Organic dyes were used very extensively as anti-infective agents before the discovery of the sulfonamides and the antibiotics. A few cationic dyes still find limited use as anti-infectives. These include the triphenylmethane dyes gentian violet and basic fuchsin and the thiazine dye methylene blue. The dyes form colorless leucobase forms under alkaline conditions. Cationic dyes are active against Gram-positive bacteria and many fungi; Gram-negative bacteria are generally resistant. The difference in susceptibility is probably related to the cellular characteristics that underlie the Gram stain.
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