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SUMMARY: Serum could be replaced in media for growing the L-phase of Proteus vulgaris by a phospholipid fraction from egg yolk and for growing the L-phase of Streptobacillus moniliforrnis by the addition of this fraction + bovine albumin. Organisms of the pleuropneumonia group grew in media in which serum was replaced by these materials, only with the further addition of cholesterol. The L-phase organisms thus differ in nutrient requirements from the pleuropneumonia group. They also differ in their requirements from the parent bacteria. Differences were noted in colonial appearances between the L-phase organisms studied and the organisms of the pleuropneumonia group. It has been shown that many bacteria can exist in an L-phase which difl'ers considerably in morphology and properties from the original bacilli (Kliene- berger-Nobel, 1951 ; Dienes & Weinberger, 1951). Morphological studies have suggested close similarities between bacteria in the L-phase and organisms of the pleuropneumonia group, and it has been stated that cultures of the two may be indistinguishable (Tulasne, 1951 ; Dienes & Weinberger, 1951). How- ever, in an investigation of the biological properties of the pleuropneumonia group significant differences in cultural appearances were noted between the L-phase of StreptobaciEZus monibiformis and organisms of the pleuropneumonia group (Edward, 1950). The relationship of the pleuropneumonia group of organisms to the L-phase of bacteria would seem to require further investiga- tion in view of its importance in determining the taxonomic position of the pleuropneumonia-like organisms. Although cultures of the L-phase of certain Gram-positiT7-e bacteria have been obtained on media not enriched with serum, the L-phases of Gram.- negative bacteria have only been grown on media enriched with serum or ascitic fluid. All except the saprophytic members of the pleuropneumonia group require media similarly enriched. Cholesterol (or another sterol) has been shown to be an essential growth factor for the pleuropneumonia group; good growth was obtained on media in which serum was replaced by chole- sterol + serum albumin or cholesterol + a phospholipid material from egg-yolk (Edward & Fitzgerald, 1951). The examination of the L-phases of two Gram- negative bacteria, to find whether serum could be replaced in the medium by the same substances, has now revealed differences in growth requirements between them and organisms of the pleuropneumonia group.

A Difference in Growth Requirements between Bacteria in the L-phase and Organisms of the Pleuropneumonia Group