Stress-associated changes in the gut microbiome of Asian elephants

Abstract

Background: The intestinal tract of practically all animals is inhabited by a diverse microorganism referred to as gut microbiome. The importance of gut microbiome to its host is well known and an increasing number of studies show that the microbes are associated with many body functions, such as the gut-brain axis function. Stress, in particular, can impact the gut-brain axis at all stages of life. Stress can reshape the gut bacterial composition through glucocorticoid secretion, inflammation, or autonomic alterations. This often leads to gut bacterial imbalance (dysbiosis), as well as low microbial diversity. Although studies on gut microbiome are increasing, the mechanisms of gut-brain axis and how microbes associate with stress still remain poorly understood. Also, the gut microbiome studies have focused mostly on humans or laboratory animals in controlled environments. It is important to widen the studies on mammals in their natural environment to better understand the complex development of gut microbiome. The aim of this thesis was to (1) determine the composition of gut microbiome of Asian elephants living in their natural environment and (2) test the hypothesis that increased stress alters the gut microbiome composition. Methods: The gut microbiome was determined from 94 semi-captive Asian elephants (Elephas maximus) in Myanmar, from which both fecal sample collection and blood sampling was possible. The gut microbiome was analyzed from the fecal samples using 16S rRNA metabarcoding approach. Stress levels were determined from the blood samples as: (I) serum cortisol (SC), (II) heterophil to lymphocyte ratio in blood samples (H:L) and (III) fecal glucocorticoid metabolites (FGM). Results: The overall gut microbe composition in Asian elephants was similar to previous studies, dominated by Firmicutes (55%) and Bacteroidetes (25%) followed by Spirochaetaes (8%). One specific genus Solibacillus was found to be significantly more abundant in this thesis compared to previous studies. There were also differences in the microbiome between elephants. The age and location of the elephant had significant effects on the gut microbe composition. The stress measure H:L ratio was also associated with gut microbiome by reducing the alpha diversity. All three stress measures were also associated with compositional changes in the gut microbiome. Conclusions: Gut microbe composition in Asian elephants is diverse and similar to other fermenters. Stress, particularly long-term stress, can shape the composition of the microbiome. This thesis provides in my knowledge the most comprehensive picture of gut microbiome in Asian elephants in their natural living environment. In addition, this thesis adds important knowledge of the microbiota-gut-brain axis and how stress levels are associated with it.