CULTURAL DIFFERENTIATION 



413 



of the human type, but has little or no effect on that of the bovine type (Figs. 

 70 and 71). Thus on glycerine agar the human bacillus gives a thick, confluent, 

 wrinkled growth, while the bovine bacillus gives only a poor, effuse, ground- 

 glass growth, or may fail to grow at all. On glycerine potato again the human 

 bacillus gives a raised, confluent, wrinkled or warty growth, while the bovine 

 bacillus gives a poor, often discrete, effuse growth ; and similarly with the other 

 media. 



Another point of distinction is that the human type often forms a yellowish or 

 orange pigment, whereas the bovine type never does. The pigment is noticeable 

 only on batches of serum that 

 have a rich yellow colour. For this 

 reason the serum should come from 

 an old cow ; pale serum from a 

 young cow, or from another species 

 of animal, is useless. On a golden- 

 yellow serum the human type often 

 produces a rich yellow or orange- 

 yellow growth ; the bovine type 

 gives a non-pigmented growth. 

 On a serum coloured only slightly 

 yellow the growth of the human 

 type is cream or light yellow in 

 colour. The differentiation of the 

 human and bovine types may take 

 iTionths to complete ; often when 

 the organisms are first isolated, 

 they grow very poorly, and it is 

 not till they have become accus- 

 tomed to saprophytic conditions 

 that they give the best growths of 

 which they are capable. 



Cultural DifTerentiatiou of the 

 Murine from the Human and Bovine 

 Bacilli. — The murine type, as ex- 

 emplified by the vole bacillus, 

 grows much more slowly than 

 either of the other two mammalian 

 types, taking four weeks or more 

 to form colonies visible to the 

 naked eye on egg medium. Little or no growth occurs in primary culture on media 

 containing glycerine, and even in subculture growth is poor. Unlike the other 

 two types, the murine bacillus grows in plain trypsinized broth, producing film-like 

 colonies on the surface and a fine deposit. It grows likewise on potato without 

 glycerine. 



Cultural Differentiation between the Human and Avian Tubercle Bacilli. — 



When first isolated from lesions in birds, the avian bacillus may closely resemble 

 the human bacillus, but as a rule after a few generations in the laboratory it takes 

 on a more rapid and luxuriant growth, so that differentiation on cultural grounds 

 alone is rendered possible. There are certain avian strains, however, that remain 



Fig. 72. — Myco. tuberculosis. 



Human type. Glycerine broth, 6 weeks, 37° C, 

 showing thick, wrinkled, surface growth, 

 spreading up the sides of the flask. 



