The gut microbiota plays a vital role in regulating the host’s energy metabolism. Until recently, studies have shown that mice without microbiota accumulate less fat and are protected from obesity induced by high-fat diet. Several proposed mechanisms suggest that the interaction between gut microbiota and host energy metabolism is a complex process with multiple factors at play. However, identifying causal relationships has been a significant experimental challenge.
Gnotobiotic mice, colonised with a simplified microbiota made up of defined species such as OligoMM12, have become a major tool to identify potential mechanisms of interaction between the microbiota and the host. Researchers from the group of Emma Slack at the Department of Health Sciences and Technology ETH Zürich compared the metabolic profile of germ-free (GF), gnotobiotic OligoMM12 and conventionally raised male C57Bl6/J mice (specific-opportunistic-pathogen-free (SPF)) with a focus on circadian rhythm.
Addressing the influence of the OligoMM12 microbiota on host metabolism has been a challenging task. Long-term experiments require hygiene barrier conditions like those required to work with GF mice. TSE Systems, in collaboration with the authors, built an isolator-housed metabolic cage system based on the TSE PhenoMaster, which allowed longitudinal monitoring of O2, CO2, and hydrogen levels over periods of several weeks in GF and gnotobiotic mice while maintaining a strict hygienic barrier. The authors could confirm that GF mice maintain their GF status over at least 10 days of accommodation in PhenoMaster cages inside an isolator.
Fig. 1. Sterility test in isolator-based indirect calorimetry system.
The results showed that OligoMM12 mice had increased fat mass compared to GF and SPF mice, and GF mice exhibited slightly increased food intake without an effect on energy expenditure. However, GF mice had a lower respiratory exchange ratio (RER) than SPF mice in both light and dark phases, indicating an increased fat/decreased glucose metabolism. Additionally, circadian changes in RER, microbiota-derived hydrogen, and short-chain fatty acids were observed.
This study demonstrates for the first time that isolator-based indirect calorimetry is possible and allows detailed analysis of the metabolism of GF and gnotobiotic mice in real time. As microbial dysbiosis is associated with various human diseases, circadian analysis of energy balance represents a crucial tool for mining microbiome data for therapeutic and diagnostic purposes.
Hoces, D., Lan, J., Sun, W., Geiser, T., Stäubli, M. L., Cappio Barazzone, E., … & Slack, E. (2022). Metabolic reconstitution of germ-free mice by a gnotobiotic microbiota varies over the circadian cycle. PLoS biology, 20(9), e3001743.