Physiology and metabolic fluxes of wild-type and riboflavin-producing Bacillus subtilis

Abstract

Continuous cultivation in a glucose-limited chemostat was used to determine the growth parameters of wild-type Bacillus subtilis and of a recombinant, riboflavin-producing strain. Maintenance coefficients of 0.45 and 0.66 mmol of glucose g-1 h-1 were determined for the wild-type and recombinant strains, respectively. However, the maximum molar growth yield of 82 to 85 g (cell dry weight)/mol of glucose was found to be almost identical in both strains. A nonlinear relationship between the specific riboflavin production rate and the dilution rate was observed, revealing a coupling of product formation and growth under strict substrate-limited conditions. Most prominently, riboflavin formation completely ceased at specific growth rates below 0.15 h-1. For molecular characterization of B. subtilis, the total amino acid composition of the wild type was experimentally determined and the complete building block requirements for biomass formation were derived. In particular, the murein sacculus was found to constitute approximately 9% of B. subtilis biomass, three- to fivefold more than in Escherichia coli. Estimation of intracellular metabolic fluxes by a refined mass balance approach revealed a substantial, growth rate-dependent flux through the oxidative branch of the pentose phosphate pathway. Furthermore, this flux is indicated to be increased in the strain engineered for riboflavin formation. Glucose catabolism at low growth rates with reduced biomass yields was supported mainly by the tricarboxylic acid cycle.