title: Microbes and the gut-brain axis.
abstract: BACKGROUND
  The 'gut-brain' or 'brain-gut axis', depending on whether we emphasize bottom-up or top-bottom pathways, is a bi-directional communication system, comprised of neural pathways, such as the enteric nervous system (ENS), vagus, sympathetic and spinal nerves, and humoral pathways, which include cytokines, hormones, and neuropeptides as signaling molecules. Recent evidence, mainly arising from animal models, supports a role of microbes as signaling components in the gut-brain axis.
  
  
  OBJECTIVE
  The purpose of this review is to summarize our current knowledge regarding the role of microbes, including commensals, probiotics and gastrointestinal pathogens, in bottom-up pathways of communication in the gut-brain axis. Although this has clear implications for psychiatric co-morbidity in functional and inflammatory conditions of the gut, the focus of this review will be to discuss the current evidence for a role of bacteria (commensals, probiotics, and pathogens) as key modulators of gut-brain communication.
  
  
  CONCLUSIONS
  The strongest evidence for a role of microbes as signaling components in the gut-brain axis currently arises from animal studies and indicate that mechanisms of communication are likely to be multiple. There is need for the concepts generated in animal models to be translated to the human in the future.
authors.affiliation: Farcombe Family Digestive Health Research Institute, McMaster University, Hamilton, Canada.
authors.forename: P
authors.initials: P
authors.lastname: Bercik
authors.forename: S M
authors.initials: SM
authors.lastname: Collins
authors.forename: E F
authors.initials: EF
authors.lastname: Verdu
mesh_terms: D000818:Animals; D001921:Brain / Q000382:microbiology*; D002490:Central Nervous System / Q000382:microbiology / Q000502:physiology; D017615:Enteric Nervous System / Q000502:physiology; D041981:Gastrointestinal Tract / Q000294:innervation* / Q000382:microbiology*; D006706:Homeostasis / Q000502:physiology; D006801:Humans; D007422:Intestines / Q000382:microbiology; D051379:Mice; D019936:Probiotics