Nutrient acquisition by mycobacteria.

The growth and nutritional requirements of mycobacteria have been intensively studied since the discovery of Mycobacterium tuberculosis more than a century ago. However, the identity of many transporters for essential nutrients of M. tuberculosis and other mycobacteria is still unknown despite a wealth of genomic data and the availability of sophisticated genetic tools. Recently, considerable progress has been made in recognizing that two lipid permeability barriers have to be overcome in order for a nutrient molecule to reach the cytoplasm of mycobacteria. Uptake processes are discussed by comparing M. tuberculosis with Mycobacterium smegmatis. For example, M. tuberculosis has only five recognizable carbohydrate transporters in the inner membrane, while M. smegmatis has 28 such transporters at its disposal. The specificities of inner-membrane transporters for sulfate, phosphate and some amino acids have been determined. Outer-membrane channel proteins in both organisms are thought to contribute to nutrient uptake. In particular, the Msp porins have been shown to be required for uptake of carbohydrates, amino acids and phosphate by M. smegmatis. The set of porins also appears to be different for M. tuberculosis and M. smegmatis. These differences likely reflect the lifestyles of these mycobacteria and the availability of nutrients in their natural habitats: the soil and the human body. The comprehensive identification and the biochemical and structural characterization of the nutrient transporters of M. tuberculosis will not only promote our understanding of the physiology of this important human pathogen, but might also be exploited to improve tuberculosis chemotherapy.