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Fig. 1 PGL is a mycobacterial cell–surface virulence factor. (A) Layers of the mycobacterial cell wall. CL = capsular layer, MM = mycomembrane, AG = arabinogalactan, PG = peptidoglycan, and PM = plasma membrane. (B) Simplified representative chemical structures of PDIM and PGL. PDIM and PGL structures are heterogeneous, and reports vary in the literature. For M. tuberculosis: X = 14–16; Y = methoxy, keto, or hydroxy; m = 3–5; n = 15–17; R = CH3 or H. For M. marinum: X = 14–16; Y = methoxy, keto, or hydroxy; m = 3–4; n = 16–18; and R = CH3. (C) Glycans of PGL vary according to the species of the mycobacteria.