1-33: Measuring rates of microbial cellulose utilization

Cellulosic ethanol has gained renewed interest in the last decade as an alternative transportation fuel.  Commerical cellulosic ethanol is likely to be produced by large-scale microbial solid substrate fermentations, the most cost-effective of which is Consolidated Bioprocessing (CBP). In CBP, micro-organisms convert solid lignocellulosic substrates to ethanol without the need of adding externally produced enzymes^1,2. The physiology of microbial cellulose utilization on solid substrates as applied in CBP is poorly understood. One hindrance is the lack of methods that discriminate between cellulose particles and microbes.

Clostridium thermocellum is an anaerobic, thermophylic model CBP organism that can convert crystalline cellulose to ethanol. For utilization of cellulose the cells adhere and colonize the substrate particles, cells can therefore not be discriminated from substrate by conventional methods. This makes accurate evaluations of solid substrate cultures difficult³.

We have developed methods based on elemental analysis and real-time fermentation data that accurately determine rates of cell growth and substrate utilization for C. thermocellum on solid substrates. We compared bioreactor-grown C. thermocellum cultures on cellulose and cellobiose. By applying our methods, we are able to determine cell growth rates and substrate utilization rates. Furthermore, we can determine maximal specific growth rates and maximal cell specific substrate utilization rates. Our findings show distinct differences between cells growing on solid substrate and soluble substrate, and give valuable insights into solid-substrate physiology.

1: Farrell et al, Science. 2006 Jan 27;311(5760):506-8.

2: Lynd et al, Curr Opin Biotechnol. 2005 Oct;16(5):577-83.

3: Lynd et al, Microbiol Mol Biol Rev. 2002 Sep;66(3):506-77.