Cellulose hydrolysis can be accomplished by cellulase enzymes acting without cells (e. g., cellulose hydrolysis in SHF), by cellulases acting in the presence of cells but with no cell — enzyme attachment (e. g., SSF), or by cellulases attached to cells (e. g., C. thermocellum in CBP). In the latter case, hydrolysis is mediated by ternary cellulose-enzyme-microbe (CEM) complexes rather than binary cellulose-enzyme (CE) complexes. For anaerobic cellulolytic bacteria, CEM complexes are commonly formed and are thought to be the major agent of cellulose hydrolysis (15). Potential benefits of CEM complexes for cellulolytic microorganisms have been suggested, including preferred access to hydrolysis products and local concentration of cellulases (15, 51-54).

Lu and coworkers (55) investigated such “cell-enzyme synergy” for C. thermocellum. Figure 16.5 shows that cellulose concentration changes are plotted against time for different cases:

• Microbial hydrolysis (CEM) — C. thermocellum culture with 100 mg/L cellulosome, 264 mg/L cell protein;

• Microbial control (CEM but non-active cells by chemical inhibitor) — a C. thermocellum culture with 100 mg/L cellulosome, 264 mg/L cell protein plus 38.5 mM sodium azide, a cell-growth inhibitor;

• CE in SSF — 100 mg/L purified C. thermocellum cellulase and active T. saccharolyticum that assimilates hydrolysis products;

• Cell-free control 1 (CE in SHF), 100 mg/L purified C. thermocellum cellulase; and

• Cell-free control 2 (CE in SHF plus chemical inhibitor), 100 mg/L purified cellulase

C. thermocellum cellulase with 38.5 mM sodium azide.

Comparison of microbial cultures (CEM) and cellulase-enzyme in the presence of T. saccharolyticum (SSF) cultures suggests that the C. thermocellum cellulase complex is substantially more effective during microbial hydrolysis compared to SSF under the con­ditions examined. Such “enzyme-microbe synergy” requires the presence of metabolically active cellulolytic microbes, and is not explained by removal of hydrolysis products from the bulk fermentation broth because (see control experiments).

From an applied perspective, the 2.7- to 4.7-fold synergistic effect reported is significant in the context of the search for strategies to decrease the cost of enzymatic hydrolysis, a focus of considerable effort since the late 1990s (3, 24, 56, 57). From a fundamental perspective, such synergy is one of the important inherent mechanisms to reduce cellulase synthesis requirement for anaerobic cultures.