An obligate aerobe adapts to hypoxia by hybridising fermentation with carbon storage

Abstract

AbstractIn soil ecosystems, obligately aerobic bacteria survive oxygen deprivation (hypoxia) by entering non-replicative persistent states. Little is known about how these bacteria rewire their metabolism to stay viable in these states. The model obligate aerobeMycobacterium smegmatismaintains redox homeostasis during hypoxia by mediating fermentative hydrogen production. However, the fate of organic carbon during fermentation, and the associated remodeling of carbon metabolism, is unresolved. Here we systematically profiled the metabolism ofM. smegmatisduring aerobic growth, hypoxic persistence, and the transition between these states. Using differential isotope labelling, and paired metabolomics and proteomics, we observed rerouting of central carbon metabolism through the pentose phosphate pathway and Entner-Doudoroff pathway during hypoxia. We show thatM. smegmatisexcretes high levels of hydrogen concurrently with upregulating triacylglyceride synthases and accumulating glycerides as carbon stores. Using electron cryotomography (cryo-ET), we observed the presence of large spheroid structures consistent with the appearance of lipid droplets. Thus, in contrast to obligately and facultative anaerobic fermentative bacteria,M. smegmatisstores rather than excretes organic carbon during hypoxia. This novel hybrid metabolism likely provides a competitive advantage in resource-variable environments by allowingM. smegmatisto simultaneously dispose excess reductant during hypoxia and maintain carbon stores to rapidly resume growth upon reoxygenation.