Growth rate of mixed ruminal bacteria as a function of energetic substrate concentration in bath culture

The objective of this study was to evaluate levels of sucrose on microbial growth. Roll tubes were filled with 15.6 mL of buffer Chen, 1 mL of inocula (rumen fluid from fistulated steer at pasture), 1 mL of Trypticase or hydrolyzed casein (1 g L -1 as final concentration) and 2.4 mL of sucrose solutions (0.00 g L -1 , 0.37 g L -1 , 0.75 g L -1 , 1.5 g L -1 , 3.0 g L -1 , 6.0 g L -1 , 12.0 g L -1 , and 24.0 g L -1 ), in triplicate. The pH was 5.56 at 0 h and 4.6 after 6 h, and 5.83 in 0 g L -1 and 4.18 above 1.5 g L -1 of sucrose. The optical density (OD-600 nm) and microbial protein (MP-595 nm) tended to stabilize after 12 h and 6 g L -1 of sucrose, with no interaction between them. The OD (6 h to 24 h mean values) was curvilinear (hyperbolic), as a function of sucrose concentration, followed the saturation kinetics of Michaelis-Menten, which is typical of enzymes systems, and was described by the following Lineweaver-Burk equation: 1/OD = 0.508*(1/sucrose) + 1.15, R² = 0.99. The theoretical maximum microbial growth (1/a = k max ) was 0.87 OD and the concentration of sucrose to reach half of k max (b/a = K s ) was 0.44 g L -1 of sucrose. The microbial growth was affected by incubation time, substrate concentration, and culture medium acidification. KEY-WORDS: Lineweaver-Burk; Michaelis-Menten; pH; saturation kinetics; sucrose.