TUBERCLE BACILLUS 429 



one type into the other, no experiments have been reported up to the 

 present time which are sufficiently conclusive and extensive to war- 

 rant the assumption that one variety has been permanently changed 

 into the other. Loss or increase of pathogenic properties of one 

 strain does not suffice to bridge the gap between it and another strain 

 habitually pathogenic for another animal. It is very probable that 

 the human, bovine, and avian strains had a common ancestor and 

 that acclimatization 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 greatest chapters in the history of medicine 

 was inaugurated by the isolation of the tubercle bacillus in pure cul- 

 ture, and the demonstration of its etiological relationship to tuber- 

 culosis. The credit for this work, which in every detail marks an 

 important epoch in bacteriology, belongs to Robert Koch. 1 



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 resembles 

 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. 2 Some observers 

 have classed the tubercle bacillus with the group of Actinomyces on 

 the basis of this branching. 3 



1 Berl. klin. Woch., 1882, No. 15; Mitt. a. d. Kais. Ges.-Amte, 1884, ii, 1. 



2 Klein, Centralbl. f. Bakt., 1890, vii, 794. 



3 Babes and Levaditi, Arch, de med. exper. et d'anat. path., 1897, ix, 1041. 



