Thus, our knockout mutants would be unchanged with respect to PAS uptake. It might just be possible that PAS is both an inhibitor of mycobactin biosynthesis as well as a folate analogue (although our personal view is that this is unlikely). This would, though, distinguish PAS from those compounds that are only antifolate compounds this website and are completely ineffective against mycobacteria. The specificity of PAS towards mycobacteria has to rest in it being

an inhibitor of some metabolic activity that is only found in the mycobacteria, and for this reason, we continue to believe that PAS is a salicylate analogue and works by inhibiting mycobactin synthesis – which, of course, is a sequence only found in the mycobacteria. The mode of action of PAS has never been particularly clear. Because it was established as an antimycobacterial agent well before the structure of mycobactin was elucidated (see Introduction), its mode of action was asserted to be that of an antifolate agent and it was thus, like the sulphonamide drugs, an analogue of PABA. However, it was never clear why the sulphonamides were completely ineffective against mycobacterial infections and why PAS was ineffective against

other bacteria and so specific for mycobacteria. (This contrary evidence was elegantly summarized by Winder 1982). Unfortunately, once the original assertion had been made selleck inhibitor that PAS was an antifolate drug, this became widely accepted and written into many standard textbooks covering the mode of action of antimicrobial agents; this view has been very hard to reverse. However, once mycobactin had been discovered and the nearly active synthesis and accumulation of salicylic acid by mycobacteria had been established, it appeared, at least to us, that PAS was more likely to be an inhibitor of mycobactin biosynthesis (Ratledge & Brown, 1972). Our subsequent work (Brown & Ratledge, 1975; Adilakshmi et al., 2000) has

provided support for this view. Of course, definitive proof of PAS being an inhibitor of mycobactin biosynthesis must await the development of appropriate assays for the individual enzymes of the pathway, but these assays may be difficult to achieve due to the complexity of the reactions and the apparent need for carrier proteins to be attached to the various intermediates (Quadri et al., 1998; Ratledge, 2004). Our hypothesis on the mode of action of PAS is now considerably strengthened with these present results. It does occur to us, though, that as the effectiveness of PAS is considerably enhanced by preventing salicylate biosynthesis – i.e. using the salicylate knockout mutants – then its efficacy as an antituberculosis agent should be similarly increased by administering it along with an inhibitor of salicylate synthase as has been achieved recently by Payne et al.