Observations on Some Biological Characteristics of Organisms of the Pleuropneumonia Group.

Within the past few years there has arisen a renewed interest in that group of bacterial forms designated as the "Pleuropneumonia Group," the prototype of which is Pleuropneumonia bovum, first described by Nocard and Roux in 1898, and for many years regarded as a virus because of its filterability. However, the cultivation of this organism on cell-free media, and the later complete description of a similar form as causing agalactia of sheep (Bridre and Donatien, 1925) removed them from this category. Of recent years the group has increased in number with some 20 different pleuropneumonia-like microorganisms being reported from highly varied sources. The common features which in general separate the pleuropneumonia organisms from the larger bacterial forms and filterable variants of the latter are: (1) A slow rate of growth, optimum for some strains being attained only after 4 to 6 days; (2) they require a high, 10-20 per cent, content of animal protein in the form of serum, etc., in the medium; (3) they stain poorly with the common aniline dyes but are well colored by the polychromatic stains, such as Giemsa; (4) morphologically they may exhibit a complex developmental cycle with asteroid, branching or granular forms being the predonminating elements. Although no exact determinations of their size have been made, the ability of these