Molecular basis underlying Mycobacterium tuberculosis D-cycloserine resistance. Is there a role for ubiquinone and menaquinone metabolic pathways?

INTRODUCTION: Tuberculosis remains a formidable threat to global public health. Multidrug-resistant tuberculosis presents increasing burden on the control strategy. D-Cycloserine (DCS) is an effective second-line drug against Mycobacterium tuberculosis (M. tuberculosis), the causative agent of tuberculosis. Though less potent than isoniazid (INH) and streptomycin, DCS is crucial for antibiotic-resistant tuberculosis. One advantage of DCS is that less drug-resistant M. tuberculosis is reported in comparison with first-line antituberculosis drugs such as INH and rifampin. AREAS COVERED: In this review, we summarise our current knowledge of DCS, and review the drug target and low-level resistance of DCS in M. tuberculosis. We summarise the metabolism of D-alanine (D-Ala) and peptidoglycan biosynthesis in bacteria. We first compared the amino acid similarity of Mycobacterium alanine racemase and D-Ala:D-alanine ligase and quite unexpectedly found that the two enzymes are highly conserved among Mycobacterium. EXPERT OPINION: We summarise the drug targets of DCS and possible mechanisms underlying its low-level resistance for the first time. One significant finding is that ubiquinone and menaquinone metabolism-related genes are novel genes underlying DCS resistance in Escherichia coli and with homologues in M. tuberculosis. Further understanding of DCS targets and basis for its low-level resistance might inspire us to improve the use of DCS or find better drug targets.