416 



MYCOBACTERIUM 



changed conditions. If, for example, a suspension of tubercle bacilli is inocu- 

 lated simultaneously in equal quantities on to a number of different media, 

 quite'a variety of colonial types will develop (Fig. 76). If the different types are 

 picked off and inoculated all on to the same medium, complete similarity of colonial 

 appearance will often result, indicating that no fundamental biological change 

 has been effected. What degree of fixity variant types may have, it is very 

 difficult to ascertain, and we believe that before further progress is possible, a 

 very careful study of the limits of transient environmental variation will have 

 to be made. Without expressing any opinion on the degree of fixity of types, 

 or of giving a critical review of the extensive bibliography that has grown up 

 on the subject of variation, we shall confine ourselves to describing two main 

 types possessing correlated morphological and cultural appearances. 



Fig. 76. — Myco. tuberculosis — bovine type. 



Surface colonies on tliree different media, inoculated at the same time from the same sus- 

 pension and incubated under identical conditions ; 37 days, 37° C. The figure illustrates the 

 dependence of the colonial form on the nature of the medium. 



Left : Modified Dorset egg medium. Middle : Egg yolk medium. Right : Egg yolk agar 

 medium. 



On Dorset or 5 per cent, glycerol egg medium many cultures of tubercle and 

 saprophytic acid-fast bacilli produce colonies that are either smooth or rough. 

 In stroke cultures the smooth forms give rise to a moist butyrous growth with 

 a smooth glistening surface, while the rough forms produce a dry, rather 

 friable growth, with a rough, dull, and often heaped-up surface, looking not un- 

 like dry bread crumbs (Fig. 77). Rubbed up in water the smooth growth yields 

 a fairly homogeneous suspension, the rough growth a granular suspension. Single 

 colonies of the smooth form tend to be circular, convex, with a generally smooth 

 surface and an entire edge ; rough colonies are irregular in outline, are often 



