Dual Nature of Acid-Fastness **

Acid-fastness is usually defined by the Ziehl-Neelsen technique: staining in carbol-fuchsin, differentiation in HCl-alcohol, and counterstaining in methylene blue.8 As a result, acid-fast substances are red, non-acid-fast materials, blue. Some authors have felt that this property of differential staining resided in one specific type of compound, the mycolic acid type of long-chain fatty acid found only in the acid-fast mycobacteria." Most authorities, however, now consider that the bulk of the evidence supports a structural rather than a chemical explanation of the phenomenon. It is certainly true that grindinge6 or crushing' tubercle bacilli abolishes their acid-fastness although no change in chemical composition may be detected. Similar, but not identical, acid-fastness has been produced in a number of usually non-acid-fast bacteria by coating them with lipid not in itself acid-fast.' Spores of bacteria are usually acid-fast9 so long as their protective capsule is present. Finally, mycolic acid, the presumed acid-fast compound in tubercle bacilli, was reexamined'9'2' and found to be somewhat acid-fast, but not strongly enough to account for the brilliant staining of intact bacilli. It has been recently noted2 that under certain conditions spermatozoa are acid-fast. When an explanation of this observation was sought, certain contradictions in the basic theory became manifest. The original report of acid-fastness was by Neisser." He reported that lepra bacilli, once stained, were resistant to acid destaining. This was in contrast to the ease with which other known bacteria were destained. A year later Ehrlich" noted a similar property in tubercle bacilli. In these latter organisms, this resistance to destaining was coupled with a resistance to staining and to chemical treatment generally.'0"15 Out of this generalized resistance of tubercle bacilli has grown the structural theory of acid-fastness.' However, neither lepra bacilli" nor spermatozoa showed any staining resistance comparable to tubercle bacilli. It therefore appeared questionable whether the structural theory of acid-fastness as presently developed was completely valid. The object of this present paper then is to study both the differences and simi-