports a single bacterium contributing overall to CD pathogenesis. Alternatively, dysbiosis, or bacterial imbalance, is more widely accepted as a leading factor in the disrupted host-immune system cross-talk resulting in subsequent intestinal inflammation. Depletion of symbiont microbes including Firmicutes, Bifidobacteria and Clostridia, in conjunction with an increase in pathobiont microbes from Bacteroidetes, Enterobacteria, is a striking feature observed in CD. No single factor has been identified as driving this dysbiosis, although diet, antibiotic exposure and possible early life events in presence of underlying genetic susceptibility may contribute. The aim of this review is to highlight the current accumulating literature on the proposed role of bacteria in the pathogenesis of CD.