THE MORPHOLOGY OF PPLO AND BACTERIAL L FORMS* 



Louis Dienes 



Departments of Medicine and Bacteriology, Massachusetts General Hospital, and the 



Robert W . Lovett Memorial Foundation for the Study of Crippling Diseases, 



Harvard Medical School, Boston, Mass. 



The smallest organisms growing without the help of other cells are found 

 in the cultures of pleuropneumonia-like organisms (PPLO). Some are as 

 small as 0.15 to 0.25 /x- The majority of the organisms in the culture is 

 considerably larger. Size is only one of the distinctive characteristics of 

 these organisms. Their structure, the appearance of their colonies, their 

 chemical makeup and their reproductive processes also differ at first sight from 

 those of other microorganisms. However, many similarities to bacteria are 

 present. Their organization is as simple as is that of the bacteria. They do 

 not have distinct nuclei. Their growth requirements, metabolism, and sensi- 

 tivity to antibiotics are quite similar to those of the bacteria. An important 

 exception is that the PPLO are not sensitive to penicillin. The basic difference 

 between PPLO and bacteria is the absence in PPLO of a rigid cell wall, and 

 most of the distinctive properties of PPLO are the consequence of the lack of 

 this structural property characteristic of bacteria. The organisms are soft, 

 fragile, and easily distorted. Their size varies within wide limits from 0.15 /x 

 to 10 M, or larger. On agar media the structure and appearance of the colonies 

 of PPLO are characteristic and differ markedly from those of bacteria. Finally, 

 the method of reproduction seems to be more complex than that of bacteria, 

 although basically it is probably similar. In the fight of these similarities and 

 differences some authors propose to create a special class for PPLO,^ while 

 others regard them as a subdivision of the class of bacteria.-'^ 



The PPLO were discovered as parasites causing disease in animals or living 

 on their mucous membranes. They were isolated also from sewage, well water 

 and soil. The saprophytic strains differ in some respects from the parasitic, 

 but we have no information to suggest that they are part of the microflora 

 other than those related to animal organisms. 



The suggestion that the PPLO might be an independent subdivision of mi- 

 croorganisms is made unlikely by the observation that bacteria under certain 

 conditions assume a growth form which presents aU the distinctive properties 

 of PPLO.-* These bacterial forms, usually designated as L forms, like the 

 PPLO are soft and fragile, lack a rigid cell waU, and are considerably smaller 

 than the usual bacteria. The appearance of the colonies, the morphology of 

 the organisms, their reproductive processes and their sensitivity to antibiotics 

 are similar to that of PPLO, and include resistance to penicillin. The best 

 illustration of the similarity between the two groups is the fact that 15 years 

 passed before it was generally recognized that the L forms were growth forms 

 of bacteria and not PPLO mixed with the cultures and thus foreign to the bac- 



* The work reported in this paper was supported by a grant from the National Institute 

 of Arthritis and Metabolic Diseases, Public Health Service, Bethesda, Md. This paper is 

 pubHcation No. 322 of the Robert W. Lovett Memorial Foundation for the Study of Crip- 

 pling Diseases. 



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