Figure 1.

Targeting mycolic acid transport in extracellular and intracellular M. abscessus. (i) While rough (R) M. abscessus aggressively clumps and grows as serpentine cords in the extracellular milieu, evading phagocytosis by macrophages, (ii) smooth (S) M. abscessus, which is present as mostly singular organisms in the extracellular milieu, is easily phagocytosed. Once inside macrophages (iii) the R form is present in large social phagosomes containing numerous bacteria. These phagosomes mature rapidly and fuse with lysosomes. However, despite the acidic and radical environment present within these phagolysosomes, the R variant continues to divide rapidly, overpowering the macrophage defenses and resulting in autophagy and apoptosis. The S variants on the other hand (iv) remain in immature phagosomes because a tight apposition of their cell walls is maintained all around with the phagosomal membrane. These bacteria are not toxic to the cells and do not impact greatly upon the survival of the infected macrophages. The S form then (v) induces phagosome-cytosol communications through disruptions in the phagosome membrane providing access to cytosolic nutrients potentially, chronically sustaining a small population of persistent bacteria. Another feature of the S form-containing phagosomes is the large electron translucent zone (ETZ) which, is observable by electron microscopy and which is almost completely absent in R form-containing phagosomes. The ETZ which is a large outermost part of the mycobacterial cell wall is dependent upon the presence of large quantities of glycopeptidolipids (GPL). Mutations, for example resulting in amino acid substitutions in critical residues of the protein MmpL4a (vi), which transports GPL from the cytosolic face of the bacterial plasma membrane where they are made to the outer membrane, result in the disappearance of a prominent ETZ. Inhibition of the mycolic acid transporter, MmpL3 (vii), by a piperidinol-based derivative (compound 1) or indole-2-carboxamides (compounds 2 and 3), leads to abrogation of arabinogalactan mycolylation and of the production of trehalose dimycolate (TDM). This efficiently stops the growth of both intracellular and extracellular M. abscessus.