Transcriptional Responses to Oxidative Stress in Clostridium thermocellum

Redox stress and imbalance are likely contributors to fermentation yield and bioproduction inefficiencies in fermentations using Clostridium thermocellum.  C. thermocellum batch fermentations at different redox potentials, cells were cultured with 0, 40, 100, 150, 200, 275, 325 and 400 mg/L methyl viologen (a terminal electron acceptor).  As proxies for putative effects of redox stress, product profiles and cell growth were monitored, O/R and carbon recovery balances were calculated.  We observe differences in culture redox potentials, maximal growth rates and cell densities achieved as well as fermentation product profiles at different methyl viologen concentrations, possibly indicating a response to oxidative cellular stress induced by this treatment.  Growth rate and maximum cell density in 400 ug/mL methyl viologen were both approximately half of those in untreated controls.  Redox stress is also indirectly evidenced during C. thermocellum fermentation by amino acid production under standard fermentation conditions, likely resulting from a need to re-oxidize nicotinamide redox cofactors reduced during glycolysis.  We will also present transcriptomic expression analysis from carbon-limited cellobiose fermentations in which methyl viologen concentration was increased from 0 to 400 mg/L, resulting in an oxidized fermentation environment.  We interpret expression differences over this range of methyl viologen concentrations as the transient transcriptional response(s) in C. thermocellum to increasing levels of oxidative stress.  These studies will be useful to elucidate electron and carbon flow in C. thermocellum in the future.