Collaborative Research: Mechanisms of Microbial Ethanol Inhibition in Clostridium thermocellum

Alcohols are one of the largest classes of chemicals produced in the bioeconomy (more than 32 billion gallons of ethanol are made each year, about half from the United States), and are currently mainly produced from starch and sugar. Switching to cellulosic feedstocks such as agricultural waste and wood would make ethanol production cheaper, more sustainable, and more reliable, but transition depends on engineering microbes that can tolerate the alcohols they produce. Alcohol toxicity remains a critical and poorly understood barrier to that transition. For decades, scientists have assumed that alcohols kill microbes primarily by disrupting cell membranes, much like a detergent. This project challenges that assumption. Using the bacterium Clostridium thermocellum, a microbe capable of breaking down plant material directly into fuel, this research will test whether metabolic disruptions inside the cell, rather than membrane damage, are the primary cause of alcohol toxicity at the concentrations encountered in industrial processes. Understanding the true mechanism of alcohol inhibition is essential for engineering microbes that can tolerate higher alcohol levels, enabling more efficient and economical production of biofuels. This project will also provide interdisciplinary training opportunities for graduate, undergraduate, and high school students at the University of Wisconsin-Madison, Dartmouth College, and partner institutions in New Hampshire, and will engage the broader public through a novel outreach program connecting biofuel research with the local brewing and distilling community. This project addresses NSF priorities in Biotechnology, as well as Advanced Materials and Manufacturing. This project investigates the mechanisms of microbial ethanol inhibition in Clostridium thermocellum, a cellulolytic thermophile with significant potential for consolidated bioprocessing of lignocellulosic biomass. The prevailing chaotropic membrane disruption model fails to explain observations in low-tolerance organisms such as C. thermocellum. Emerging evidence instead implicates metabolic redox imbalance (specifically, elevated NADH/NAD+ ratios driven by alcohol dehydrogenase (AdhE) activity leading to glycolytic blockage) as a dominant inhibitory mechanism at low to moderate ethanol concentrations. This project will pursue three integrated aims: (1) define how and when AdhE-driven redox imbalance causes glycolytic blockage using metabolomics, isotope tracing, and targeted genetic perturbations; (2) dissect the genetic architecture of ethanol tolerance with a focus on cofactor specificity (NADH vs. NADPH) in alcohol dehydrogenases through adaptive laboratory evolution and rational engineering; and (3) quantify concentration-dependent membrane metabolite leakage and lipidomic adaptations using lipidomics and biophysical measurements. Data from all three aims will be integrated into a unified, concentration-stratified mechanistic model that delineates when metabolic versus membrane mechanisms dominate, and provides design rules for engineering strains with enhanced solvent tolerance. This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.