Phenotype Alterations in the Cecal Ecosystem Involved in the Asymptomatic Intestinal Persistence of Paratyphoid Salmonella in Chickens

SIMPLE SUMMARY: To combat infections, hosts employ a combination of antagonistic and cooperative host defense strategies. The former refers to pathogen killing mediated by host immunity (disease resistance), while the latter refers to physiological defense mechanisms that promote host health during infection independent of pathogen killing, leading to a cooperation between the host and the pathogen (diseases tolerance). In chickens, the paratyphoid Salmonella evolved the capacity to survive the initial robust immune response and persist in the avian ceca for months without triggering clinical signs. The persistent or carrier phase of a Salmonella infection in the avian host involves a complex balance of antagonistic and cooperative host defense strategies. Initially, the host reacts with a normal inflammatory response that controls bacterial invasion. After 3–4 days of inflammation, the host response changes to a more anti-inflammatory response characterized by changes in the local intestinal physiology that is no longer antagonistic to the bacterial pathogen, but instead ‘tolerates’ its presence. Thus, the chicken becomes a carrier of the pathogen allowing it to survive in the intestine without causing disease. It is hoped that understanding these mechanisms of pathogen survival in the chicken will allow future research to exploit these novel disease tolerance mechanisms to impact poultry health and reduced foodborne infections of Salmonella. ABSTRACT: The gastrointestinal ecosystem involves interactions between the host, gut microbiota, and external environment. To colonize the gut of poultry, Salmonella must surmount barriers levied by the intestine including mucosal innate immune responses and microbiota-mediated niche restrictions. Accordingly, comprehending Salmonella intestinal colonization in poultry requires an understanding of how the pathogen interacts with the intestinal ecosystem. In chickens, the paratyphoid Salmonella have evolved the capacity to survive the initial immune response and persist in the avian ceca for months without triggering clinical signs. The persistence of a Salmonella infection in the avian host involves both host defenses and tolerogenic defense strategies. The initial phase of the Salmonella–gut ecosystem interaction is characteristically an innate pro-inflammatory response that controls bacterial invasion. The second phase is initiated by an expansion of the T regulatory cell population in the cecum of Salmonella-infected chickens accompanied by well-defined shifts in the enteric neuro-immunometabolic pathways that changes the local phenotype from pro-inflammatory to an anti-inflammatory environment. Thus, paratyphoid Salmonella in chickens have evolved a unique survival strategy that minimizes the inflammatory response (disease resistance) during the initial infection and then induces an immunometabolic reprogramming in the cecum that alters the host defense to disease tolerance that provides an environment conducive to drive asymptomatic carriage of the bacterial pathogen.