ACID-FAST BACILLI 253 



and others). Diffuse daylight has the same effect but in a less 

 degree. Obviously, however, these results are not obtained when 

 the bacillus is protected by the medium in which it is present, as 

 in the case of milk for example. 



(iv.) Desiccation. — In ordinary dust, Cornet was the first to 

 demonstrate that the tubercle bacillus might retain its vitality and 

 virulence for long periods of time. Sawizky put this at two and a 

 half months, but there is little doubt that tubercular sputum in a 

 dried form may remain virulent for many months (Schill and 

 Fisher, Galtier, Swithinbank, and others). 



(v.) Chemical substances.— Koch, Yersin, Crookshank, and 

 many others have demonstrated the germicidal effect of carbolic 

 acid on tubercle bacilli. It is generally accepted that these bacilli 

 are killed by a 5 per cent, solution in one minute. Absolute 

 alcohol kills them in five minutes; mercuric chloride (i-iooo) in 

 ten minutes ; and tricresol (i per cent), formalin (5 per cent), lysol 

 (2 per cent.), and chloride of lime (2 per cent.) are also held to be 

 germicidal to the tubercle bacillus. 



Acid-fast Bacilli allied to the Tubercle Bacillus 

 We are now in a position to consider the group of organisms 

 morphologically and tinctorially similar to the bacillus tuberculosis. 

 This group is known as that of the acid-fast bacilli on account of 

 the fact that in staining by the Ziehl-Neelsen method {see p. 60) 

 these organisms possess, like the tubercle bacillus, the power of 

 resisting decolorisation by the acid following the red stain.^ In 

 England such bacilli are termed acid-fast, in Germany saurefeste, 

 and in France acidophile. The group is one of great importance, 

 partly on account of the ease with which its members may be 

 mistaken for the " true " tubercle bacillus, and partly on account of 

 the relationship which appears to exist between them and the 

 tubercle bacillus. Some bacteriologists hold that possibly these 

 acid-fast bacilli represent a saprophytic stage in the life-history of 

 the true tubercle bacillus. 



Classification of acid-fast bacilli.— These bacilli may be 

 divided provisionally and for convenience into four chief sub- 

 divisions : — 



{a) The acid-fast bacilli of other diseases or conditions affecting 



1 Acid-fastness is due, in all probability, not to fat in the bacillus, but to a 

 substance of the nature of wax, which can be extracted by acid-alcohol, ether, 

 or other wax solvents. For a discussion of this subject see Trans. British 

 Congress of Tuberculosis, 1901, voL iii., pp. 498-502 (Bulloch). 



