940 THE PLEUROPNEUMONIA GROUP OF ORGANISMS 



The systematic position of these organisms has given rise to much discussion. 

 The curious association of LI with Streptobacillus moniliformis has been, in particular, 

 the subject of wide speculation. Though Klieneberger herself regards it as a 

 symbiont, other workers, notably Dienes (1939) and Heilman (1941), maintain 

 that it is merely a variant form of the bacillus. It is too early as yet to form 

 any sound judgment on these conflicting views. The fact, however, that none 

 of the numerous other pleuropneumonia-like organisms has been found accompany- 

 ing a given bacillus does tend, in our opinion, to support Klieneberger's contention. 

 In this regard the close association of the genetically distinct fusiform bacilli and 

 spirochsetes found in Vincent's angina will be recalled to mind. 



Apart from their peculiar mode of reproduction, the pleuropneumonia group 

 of organisms seems to fall in between the bacteria on the one hand and the Rickettsia 

 and filtrable virus group of organisms on the other. They are distinguished from 

 Rickettsia mainly by their extreme pleomorphism and by their ability to grow 

 on nutrient media in the absence of living cells. From bacteria they seem to 

 differ less fundamentally, but the filtrability of their smallest elements and their 

 limited metabolic powers assign them a place at the lowest end of the bacterial 

 scale. What relation they bear to the group of cocco-bacilliform bodies believed 

 by Nelson (1936rt, b, 1937, 1940) to be responsible for fowl coryza and for infectious 

 catarrh of mice and rats it is impossible to say, though it may be noted that the 

 organisms described by Nelson were less pleomorphic and grew only in the presence 

 of living cells. 



The nomenclature and classification of the pleuropneumonia group of organisms 

 present grave difficulties. Ledingham (1933) would place them in the family 

 Actinomycetacece, Turner (1935) in a new order oi Borrelomycetales, and Sabin (1941) 

 in a new class of Paramycetes. Klieneberger and Smiles (1942) agree with Sabin 

 that their inclusion among the Schizomycetes is hardly justifiable. Until we know 

 more about the genetic relationship of these organisms to the common bacteria, 

 it seems wiser to refrain from premature commitments and to suffer the discomfort 

 of using a clumsy, though comprehensive, tcm like the pleuropneumonia group. 



In the remainder of this chapter we shall give a fairly full description of the 

 original type species, and brief accounts of the others. Those who wish for further 

 information should consult the review by Sabin (1941), to which we ourselves 

 are indebted. 



The Organism of Pleuropneumonia 



Cultivation. — This organism, which is sometimes referred to as Asterococciis 

 mycoides (Borrel et al. 1910), can be cultivated on a number of different media, 

 but most workers have used serum broth or serum agar. Growth is said to occur 

 under both aerobic and anaerobic conditions ; according to Turner (1935), micro- 

 aerophilic conditions are most suitable. The optimum temperature for develop- 

 ment is 37° C. ; no growth occurs below 30° C. (Tang et al. 1935). With freshly 

 isolated strains, 2-3 day serum broth cultures often contain distinctive mucoid 

 islands and threads visible to the naked eye, while dark-ground examination may 

 reveal the ])res('iice of minute colonies, the smallest of which is about 12 /« in 

 diameter (Tang et al. 1936). With older strains only a general cloudiness of the 

 medium is seen. On solid media dew-drop colonies appear in 5 or 6 days. Under 

 the microscope these are often umbonate, and consist of a yellowish-brown granular 



