THE TUBERCLE BACILLUS 189 



sistant to heat and drying than others in which they were not 

 found and that consequently they could not be regarded as spores. 



Occasionally long thread-like branching forms, sometimes with 

 swollen ends, are seen. They are considered by some observ- 

 ers as involution forms; others regard them as normal and on 

 this basis class the tubercle bacilli either with the higher bacteria 

 (trichomycetes) or with the true molds; still others place them 

 and closely related forms in a separate group intermediate between 

 the ordinary bacteria and the higher forms. Their classification 

 is still an unsettled question. 



Another peculiarity of the tubercle bacillus is the possession of 

 a waxy envelope, which not only confers upon it an additional 

 degree of resistance, but which also prevents it from readily taking 

 up the ordinary anilin dyes. Once stained, however, it is with diffi- 

 culty that the color can be removed even by the use of strong 

 acids. The property is so characteristic that the tubercle bacillus 

 and certain other closely related organisms are termed " acid- 

 fast." This staining peculiarity makes it possible to recognize 

 the organism immediately in film preparations from pus, sputum, 

 etc. Details of the special stains and methods employed have al- 

 ready been described. 



Cultivation. On account of their slow growth tubercle bacilli 

 are difficult to cultivate and even more difficult to isolate. Koch 

 succeeded in obtaining a pure culture by carefully rubbing tuber- 

 culous tissue over the surface of coagulated beef serum, and then 

 after about two weeks' incubation there appeared minute points of 

 irregular whitish growth which he compared to small, dry scales. 

 Once isolated the organism will grow readily on egg medium or 

 on agar containing 3 to 5 per cent glycerin ; in from ten to fourteen 

 days growth appears as a dull, whitish, wrinkled film. In thin 

 layers of glycerin broth, if a small amount of growth is carefully 

 placed on the surface it spreads as a wrinkled pellicle from one 

 side of the container to the other; this method of cultivation is 

 usually employed for the production of tuberculin. 



It frequently happens that in tuberculous tissue tubercle bacilli 

 are found free from contaminating organisms and a practically 



