The human gastrointestinal tract is heavily colonized by millions of bacteria, termed the microbiota, which is closely involved in host metabolic and immunological processes. This beneficial relationship is dependent upon the community structure of the microbiota, as defined by the relative proportions and diversity of the groups within the community. External or internal factors can impact these population abundances, which can have deleterious effects upon host health. This condition, termed dysbiosis, has been associated with inflammatory bowel disease as well as obesity. The development of these dysbiotic profiles and their involvement in deleterious health outcomes are topics that are not yet well understood. Thus, we developed this study to ascertain factors that influence mammalian hosts to acquire dysbiotic microbiota, and to associate dysbiosis with changes in host health.
Psychological stressor exposure is a primary correlate of symptomatic episodes (e.g., pain, bleeding) in patients with inflammatory bowel disease, though it is not understood how this occurs. Previous studies have shown that exposing mammals to stress can affect the microbiota which are associated with GI mucosal immunity, but the changes have been not been well-defined, particularly in mice. Thus, mice were exposed to a long-term stressor known as restraint in order to evaluate how stress exposure affects the murine gastrointestinal microbiota. In addition, mice were subjected to a social stressor, social disruption, in order to determine if the type and duration of the stressor affects microbiota community structure differently. Colonic mucosal and luminal contents were collected and 16S rRNA sequences were analyzed. Mucosal and luminal contents had unique community structures that clustered separately on a principle coordinate analysis cluster plot. Restraint stress affected the mucosal associated populations to a greater extent and reduced beneficial groups, including Lactobacillus. Likewise, social disruption affected the mucosal-associated microbiota significantly after only 2 hours of exposure, and this effect compounded with repeated exposures. As with restraint stress, Lactobacillus was reduced in Social Disruption (SDR)-exposed mice. qPCR analysis indicated that the immunomodulatory species, Lactobacillus reuteri, was reduced in absolute abundance in SDR-exposed mice, but not in restraint-exposed mice.
Mice exposed to either restraint or SDR had significant increases in severity of infection by an enteric pathogen, Citrobacter rodentium. iNOS, IL-1beta, CCL2, and TNF-alpha were significantly increased in mice exposed to the stressors, and overall colitic pathology was also increased in the stress-exposed mice. In order to determine if there is an association between stress-induced changes to the microbiota, and stress-induced aggravation of colitic inflammation during pathogen challenge, germ-free mice were given oral gavage of the fecal slurry from stressor-exposed conventional mice, and then challenged with C. rodentium. Mice that received the stress-exposed microbiota had increased pro-inflammatory transcript levels as well as heightened colitic pathology, indicating that stressor-induced changes to the microbiota are associated with changes in gastrointestinal immune function.
Probiotic Lactobacillus reuteri can act as an ameliorative agent upon stressor-exacerbated colitis. Since L. reuteri can increase colonic diversity, we determined if L. reuteri is stabilizing the host microbiota as a primary mechanism in reversing the heightened inflammatory state in stressor-exposed C. rodentium-infected mice. SDR-exposed mice that received the probiotic gavage had no change in microbial diversity or composition within the colon after one day of infection compared to control, but stress and infection-associated changes to Lactobacillus, Parabacteroides, and S24-7 were observed. Additionally, stress and infection had lasting effects upon the microbiota, which suggests that these impacts can have long-term outcomes upon host health as mediated by the microbiota.
Though these findings make evident that external factors such as psychological stress can disturb healthy microbiota structures, which can then feed deleteriously back upon murine host health, it is not known whether such extrinsic impacts can shift microbiota profiles in human hosts. Host-to-host transmission of microbiota communities is another possible mechanism by which a host might incorporate a dysbiotic profile. Mothers are a primary source of microbes for their offspring in early life, and maternal obesity is a strong antecedent for obesity in later life for the offspring. Thus, we hypothesized that children born to obese mothers will have a unique microbiota structure. Indeed, using UniFrac unweighted distances, children, aged around two years and born to high-income obese mothers clustered separately from children born to high-income non-obese mothers. Clustering was not seen in children born to low-income mothers. These data indicate that both socioeconomic status of the mother, and maternal obesity associate with the community structure of their children. In sum, these combined data of psychological stress within mice and obese microbiota associations between mother and child in humans lend credence to the overarching hypothesis that external factors can significantly impact microbiota community structure, resulting in changes to health-associated microbial groups and disturbing normal host immune or physiological activity.