76' BACTERIOLOGY 



is forced into the bacilli by heat and the use of a powerful 

 mordant in this case carbolic acid) they retain the colour 

 when exposed to the action of a strong mineral acid, such 

 as sulphuric or nitric, even when applied in a concentration 

 of 20 per cent, or more. They are therefore called "acid- 

 proof" or sometimes, though rather inaccurately, "acid-fast." 

 Some of the members of this group, when so stained, resist 

 also the decolorising action of alcohol, whilst others do not. 

 The extremely important causal organisms of Tuberculosis 

 and of Leprosy are found in this group, whilst there are 

 in it also numerous other bacilli, either only feebly patho- 

 genic or not causing disease. These latter are widely diffused 

 in nature, and are found in milk, butter, cheese, ham, water, 

 manure, and on plants and vegetables, as for example in the case 

 of the Timothy grass bacillus. One of them, the Smegma 

 bacillus, is often found upon the surface of the body, especially 

 about the genital region. It is, therefore, highly important to 

 be able to distinguish these more or less harmless saprophytic 

 organisms from the Tubercle bacillus. They usually grow 

 much more rapidly and easily upon culture-media, but the 

 most certain differential test is the inoculation of the guinea- 

 pig, which is extremely susceptible to tuberculosis, but is 

 unaffected or not affected in the same way by the others. 



One point of extreme importance in connection with acid- 

 proof bacilli is the possibility that they are merely one stage 

 in the life-history of organisms belonging to a higher bacterial 

 group, the Streptothriceae, and that other phases of their 

 growth and development during which they are not acid-proof 

 may exist. This has been definitely proved in the case of the 

 Leprosy bacillus (see p. 82), and is suspected to occur in that 

 of the Tubercle bacillus itself. 



Bacillus tuberculosis. Little need be said here to 

 emphasize the widespread havoc and destruction wrought by 

 the Tubercle bacillus in man and animals throughout the 

 world. Tuberculosis is the commonest and most important 

 of all bacterial diseases. It can occur at any age ; it may 

 affect any organ or tissue of the body, and in many different 

 ways. One in every ten deaths occurring in this country from 

 all causes, is due directly to tuberculosis, and this does not 

 include cases where the disease may be present undiagnosed 

 in persons who die from other diseases a not uncommon 

 phenomenon. In the decade between 1891 and 1900, the mean 

 population of England and Wales was slightly over thirty 

 millions, and in that period 616,006 persons died of tuber- 

 culosis. In the United States in the year 1900 alone, 111,059 

 tuberculous deaths were reported, and it is estimated that 



