Characterization of lignocellulose particles during lignocellulose solubilization by Clostridium thermocellum

Metabolome Analysis of Constituents in Membrane Vesicles for Clostridium thermocellum Growth Stimulation

The cultivation of the cellulolytic bacterium, Clostridium thermocellum, can have cost-effective cellulosic biomass utilizations, such as consolidated bioprocessing, simultaneous biological enzyme production and saccharification. However, these processes require a longer cultivation term of approximately 1 week. We demonstrate that constituents of the C. thermocellum membrane vesicle fraction significantly promoted the growth rate of C. thermocellum. Similarly, cell-free Bacillus subtilis broth was able to increase C. thermocellum growth rate, while several B. subtilis single-gene deletion mutants, e.g., yxeJ, yxeH, ahpC, yxdK, iolF, decreased the growth stimulation ability. Metabolome analysis revealed signal compounds for cell–cell communication in the C. thermocellum membrane vesicle fraction (ethyl 2-decenoate, ethyl 4-decenoate, and 2-dodecenoic acid) and B. subtilis broth (nicotinamide, indole-3-carboxaldehyde, urocanic acid, nopaline, and 6-paradol). These findings suggest that the constituents in membrane vesicles from C. thermocellum and B. subtilis could promote C. thermocellum growth, leading to improved efficiency of cellulosic biomass utilization.

Cellulosomes localise on the surface of membrane vesicles from the cellulolytic bacterium Clostridium thermocellum

Membrane vesicles released from bacteria contribute to cell–cell communication by carrying various cargos such as proteins, nucleic acids and signaling molecules. Cellulolytic bacteria have been isolated from many environments, yet the function of membrane vesicles for cellulolytic ability has been rarely described. Here, we show that a Gram-positive cellulolytic bacterium Clostridium thermocellum released membrane vesicles, each approximately 50–300 nm in diameter, into the broth. The observations with immunoelectron microscopy also revealed that cellulosomes, which are carbohydrate-active enzyme complexes that give C. thermocellum high cellulolytic activity, localized on the surface of the membrane vesicles. The membrane vesicles collected by ultracentrifugation maintained the cellulolytic activity. Supplementation with the biosurfactant surfactin or sonication treatment disrupted the membrane vesicles in the exoproteome of C. thermocellum and significantly decreased the degradation activity of the exoproteome for microcrystalline cellulose. However, these did not affect the degradation activity for soluble carboxymethyl cellulose. These results suggest a novel function of membrane vesicles: C. thermocellum releases cellulolytic enzymes on the surface of membrane vesicles to enhance the cellulolytic activity of C. thermocellum for crystalline cellulose.