Characterization of C. debilis GB1 a thermophilic facultative anaerobe capable of lending aerotolerance in co-culture with C. thermocellum
Cellulosic ethanol production is mainly described as an anaerobic process. Through use of aerobic selective pressure, using inoculate from samples taken from a farm in South-Eastern Saskatchewan, Canada, we were able to create highly cellulolytic aerotolerant enrichments which produced both hydrogen and ethanol. We were able to recreate this highly cellulolytic aerotolerant phenotype using C. thermocellum DSM 1237 and one of the Caldibacillus debilis strains isolated from the aerotolerant enrichments, strain GB1. Caldibacillus debilis GB1 is a thermophilic facultative anaerobe. C. debilis GB1 displayed a physiology distinct from that of the type strain Tf which is an obligate aerobe. Under oxygen limiting conditions, both GB1 and Tf produce end-products lactate, acetate, formate and CO2; however, GB1 alone produces ethanol. A key feature necessary for aerotolerant growth of the co-culture with C. thermocellum was the capacity of C. debilis GB1 to grow both aerobically and anaerobically. In order to characterize GB1, we sequenced and annotated the genome then used high throughput proteomics to characterise the protein expression changes between aerobic and anaerobic metabolism. We found C. debilis GB1 has typical mixed fermentation pathways and aerobic/anaerobic regulation similar to those found in E. coli K-12 and B. cereus. In order to explain the lack of ethanol production in the type strain, C. debilis Tf, we did a genomic comparison of both strains Tf and GB1, focusing on the genes concerning pyruvate metabolism. In particular, there were amino acid changes in the aldehyde-alcohol dehydrogenase ADHE that could lead to inactivity in strain Tf. This genomic evidence, plus Tf having physiology consistent with an adhE deletion mutant in the closely related Geobacillus thermoglucosidasius, leads us to propose that the adhE gene is either differently regulated or inactive in C. debilis Tf. From our work, we showed that C. debilis GB1 allowed us to further define and characterize the physiology of the genus Caldibacillus, find a new aerotolerant mode of mode of Consolidated bioprocessing using C. thermocellum, and further understanding of aerobic/anaerobic regulation in thermophilic facultative anaerobes.