Mycolic acid

Mycolic acids are long-chain fatty acids in the cell envelopes of mycobacteria and some related bacteria. Their waxy, hydrophobic character helps form a permeability barrier.

Mycobacterial cell wall: The layered envelope surrounding mycobacteria, including peptidoglycan, arabinogalactan, and a lipid-rich outer membrane. Mycolic acids are covalently linked to arabinogalactan and help form its outer membrane.
Acid-fast staining: A staining method that identifies bacteria retaining dye after treatment with acid-alcohol. The lipid-rich, mycolic-acid-containing envelope helps mycobacteria retain the primary stain.
Fatty acid: A carboxylic acid with a hydrocarbon chain, used in biological lipids and metabolism. Mycolic acids are an unusually large and structurally complex fatty-acid family.
Lipopolysaccharide: A large glycolipid that forms the outer leaflet of the outer membrane in most Gram-negative bacteria. It serves an outer-membrane role in Gram-negative bacteria unlike the mycolate-based mycomembrane.
Mycomembrane: The lipid-rich outer membrane of mycobacteria, built largely from mycolic acids and associated lipids. Mycolic acids form a major structural framework of this distinctive membrane.
Ziehl–Neelsen stain: A heated acid-fast staining method using carbol fuchsin and an acid-alcohol decolorizer. It reveals the acid-fast phenotype associated with mycolic-acid-rich envelopes.
Hydroxy fatty acid: A fatty acid bearing one or more hydroxyl groups on its hydrocarbon chain. Mycolic acids contain a hydroxyl group that distinguishes their core structure.
Lipoarabinomannan: A lipoglycan in mycobacterial envelopes and membranes, consisting of a lipid anchor and arabinan-rich carbohydrate. It is a major envelope lipid-related molecule, but its defining structure is a lipoglycan, not a fatty acid.
Arabinogalactan: A polysaccharide in the mycobacterial cell envelope that links peptidoglycan to mycolic acids. It anchors covalently attached mycolic acids to the cell-wall core.
Isoniazid: An antimycobacterial drug that inhibits mycolic acid synthesis after activation by KatG. Its activity disrupts the production of a key envelope component.

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