406 



MYCOBACTERIUM 



the tubercle bacilli are capable of producing filaments — particularly in liquid media 

 — but these forms are not encountered under ordinary conditions. Clubbed forms, 

 resembling the typical clubs of the diphtheria bacillus, are not uncommon in cul- 

 ture ; branched forms have been described by some authors, but are probably 

 infrequent, except in the avian bacilli. Staining is either uniform or granular ; 

 in the latter type the granules may be restricted to the poles, or they may be 

 evenly distributed throughout the length of the bacillus — the so-called beaded 

 form. In bacilli that appear to be undergoing degeneration, the staining is often 

 irregular both in depth and in situation. In young cultures it is common to find 

 a certain proportion of non-acid-fast forms. The morphology of the developing 

 organisms may be studied by Pryce's (1941) slide-cell technique of micro-culture. 

 On the average human bacilli tend to be long, thin, and curved, and to show 

 granular staining, while bovine bacilli tend to be short, straight, and thick, and to 

 show uniform staining. Their morphology, however, is so variable, and is so 

 dependent upon environmental factors, that no weight can be attached to these 



<« . 



' / 



4? *y . 



'\ ♦ 







* ^ 

 ^—' 



.^^ 





'^ , 



, z' 



C . 



1 



' /•. 



' J 



Fig. 68. — Myco. tuberculosis. 

 Glycerine egg-culture, 4 weeks, 37° C, show- 

 ing short, straight forms of bacilli ( X 1000). 



Fig. 69. — Myco. tuberculosis. 

 Glycerine agar culture, 4 weeks, at 37° C, 

 showing some short, straight forms, and 

 some longer curved forms ( X 1000). 



criteria in the identification of individual strains. Murine bacilli, according to 

 Griffith (1942), are slender and often longer than human bacilli. Curved forms 

 of the shepherd's crook, sickle, spiral, and S-type are abundant and characteristic. 

 Some organisms show fine granulation and vacuolation along their whole length. 

 The acid-fast bacilli are Gram-positive. Staining is not always easy, but with 

 a 5 per cent, solution of gentian violet in alcohol and aniline oil, aided by gentle 

 warming, it is usually possible to obtain satisfactory preparations. According 

 to Kretschmer (1934), the Gram-positiveness is independent of treatment with 

 iodine, and is closely bound up with the property of acid-fastness. 



The organisms are resistant to simple solutions of the aniline dyes. To overcome 

 this difficulty several methods of staining have been devised. Koch (1882) first 

 stained the tubercle bacillus by immersion for 24 hours in an alkaline solution of 

 methylene blue. Ehrlich (1882) improved on this by using aniline oil basic fuchsin 

 or aniline oil methyl violet. By this means the bacilli were stained in 15 to 30 

 minutes and subsequently resisted decolorization with 33 per cent. HNO3 for a few 



