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Certain strains of pleuropneumonialike organisms fail to multiply in vitro in the absence of factors found principally in mammalian sera. Results of attempts to isolate and characterize this requirement have led to two conclusions. Smith and Morton (1951, 1952) found that a heat stable protein permitted growth to occur when added to a complex basal medium. Edward and Fitzgerald (1951) and Edward (1953) succefully cultivated certain strains on a complex basal medium to which were added cholesterol, bovine serum albumin, and an acetone insoluble lipid fraction derived from egg yolk. Additional study was therefore required to elucidate these seemingly divergent results. Consequently, the protein described by Smith and Morton has been studied chemically and biologically with the eventual finding that it is a lipoprotein similar to or a degradation product of alpha-1 lipoprotein found in mammalian sera.

MIany of the pleuropneumonialike organisms (PPLO) require for growth a factor found in mammalian blood sera. This factor has been isolated and characterized as a lipoprotein (Smith et al., 1954). The protein moiety, cholesterol ester, and phospholipid, comprising this lipoprotein, are all growth requirements. Edward and Fitzgerald (1951) reported that cholesterol was active in promoting growth and that cholestanol and stigmasterol were as effective as cholesterol, whereas ergosterol, coprosterol, and certain cholesterol esters were inactive. They concluded that both lipid and protein fractions of serum were concerned in promotion of growth. Smith et al. (1954) reported that cholesterol and only cholesterol laurate of 5 cholesterol esters examined satisfied the sterol requirement. It has been shown that various proteins can substitute for the protein moiety of the lipoprotein factor and that the ability of a given protein to support growth apparently depends upon its ability to bind sterol (Smith et al., 1954). However, no detailed quantitative analysis of the lipid requirement of pleuropneumonialike organisms has been made. As a first step in an attempt to clarify the function of the lipoprotein growth factor, the quantitative growth response to various lipids was determined and the necessary molecular configuration of the sterol requirement was elucidated. MATERIALS AND METHODS Five representative strains of pleuropneumonialike organisms were employed: 07, 39, Campo, and 48, of human origin; J, of poultry origin. Strain 07 served as the test strain, whereas the others were used for comparative purposes. The cholesterol esters of fatty acids were pre-' This work was supported in part by National Science Foundation grant G-3026.

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Early studies with the pleuropneumonialike organisms concerned themselves with methods of isolation of these organisms and their possible relationship to various pathological conditions. More recently, investigators have included studies of nutritional requirements and some metabolic activities. While these microorganisms can be cultivated in artificial media, their smallest elements are capable of passing through filters designed to retain bacteria. The small size, slow growth, and sluggish metabolic activity, as well as their fragility and poor staining qualities have helped distinguish the pleuropneumonialike organisms from bacteria. The findings that some bacteria undergo variation to an L phase, which may resemble morphologically and culturally the pleuropneumonialike organisms, stress the need for more detailed information about these microorganisms. The present study concerns itself with the determination of the nucleic acid content and composition of a strain of pleuropneumonialike organisms isolated from the human urethra.