378 Annals New York Academy of Sciences 



outside the large body. The agar seems to offer a suitable physical environ- 

 ment similar perhaps to that present in the large bodies and necessary for 

 multiplication of L forms. 



Bacterial large bodies have been known since the beginning of bacteriology 

 and are usually referred to as involution or dying forms. They are produced 

 by a great variety of influences on the bacteria that prevent normal multipU- 

 cation. Large bodies occur in the natural environment of bacteria. In some 

 cases, in contrast to older opinion, it is apparent that they remain viable and 

 able to reproduce for a longer period than single bacteria. Hence, the forma- 

 tion of large bodies is probably a useful process for bacteria in their natural en- 

 vironment and can be thought of as a phenomenon of adaptation and not 

 merely the result of degeneration. 



At present, L forms can not be regarded in the same light. In most in- 

 stances they develop and can be propagated only under artificial conditions. 

 It seems likely that they represent the growth of forms in the laboratory that 

 naturally occur only in the large bodies derived from bacteria. Small size, 

 growth into agar, and a tendency to produce large bodies (characteristics of L 

 forms) may be the result of this natural site of growth. 



It is remarkable that bacteria cultivated directly from pathological processes 

 relatively often have the tendency to grow into large forms and to produce L 

 colonies. This may be the result of injury to the organism by the defensive 

 forces of the host. On the other hand, it may be an adaptation of the bac- 

 teria to parasitism. In one case of peritonitis, for example,^ it seemed that a 

 bacteroides strain continued to multiply in the L form inside the phagocytic 

 cells of the host. Such an observation suggests that although L forms may be 

 produced under artificial conditions, this process might occur naturally and 

 thus might have been the origin by stabilization of strains of PPLO that have 

 continued life in this form. The PPLO not only appear to be bacteria without 

 the usual cell wall but also bacteria that have passed through the processes in- 

 volved in the growth of the large bodies. The most marked difference between 

 L forms and PPLO is that PPLO are better adapted to grow in artificial media 

 and especially to grow in the small granular form. The L forms grow usually 

 only from large inocula and have a pronounced tendency to grow into large 

 bodies as well as to undergo autolysis. 



At present PPLO do not seem to be of the main stream of phylogenetic 

 development or to be a link in it. These organisms probably represent the 

 result of the simplification of the structure of bacteria as a consequence of 

 parasitism. They are not complex and occasionally are very small but, like 

 the viruses, they offer no direct clues for the origin of life. 



For illustration of the morphology of PPLO and L forms we refer to articles 

 previously published in the Annals of this Academy.^ ■!" 



References 



1. Sabin, a. B. 1941. The filterable microorganisms of the pleuropneumonia group. 



Bacteriol. Rev. 5: 331. 



2. Turner, A. W. 1935. A study on the morphology and life cycles of the organism 



of pleuropneumonia contagiosa bovum (Borrelomyces peripneumoniae nov. gen.) by 

 observation in the living state under dark ground illumination. J. Pathol. Bacteriol. 

 45: 1. 



