A team of engineers from the National University of Singapore (NUS) recently discovered that a naturally occurring bacterium, Thermoanaerobacterium thermosaccharolyticum TG57, isolated from waste generated after harvesting mushrooms, is capable of directly converting cellulose to biobutanol.

In an open-access paper in the journal Science Advances, the research team, led by Associate Professor He Jianzhong, reported that TG57 is capable of using microcrystalline cellulose directly to produce butanol (1.93 g/liter) as the only final product (without any acetone or ethanol produced)—comparable to that of engineered microbes thus far.

Fluorescence micrograph showing the adhesion of cells to a crystalline cellulose particle. The adhesion of Thermoanaerobacterium thermosaccharolyticum TG57 cells to cellulose particles—a fundamental stress adaption for such microorganisms—facilitates the efficient utilisation of cellulose for biobutanol production. The unique genome of the TG57 strain enables the bacterium to produce enzymes that enhances the synthesis of biobutanol. Credit: National University of Singapore. Click to enlarge.

The team first discovered the novel TG57 strain in 2015. The researchers went on to culture the strain to elucidate its properties and to provide a genome-level understanding of how cellulose is metabolized by T. thermosaccharolyticum.

They found that strain TG57 exhibits significant advances including unique genes responsible for a new butyrate synthesis pathway, no carbon catabolite repression, and the absence of genes responsible for acetone synthesis (which is observed as the main by-product in most Clostridium strains known today).

Further, they found that the use of glucose analog 2-deoxyglucose posed a selection pressure to facilitate isolation of strain TG57 with deletion/silencing of carbon catabolite repressor genes and thus is able to ferment simultaneously glucose, xylose, and arabinose to produce butanol (7.33 g/liter) as the sole product.

Combined analysis of genomic and transcriptomic data revealed unusual aspects of genome organization, numerous determinants for unique bioconversions, regulation of central metabolic pathways, and distinct transcriptomic profiles.

Moving forward, the research team will continue to optimize the performance of the TG57 strain, and to engineer it to enhance biobutanol ratio and yield using molecular genetic tools.

Resources

• Tinggang Li, Chen Zhang, Kun-Lin Yang and Jianzhong He (2018) “Unique genetic cassettes in a Thermoanaerobacterium contribute to simultaneous conversion of cellulose and monosugars into butanol” Science Advances Vol. 4, no. 3, e1701475 doi: 10.1126/sciadv.1701475