250 MICROBIOLOGY 



Glycerin agar. Upon glycerin agar (glycerin 3 to 6 per 

 cent) at 37.5 C., colonies become visible at the end of from 

 ten days to two weeks, at first as dry, white spots; later as 

 delicately corrugated membranes.. The colonies of the avian 

 variety have a delicate spreading border about the thicker 

 center. 



Glycerin bouillon (made of beef or veal with peptone one 

 per cent, glycerin 5 per cent, and rendered slightly alkaline) 

 is an extremely favorable medium. The bouillon should be in 

 shallow layers, in wide mouthed flasks, as the free access of 

 oxygen is essential for growth. The inoculation of this medium 

 should be made by carefully floating flakes of the growth upon 

 the surface. In multiplying, the bacteria will spread out upon 

 the surface, at first as a thin, opaque, floating membrane which 

 rapidly thickens into a whitish wrinkled or granular layer, 

 covering the entire surface of the fluid in from four to six 

 weeks. Later, portions of the membrane may sink to the bot- 

 tom. In old cultures, the membrane assumes a yellowish hue. 

 These cultures emit a peculiar aromatic odor. 



Pathogenesis. The pathogenesis of the different varie- 

 ties of Bacterium tuberculosis varies. The human and bovine 

 varieties appear to be equally virulent for guinea pigs. The 

 lesions produced are quite characteristic and can be identified 

 usually by the gross appearance or microscopic examination 

 .of the structure of the tissue changes. The human variety is 

 not usually fatal to rabbits, cattle, or other animals and fre- 

 quently the tissue changes which it produces are very slight 

 and restricted to the point of inoculation. The bovine variety 

 is usually fatal to rabbits and cattle. When fed milk contain- 

 ing tubercle bacteria horses frequently develop tuberculosis. 

 Swine are very susceptible to the bovine variety. Sheep, goats, 

 cats and dogs are less susceptible. The avian variety is viru- 

 lent for fowls. Moore was unable to produce tuberculosis in 

 guinea pigs with the avian culture or to produce tuberculosis 

 in fowls with the human and bovine varieties by feeding or 

 by subcutaneous inoculation of pure cultures or of tubercu- 



