466 THE TUBERCLE BACILLUS GROUP 
the human, bovine, and avian strains had a common ancestor and 
that acchmatization in different animals has led to the perpetuation 
of three culturally and pathogenically stable varieties. The ichthic 
type is much more closely related to the non-pathogenic grass and 
dung bacilli than to the true tubercle bacilli. 
TUBERCLE BACILLUS. ^ 
Historical.— One of the great chapters in medicine was inaugurated 
at a meeting of the French Academy of Medicine December 5, 1865, 
when Jean Antoine Yillemin announced his successful infection of 
rabbits with tuberculous material from human cases of this dread 
disease. In a second communication in 1S6() he reaffirmed his thesis 
that tuberculosis is a specific infection. The violent opposition which 
this momentous discovery aroused was not stilled until Robert Koch- 
reported on March 24, 1882, the isolation of the tubercle bacillus in 
pure culture, and demonstrated its etiological relationship to tuber- 
culosis. This work of Robert Koch marks the beginning of a most 
important epoch in Bacteriology. 
Morphology.— The tubercle bacillus is a slender, straight or slightly 
curved rod measuring from 0.2 to 0.6 micron in diameter and from 
1.5 to 6 microns in length. The size and appearance of the organism 
varies somewhat according to the source. In sputum it frequently 
occurs in small clumps, often with the long axes of the bacilli parallel. 
Occasionally a pair of bacilli are arranged at an angle like the letter 
"V." The bacilli are typically isodiametric, but irregularities of 
outline are not uncommon; these irregularities are due to nodules 
which cause the organism to swell or bulge wherever they occur. 
These nodules frequently stain deeply, and between them are areas 
which stain lightly or not at all, thus giving the rod a beaded or vacuo- 
lated appearance which may be so marked that the organism resem- 
bles a chain of cocci. These "beaded" forms are frequently observed 
in the sputum of consumptives and occasionally in old growths on 
artificial media. 
True branching is also occasionally exhibited by tubercle bacilli 
derived both from the sputum and from culture.^ Some observers 
have classed the tubercle bacillus with the group of Actinomyces on 
the basis of this branching.^ 
The tubercle bacillus is non-motile and possesses no flagella. It 
forms no capsule but possesses a waxy envelope which confers upon 
the organism unusual resistance to desiccation and to the action of 
chemicals. No spores have been definitely demonstrated, but Koch^ 
1 An excellent summary of the significant literature of tuberculosis and the tubercle 
bacillus will be found in Calmette (L'Infection Bacillaire et la Tuberculosa chez I'homme 
et chez les animaux, Paris, 1920). 
2 Berl. klin. Wchnschr., 1882, 19, 221; Mitt. a. d. Kais. Gesamte, 1884, 2, 1. 
3 Klein: Centralbl. f. Bakteriol., 1890, 7, 785. 
■• Babes and Levaditi: Arch. d. med. exper. et d'anat. path., 1897, 9, 1041. 
6 Mitt. a. d. kais. Gesamte, 1884, 2, 22. 
