The Nature of Acid-Fastness

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Abstract
In cells stained by the Ziehl-Neelsen technique, the dye in the cell may be precipitated as beads, and later redissolved. No structures about which these beads might form can be demonstrated, and furthermore they vary in location in the same cell after dissolution and reformation. The dye may be removed from cells without destroying their acid-fastness. The same results may be obtained with acid-fast structures other than mycobacteria. These facts support the authors'' explanation of the nature of acid-fastness, namely, that in the Ziehl-Neelsen technique the dye enters the cell interior through the cytoplasmic membrane. A small portion is absorbed by the cytoplasm, but the main bulk is held in the cell because it cannot diffuse through the cytoplasmic membrane, and it is this free dye[long dash]demonstrated above[long dash]which gives the characteristic red color to the stained bacillus.