376 Annals New York Academy of Sciences 



teria with which they were associated. At present the impression of the ma- 

 jority of bacteriologists interested in these organisms is that, although they 

 are fixed in their form of growth, PPLO derived from the bacteria at some time 

 in the past. This status would correspond to that of the fungi imperfedi. The 

 impression of some investigators is that PPLO may represent a primitive stage 

 in the phylogenetic development of bacteria to which under certain conditions 

 bacteria may return. It should also be mentioned that some authors^ '^'^ re- 

 gard the similarity between PPLO and bacteria as superficial and without 

 significance. 



Information on morphology and reproductive processes of PPLO has been 

 confused for a long time, and to some extent it still is today. This confusion 

 exists not so much because of their small size but because of their fragility and 

 the ease with which they may be distorted. For these reasons, use of the 

 electron microscope thus far has yielded hardly more information than a 

 better definition of the smallest elements in the cultures of PPLO. 



In 1935, Turner- gave an excellent description of the morphology of the 

 organism of bovine pleuropneumonia in broth cultures with dark field illumina- 

 tion. His basic observations are as follows: "An old broth culture contains 

 only small granules less than 0.5 /jl in diameter. Transferred to fresh media 

 these granules increase in size to about 1 n. One or more areas appear on their 

 peripheries from which short filaments may grow out. The structures thus 

 formed suggested the first name of the organism "Asterococcus." The ends 

 of the short filaments grow to a larger size and repeat a similar reproductive 

 process. The filaments may grow longer and either differentiate into small 

 granules or develop swellings from which filaments again grow out. In addi- 

 tion, rather large spherical or irregularly-shaped bodies, several m in diameter 

 appear in the culture. Under appropriate conditions these reproduce the 

 granules and filaments. Very long straight filaments, sometimes visible in the 

 cultures, are apparently artefacts." 



The development of colonies of PPLO in agar cultures was carefully studied 

 by Liebermeister.'^ With the phase microscope he examined several strains. 

 Like Turner, he observed the extrusion of short filaments from the granules 

 and the development of new organisms at the end of the filament. It is char- 

 acteristic that the smallest organisms seem to divide but that the daughter 

 organisms usually are not closely associated but seem to be at the ends of a 

 short rod. Liebermeister did not observe the development of multiple fila- 

 ments from a granule nor the development of long filaments in the strains 

 which he studied. 



The size of the organisms, especially on the surface of agar colonies, varies in 

 the cultures, and the smallest forms visible with the light microscope usually 

 make up a very small fraction of the culture. Autolysis and deformation of 

 the larger forms often produce a bewildering pleomorphism in aging cultures. 



Klieneberger^ has published beautiful photographs indicating that the larger 

 organisms are aggregates of small ones enclosed in a common envelope. This 

 structure of the large forms is clearly visible in electron micrographs. Under 

 appropriate conditions granules grow out from the large bodies. 



From this short discussion it seems that the morphology and reproductive 



