New synthetic fungal-bacterial consortia for direct production of isobutanol from biomass
20 August 2013
A team from the University of Michigan, Michigan State, and UCLA has designed synthetic fungal-bacterial consortia for the direct production of isobutanol from biomass. The required biological functions are divided between two specialists: the fungus Trichoderma reesei, which secretes cellulase enzymes to hydrolyze lignocellulosic biomass into soluble saccharides, and the bacterium Escherichia coli, which metabolizes soluble saccharides into the desired products.
In experiments reported in an open access paper published in the Proceedings of the National Academies (PNAS), they achieved isobutanol titers up to 1.88 g/L and yields up to 62% of theoretical maximum from the direct conversion of microcrystalline cellulose and pretreated corn stover to isobutanol.
Synergistic microbial communities are ubiquitous in nature and exhibit appealing features, such as sophisticated metabolic capabilities and robustness. This has inspired fast-growing interest in engineering synthetic microbial consortia for biotechnology development. However, there are relatively few reports of their use in real-world applications, and achieving population stability and regulation has proven to be challenging. In this work, we bridge ecology theory with engineering principles to develop robust synthetic fungal-bacterial consortia for efficient biosynthesis of valuable products from lignocellulosic feedstocks.
...We developed and experimentally validated a comprehensive mathematical model for T. reesei/E. coli consortia, providing insights on key determinants of the system’s performance. To illustrate the bioprocessing potential of this consortium, we demonstrate direct conversion of microcrystalline cellulose and pretreated corn stover to isobutanol...we show that cooperator–cheater dynamics within T. reesei/E. coli consortia lead to stable population equilibria and provide a mechanism for tuning composition. Although we offer isobutanol production as a proof-of-concept application, our modular system could be readily adapted for production of many other valuable biochemicals.
While much previous research has focused on trying to create a “superbug” that could tackle the whole job of processing waste plant materials into biofuels, Xiaoxia “Nina” Lin, assistant professor of chemical engineering at the University of Michigan and her colleagues argue that a team of microbial specialists can do better.
The fungus Trichoderma reesei is already very good at breaking down tough plant material into sugars. Dr. James Liao’s lab at the University of California-Los Angeles provided E. coli bacteria that had been engineered to convert sugars into isobutanol.
The Lin group put both microbe species into a bioreactor and served up corn stalks and leaves. Colleagues at Michigan State University had pre-treated the roughage to make it easier to digest.
You can put everything in one pot. The capital investment will be much lower, and also the operating cost will be much lower, so hopefully this will make the whole process much more likely to become economically viable.—Dr. Lin
Lin’s team used game theory to analyze the relationship between the fungi and bacteria. Breaking cellulose down into sugar is hard work, so T. reesei’s tendency to do this and then share the spoils mark it as a “cooperator”. Meanwhile, the E. coli use the sugars without offering the fungus anything in return, which makes it a “cheater”.
Even so, the bacteria didn’t take over the colony because the fungi produce the sugars near their cell membranes, which gives them the first crack at using the sugars. The researchers can control E. coli’s advantage by tweaking how quickly the bacteria grow.
Others in Lin’s group are now trying to improve on their energy conversion rate and increase the tolerance of the T. reesei and E. coli to isobutanol.
The work was funded by the National Science Foundation, the Department of Energy and the U-M Office of the Vice President for Research. The university is seeking commercialization partners to help bring the technology to market.
Jeremy J. Minty, Marc E. Singer, Scott A. Scholz, Chang-Hoon Bae, Jung-Ho Ahn, Clifton E. Foster, James C. Liao, and Xiaoxia Nina Lin (2013) Design and characterization of synthetic fungal-bacterial consortia for direct production of isobutanol from cellulosic biomass. PNASdoi: 10.1073/pnas.1218447110
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