BIOLOGY OF TUBERCLE BACILLUS 233 



three facts which have complicated our knowledge of the biology 

 of the B. tuberculosis. Before discussing this subject in so far as 

 it affects the bacteriology of milk, it may be desirable to state 

 briefly the chief biological facts known respecting the true tubercle 

 bacillus.^ 



The B. tuberculosis of artificial culture is usually an unbranched, 

 slender, immotile rod, 1-5 to 4 /i long and -4 (x broad, often slightly 

 bent. In sputum and tissues the bacillus may appear branched and 

 in thread forms. The protoplasm of the bacillus consists oifat and 

 wax (26 per cent.), protmnin (24 per cent.), nucleo-proteid (23 per 

 cent.), nucleic acid (8 per cent), and the remainder of mineral and 

 proteinoid (chitin) substances. The protoplasm is frequently vacuo- 

 lated and irregularly segmented, and this becomes particularly 

 obvious after staining. As to staining, the bacillus is acid-proof, 

 and stains well with Ziehl-Neelsen or Gram. Growth does not 

 occur in the absence of oxygen, is most favoured by a temperature 

 varying from 29' C. to 42^ C, and is at all times slow on artificial 

 media. The common media used are such as agar, blood-serum, 

 potato, and fluid media. The addition of glycerine (4 to 6 per 

 cent.) favours growth. The organism is resistant in a high degree 

 to various kinds of physical agencies. It is retentive of vitality. 

 Its distribution outside the body is probably wide. All kinds of 

 animals seem liable to its infection, and the bacillus has been 

 isolated from dust, milk, etc. As one result of so wide a distribution, 

 it will be understood that man is very liable to infection. It is said 

 that of apparently healthy men, about 30 per cent, are infected at 

 some time or other in their lives by this organism in a greater or 

 lesser degree. The virulence of the human tubercle bacillus for 

 animals is much less than for man, and different strains of the 

 human bacillus may differ from each other very considerably in 

 their degree of virulence. Various external conditions also affect 

 the virulence, which is greath- reduced by continued cultivation on 

 artificial media. 



The tubercle bacillus may attack almost any organ of the body. 

 The lymphatic system is constantly involved, and as a rule the 

 organs most affected are those in primary contact or relationship to 

 the site of introduction. When tuberculosis from a human source 

 has been ingrafted upon a calf, it gains enormously in virulence by 

 being reinoculated upon a second calf (Hamilton). 



There is an important biological fact which may be mentioned 



* An account of the organism will be found in the chapter describing the 

 various species of bacteria found in milk, see p. 432. 



