• microbiota;
  • inflammation;
  • pain;
  • stress;
  • behavior;
  • germ-free mice

Summary:  Mammals are subject to colonization by an astronomical number of mutualistic and commensal microorganisms on their environmental exposed surfaces. These mutualistic species build up a complex community, called the indigenous microbiota, which aid their hosts in several physiological activities. In this review, we show that the transition between a non-colonized and a colonized state is associated with modification on the pattern of host inflammatory and behavioral responsiveness. There is a shift from innate anti-inflammatory cytokine production to efficient release of proinflammatory mediators and rapid mobilization of leukocytes upon infection or other stimuli. In addition, host responses to hypernociceptive and stressful stimuli are modulated by indigenous microbiota, partly due to the altered pattern of innate and acquired immune responsiveness of the non-colonized host. These altered responses ultimately lead to significant alteration in host behavior to environmental threats. Therefore, host colonization by indigenous microbiota modifies the way the host perceives and reacts to environmental stimuli, improving resilience of the entire host–microorganism consortium to environmental stresses.