12 THE MORPHOLOGY OF BACTERIA 



in other organs. Later Wolbach, Todd, and Palfrey^ emphasized the specificity of the 

 rickettsiae for certain insect hosts. Hertig and Wolbach further ampHfied the defini- 

 tion of the rickettsiae, suggesting that the term should be limited to proved pathogen- 

 ic organisms which are pleomorphic, non-motile, gram negative, stain rather lightly 

 with the aniline dyes, and have a tendency to an intracellular habitat. 



The rickettsiae have been cultivated in only a few instances. Noller^ has been 

 able to grow Rickettsia melophagi on blood agar inactivated by heating to 57° C, and 

 Sellards^ recently has cultivated a rickettsia-like micro-organism from tsutsugamushi 

 disease. This organism was pathogenic and was named by him Rickettsia nipponica. 

 In consequence it is difficult to come to any definite conclusions as to their nature and 

 proper classification. It is quite clear that they are not mitachondria (Cowdry),^ and 

 in many of their characteristics they resemble the bacteria very closely. Their pleo- 

 morphism is not greater than that of many bacterial species. In size they cor- 

 respond to many of the smaller bacteria. They have no definite internal structure al- 

 though a morphological nucleus has been claimed by Epstein^ for Rickettsia prowazeki. 

 They stain badly by the ordinary aniline dyes but no more so than certain bacterial 

 species like the glanders bacillus and the cholera vibrio. When cultivated they grow 

 on media similar to those used for the majority of bacteria. For the present the rickett- 

 siae should probably be included among the bacteria. In this connection the obser- 

 vations of Cowdry and of Wolbach and Schlesinger are of great importance. Cowdry^ 

 has described a rickettsia (Rickettsia ruminantium) in Amblyomma hebraeum, the 

 bont tick, which transmits the virus of the disease of sheep, goats, and cattle known as 

 "heart water." These organisms lay in the endothelial cells of the renal glomeruli and 

 in the superficial gray matter of the cerebral cortex. They were uniform coccus-shaped 

 bodies, 0.2-0.5 ix in diameter, sometimes in diplo formation. They stained deep clear 

 blue by Giemsa, and easily by Lofifler's methylene blue and other aniline dyes. They 

 were gram negative. Such organisms would naturally be included in the group of 

 rickettsiae as above outlined. Cowdry^ further described in a number of ticks (Ar- 

 gasidae and Ixodidae) certain non-pathogenic, gram negative organisms characterized 

 by their resemblance to bacteria morphologically, their large size and their intracel- 

 lular habitat. Obviously such organisms would not meet the exacting requirements 

 laid down by Hertig and Wolbach for the rickettsiae. Size and pathogenicity are 

 somewhat doubtful characteristics for classification, and it is difiicult to exclude from 

 any group organisms which possess in the main the character of the group but are 

 larger and endowed with pathogenic action. For the present the non-pathogenic 

 organisms described by Cowdry should probably be regarded as rickettsiae and 

 included with them in a larger group of pathogens and non-pathogens. 



• Wolbach, S. B., Todd, J. L., and Palfrey, F. W.: The Etiology and Pathology of Typhus. Cam- 

 bridge, 1922. 



' NoUer, W.: Arch.f. Schijfs- n. Tropen-Hyg., 21, 53. 1917. 

 ^Sellards, A. W.: Am. J. Trop. Med., 3, 529. 1923. 

 "Cowdry, E. V.: he. cil. 

 'Epstein, H.: Centralbl.f. Bakteriol., 87, ^s^. 1922. 



* Cowdry, E. V.: /. Exper. Med., 42, 231, 253. 1925. 

 T Ibid., 41, 817. 1925 



