MYCOBACTERIUM TUBERCULOSIS. HUMAN TYPE 433 



firmly of the opinion that they represent no more than modifications of the fixed 

 types ; they do not represent transitional forms between the types. By passage 

 through suitable animals it is often possible to restore the normal properties of 

 their type to these atypical strains ; but it has never yet proved possible to modify 

 them in such a way that they change into a different type. Thus an attenuated 

 dysgonic bovine strain, if passed through a series of rabbits, will often regain the 

 standard virulence of the bovine type ; it will not take on a eugonic growth and 

 come to resemble the human type (Griffith 1924). The extensive observations of 

 Jensen and Frimodt-Moller (1936) and of Frimodt-Moller (1939) are in general 

 accord with Griffith's findings. 



We may conclude therefore that the types of tubercle bacilli are fixed ; that 

 occasionally modifications due to residence in a particular environment may result ; 

 but that no alteration of environment or any other factor has yet been successful 

 under experimental conditions of changing an organism of one type into that of 

 another. 



Classification of the saprophytic acid-fast strains is very unsatisfactory. 

 Numerous types have been described, and have been differentiated on morphological 

 and cultural appearances, the degree of acid-fastness, the formation of pigment, 

 and the optimum temperature of growth (see Haag 1927, Pinner 1932, Thomson 

 1932, Schwabacher 19336, Gordon and Hagan 1938). 



A large amount of detailed information, much of which has been overlooked by 

 subsequent workers, on the various properties of the tubercle bacilli, is contained 

 in the Keports (1907, 1909, 1911, 1913) of the Koyal Commission on Tuberculosis. 

 Those anxious to learn more about these organisms are advised to study these 

 reports carefully, as well as the numerous subsequent publications of Dr. A. Stanley 

 Griffith, many of which have appeared in the Journal of Pathology and Bacteriology 

 and the Journal of Hygiene during the past thirty years. 



Mycobacterium tuberculosis. Human type 



Isolation. — By Koch in 1882 from human tuberculous lesions. 



Habitat. — Strict parasite, causing tuberculosis in man, pigs, monkeys, dogs, and parrots. 



Morphology.- — Rod-shaped organisms, 1-4 f^i long and 0-2-0 -8 /j, broad, straight or slightly 

 curved, with parallel or irregular sides, and rounded ends ; arranged singly or in 

 small clumps ; non-motile, non-sporing, and non-capsulated. Fail to stain with 

 simple dyes except after prolonged exposure. Stain best with hot carbol-fuchsin ; 

 resist decolorization with 25 per cent. H2SO4 and with absolute alcohol for 10 

 minutes. Gram-positive ; staining may be even or granular ; beaded forms are 

 common. In the animal body the bacilli are larger than in culture. Non-acid-fast 

 and clubbed forms are not uncommon in culture. 



Coagulated Ox serum. — 4 weeks, 37° C. Thin, effuse, confluent, greyish-yellow growth, 

 with a finely granular surface, looking like ground glass ; colour may be golden 

 yellow ; consistency friable ; emulsifies with great difficulty. 



5 per cent. Glycerine Ox serum.- — 4 weeks, 37° C. More luxuriant, tliicker, raised, con- 

 fluent, yellow or golden-yellow growth, with a coarsely granular surface ; irregu- 

 larly heaped-up in places ; often a granular film over the water of condensation. 

 Consistency friable, emulsifies with great difficulty. 



Dorset Egg. — 4 weeks, 37° C. Rather poor, discrete or confluent, slightly raised, greyish- 

 yellow growth, with a finely granular surface. 



5 per cent. Glycerine Egg. — 4 weeks, 37° C. More luxuriant, raised, confluent, greyish-yellow 

 growth, with a coarsely granular surface ; growth irregularly nodular and heaped- 

 up in places. 



