Alanine racemase mutants of Mycobacterium tuberculosis require D-alanine for growth and are defective for survival in macrophages and mice

作者：Awasthy Disha; Bharath Sowmya; Subbulakshmi Venkita; Sharma Umender^*

来源：Microbiology-SGM, 2012, 158(2): 319-327.

DOI：10.1099/mic.0.054064-0

Alanine racemase (Alr) is an essential enzyme in most bacteria; however, some species (e.g. Listeria monocytogenes) can utilize D-amino acid transaminase (Dat) to generate D-alanine, which renders Alr non-essential. In addition to the conflicting reports on gene knockout of air in Mycobacterium smegmatis, a recent study concluded that depletion of Alr does not affect the growth of M. smegmatis. In order to get an unambiguous answer on the essentiality of Alr in Mycobacterium tuberculosis and validate it as a drug target in vitro and in vivo, we have inactivated the air gene of M. tuberculosis and found that it was not possible to generate an air knockout in the absence of a complementing gene copy or D-alanine in the growth medium. The growth kinetics of the air mutant revealed that M. tuberculosis requires very low amounts of D-alanine (5-10 mu g ml(-1)) for optimum growth. Survival kinetics of the mutant in the absence of D-alanine indicated that depletion of this amino acid results in rapid loss of viability. The air mutant was found to be defective for growth in macrophages. Analysis of phenotype in mice suggested that non-availability of D-alanine in mice leads to clearance of bacteria followed by stabilization of bacterial number in lungs and spleen. Additionally, reversal of D-cycloserine inhibition in the presence of D-alanine in M. tuberculosis suggested that Alr is the primary target of D-cycloserine. Thus, Alr of M. tuberculosis is a valid drug target and inhibition of Air alone should result in loss of viability in vitro and in vivo.

• 出版日期2012-2