414 MYCOBACTERIUM 



permanently like the human type ; on solid media these strains are indistinguishable 

 from human strains, but differentiation is generally possible by the growth in 

 glycerine broth ; in this medium the human bacillus forms a thick wrinkled surface 

 pellicle (Fig. 72), whilst the avian bacillus grows at the bottom of the flask forming a 

 granular deposit, or sometimes spreading out in a veil-like manner over the bottom, 

 and part- way up the sides of the flask. Occasionally the avian bacilli give rise to a 

 diffuse turbidity in broth. Why it is that some bacilli grow as a pellicle on the 

 surface, others form a veil over the bottom, while still others grow diffusely, is 

 not known. It may depend on the oxygen pressure most suitable for growth, 

 or, as some workers think, on the surface tension of the medium. By lowering 

 the surface tension to below 42 dynes, Larson (1926) states that it is possible 

 to induce organisms, which usually form a surface pellicle, to grow diffusely 

 or at the bottom of the flask. 



Most avian strains grow more rapidly and more profusely than human 

 strains, and tend on solid media to give a more creamy, homogeneous, and 

 less granular growth than that of the human bacillus (Fig. 73). Though growth 

 is favoured by glycerine, it is possible to get the avian bacillus to grow on 

 simple media like nutrient agar or broth without the addition of any glycerine ; 

 this is more difficult with the human bacillus. Another point of difference is 

 that the human strains do not grow at a temperature above 40° C, whereas 

 the avian strains will grow up to 43^" or 45° C. A point of not much im- 

 portance is that avian cultures generally live .longer than human ones ; they 

 may be found viable after 1 or even 2 years ; human cultures are often dead 

 in a couple of months, though occasionally they may survive for much longer. 

 Another point of small importance is that a few avian strains when grown on 

 glycerine egg medium give a faint pink-coloured growth ; human bacilli never 

 give a pink coloration. 



Cultural Differentiation between the Avian and Cold-blooded Tubercle Bacilli. — 



In culture the avian and the cold-blooded bacilli resemble each other closely. On 

 first isolation they can easily be distinguished by the difference in their optimum 

 temperatures of growth, the avian bacillus growing best at about 40° C, the cold- 

 blooded at 25'^ C. After prolonged subculture in the laboratory this difference 

 is partly obliterated; the cold-blooded bacillus comes to grow quite well at 37° C, 

 though the avian bacillus will rarely grow below 30° C. Practical differentiation 

 can therefore be made by growing the organisms at 25° C. ; if growth is as good at 

 this temperature as at 37° C, the strain is a cold-blooded one ; if growth occurs 

 freely at 37° C, but fails to occur at 25° C, the strain is an avian one. Apart 

 from the differences in optimum temperature there are no definite cultural char- 

 acteristics distinguishing the two types, except the pink coloration of certain avian 

 strains when grown on glycerine egg. Minor differences such as the worm-cast 

 growth of the cold-blooded type on glycerine potato, and the denser nature of the 

 growth on the bottom of flasks of glycerine broth are not sufficiently constant to be 

 reliable. As a rule the cold-blooded bacillus grows more rapidly than the avian 

 bacillus ; cold-blooded strains often give a definite layer of growth in 2 or 3 days : 

 growth of avian strains is often not visible for 4 or 5 days. 



Cultural Characteristics of the Saprophytic Acid-fast Bacilli. — These organisms 

 grow rapidly at room temperature, giving rise in 2 or 3 days to a profuse confluent 

 growth. Dift'erent strains vary in their cultural reactions, but on the whole their 

 appearance on solid media is very characteristic. On glycerine agar there is a 



