In this dormant metabolic state, the bacterial cell wall thickness is increased, protein and nucleic acid syntheses are significantly downregulated and lipid metabolism appears to be the primary energy source (Wayne & Sohaskey, 2001; Timm et al., 2003). These changes are accompanied by characteristic up-regulation of a set of 48 genes, referred to as the dosR regulon (Voskuil et al., 2003). This major remodeling of key metabolic pathways leads to decreased sensitivity for currently used

antibiotics (Gomez & McKinney, 2004), and is thus an important factor responsible for the extended tuberculosis treatment time in patients (6–9 months). In spite of the dormant phenotype, these bacteria still have basal energy requirements to maintain critical metabolic functions SP600125 manufacturer (Koul et al., 2008). In recent years, significant information has been gained on the essentiality of respiratory chain components in dormant Belnacasan mouse as well as in replicating bacteria. The identification of new candidate drugs targeting the ATP-producing machinery illustrates the therapeutic potential of blocking mycobacterial energy conversion (Andries et al., 2005; Weinstein et al., 2005). Many bacteria, such as Escherichia coli and Bacillus subtilis,

can synthesize sufficient ATP for growth using substrate-level phosphorylation of fermentable carbon sources (Friedl et al., 1983; Santana et al., 1994). However, in the case of M. tuberculosis, ATP synthase is required for optimal growth as revealed by high-density

mutagenesis (Sassetti et al., 2003). Moreover, in Mycobacterium smegmatis deletion mutants indicated an essential function of ATP synthase for growth on fermentable as well as nonfermentable carbon sources (Tran & Cook, 2005). These findings suggest that mycobacteria cannot gain enough energy by substrate-level phosphorylation and need respiratory ATP synthesis for growth. In the respiratory chain, two types of NADH dehydrogenases are present in most mycobacteria Rho for NADH oxidation and for feeding reducing equivalents into the electron transport pathway (Fig. 1). However, the proton-transporting type-I NADH dehydrogenase (NDH-1), encoded by the nuo operon, is not essential in M. tuberculosis (Sassetti et al., 2003; Rao et al., 2008) and is largely deleted from the genome of Mycobacterium leprae (Cole et al., 2001). Alternatively, NADH can be oxidized by a non-proton-translocating, type-II NADH dehydrogenase (NDH-2), using menaquinone as an electron acceptor (Fig. 1). In M. tuberculosis, NDH-2 is present in two copies, referred to as Ndh and NdhA, whereas in M. smegmatis, only one copy is found (Weinstein et al., 2005). Mutagenesis studies in M. smegmatis indicated an essential function of NDH-2 for survival (Miesel et al., 1998). Chemical inhibition of NDH-2 was reported to be bactericidal for M. tuberculosis, whereas typical inhibitors of the NDH-1 did not have a significant effect (Rao et al., 2008).