Background: An impediment to the rational development of novel drugs against tuberculosis (TB) is a general paucity of knowledge concerning the metabolism of Mycobacterium tuberculosis, particularly during infection. Constraint-based modeling provides a novel approach to investigating microbial metabolism but has not yet been applied to genome-scale modeling of M. tuberculosis.

An experimental system of Mycobacterium tuberculosis growth in a carbon-limited chemostat has been established by the use of Mycobacterium bovis BCG as a model organism. For this model, carbon-limited chemostats with low concentrations of glycerol were used to simulate possible growth rates during different stages of tuberculosis. A doubling time of 23 h (D ‫؍‬ 0.03 h ؊1 ) was adopted to represent cells during the acute phase of infection, whereas a lower dilution rate equivalent to a doubling time of 69 h (D ‫؍‬ 0.01 h ؊1 ) was used to model mycobacterial persistence. This chemostat model allowed the specific response of the mycobacterial cell to carbon limitation at different growth rates to be elucidated. The macromolecular (RNA, DNA, carbohydrate, and lipid) and elemental (C, H, and N) compositions of the biomass were determined for steady-state cultures, revealing that carbohydrates and lipids comprised more than half of the dry mass of the BCG cell, with only a quarter of the dry weight consisting of protein and RNA. Consistent with studies of other bacteria, the specific growth rate impacts on the macromolecular content of BCG and the proportions of lipid, RNA, and protein increased significantly with the growth rate. The correlation of RNA content with the growth rate indicates that ribosome production in carbon-limited M. bovis BCG cells is subject to growth rate-dependent control. The results also clearly show that the proportion of lipids in the mycobacterial cell is very sensitive to changes in the growth rate, probably reflecting changes in the amounts of storage lipids. Finally, this study demonstrates the utility of the chemostat model of mycobacterial growth for functional genomic, physiology, and systems biology studies.With three million people dying from tuberculosis (TB) annually, Mycobacterium tuberculosis remains a formidable pathogen. Tuberculosis ranks among the top 10 causes of global mortality and morbidity and is the leading cause of infectious disease (66). The ability of M. tuberculosis to adapt to and survive harsh environmental conditions in order to establish and maintain long-term infections within its human host is fundamental to this organism's success. Modification of the mycobacterial cell in response to changes in the environment is crucial to this adaptive process, but detailed information about how M. tuberculosis changes its macromolecular composition in response to its environment and growth rate is lacking.The genome sequence of M. tuberculosis has been available since 1998 (9). Although information obtained from the genome sequence provided new and valuable insights into the biology of the tubercle bacillus, the genome itself provides few clues regarding how the pathogen responds to its environment by changing its cellular composition. One of the principal tasks of postgenomic biological studies of M. tuberculosis is to understand how the genome orchestrates the structure and dynamics of the cell in response to changes in the environment. This task requires an integrat...