Acid-fast bacteria in the boundary surface between salt solution and a test oil (tricaprylin) are spontaneously wet and enveloped by the oil. This behavior contrasts with that of all other cells studied by the interfacial tension method.

Four strains of human tubercle bacillus and an atypical bovine strain are an exception to the first statement above. These have possessed stability in the saline-oil interface; this stability is slight, however, and not comparable with that of non-acid-fast bacteria.

Acid-fast bacteria subjected to prolonged extraction with alcohol show resistance to wetting by oil comparable to that of non-acid-fast bacteria. These "defatted" bacteria nevertheless retain their acid-fast staining properties. Acid fastness cannot then depend on the integrity of a surface membrane.

Study of the cataphoresis of acid-fast bacteria by Freund has rendered the presence of protein in the surface highly probable. We are forced then to regard the surface of acid-fast bacteria as complex, containing at least lipoid and protein. Not improbably also carbohydrate is present.

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