314 THE TUBERCLE BACILLUS' 



ground for grouping the tubercle bacillus with the streptothrices. Coppen- 

 Jones [and Eastwood] have described ray structures with club-shaped ends in 

 tuberculous lesions exactly similar to the structures seen in the grains of 

 actinomycosis (figs. 205 and 206). 



In sputum and in tuberculous tissues the bacilli are found singly or arranged 

 in groups and in the latter case may lie parallel to one another. Occasionally 

 two bacilli are seen crossing one another at a more or less acute angle or 

 arranged like a V. 



[Bacilli obtained from the living tissues are longer and not so uniform in 

 appearance as bacilli cultivated on serum (Eastwood). 



[Both mammalian and avian bacilli when growing freely in the tissues of 

 their host are usually shorter and more uniformly stained than those which 

 are growing under adverse conditions.] 



2. Cultural characteristics. 

 A. Conditions of growth. 



The tubercle bacillus only grows in artificial culture provided that the 

 medium contains serum (Koch), glycerin (Nocard and Roux), yolk of egg 

 (Dorset), or fragments of tissues (Lumiere). 



It is an aerobic organism and only grows at temperatures above 30 C. In 

 the case of human tubercle bacilli growth ceases at 41 C. and in the case of 

 bovine bacilli at 44-45 C. The optimum temperature is 38 C. 



Certain precautions must be observed in sowing the tubercle bacillus. For 

 preference, the material will be taken from a lesion in the guinea-pig or rabbit 

 (bacilli taken directly from human tissues grow badly on artificial media), 

 rubbed up in a sterile mortar and portions of it transferred with a stout wire to 

 tubes of coagulated serum. It is better [when sowing cultures of the human 

 tubercle bacillus] to use serum to which 4 per cent, of glycerin has been added 

 before coagulation or blood agar. Never sow tuberculous tissue directly on 

 to glycerin agar : the cultures are more than likely to fail. It is immaterial 

 if the surface of the medium be slightly torn. Sow a large number of tubes 

 as many of them will remain sterile [and others are likely to be contaminated 

 with other organisms]. Incubate at 37-38 C. Growth only becomes visible 

 to the naked eye after an interval of 12 days or so but continues to increase 

 for about 4 weeks. As soon as colonies appear in any tube cover the mouth 

 with an india-rubber cap to prevent evaporation and the consequent drying 

 up of the medium. [It is perhaps even better to seal the tube with paraffin 

 or sealing-wax.] 



When a growth has been obtained, sub-cultures can be sown on various 

 media ; it is always advisable to sow a good deal of material and [until a 

 fair amount of experience has been acquired] to sow several tubes. 



[The human tubercle bacillus grows more luxuriantly than the bovine 

 tubercle bacillus in artificial culture so that the former is sometimes described 

 as the eugonic and the latter as the dysgonic tubercle bacillus.] 



Cultures of the tubercle bacillus have a characteristic but rather pleasant 

 odour. 



B. Characters of growth on various media. 



1. Coagulated serum, (a) The bacillus of the human type. After the 

 culture has been incubating at 37-38 C. for 12 days or so a number of 

 small, white, round, scaly, dry-looking colonies are seen scattered over the 

 surface of the medium. On further incubation the colonies become raised 

 but retain their scaly appearance, and the margins are irregular in outline. 

 Generally speaking, and especially when recently isolated, the colonies do 



