Isolation, Chemical Composition, and Ultrastructural Features of the Cell Membrane of the Mycoplasma-Like Organism Spiroplasma citri

Abstract

Thin sections of Spiroplasma citri, a mycoplasma-like organism isolated from citrus infected with "Stubborn" disease, showed the organisms to be limited by a single trilaminar plasma membrane. An additional outer layer could, however, be frequently seen in freeze-etched preparations of unwashed cells. The organisms were found to be extremely sensitive to lysis by osmotic shock. The cell membrane of S. citri isolated in this way resembled that of mycoplasmas in ultrastructure and gross chemical composition. The isolated membranes showed the characteristic trilaminar shape in section and the typical particle-studded fracture faces in freeze-etched preparations. Protein and lipid formed over 80% of the total dry weight of the membrane, which had a density of-1.180 g/cm3. Cholesterol constituted over 20% of the total membrane lipid. Phosphatidylglycerol, synthesized by the organisms, was the major phospholipid. Significant amounts of hexosamine (15 to 35 ,ug/mg of membrane protein) could be found in the membrane preparations. Our results support the thesis that S. citri does not possess a cell wall, either of the gram-positive or the gram-negative type, though it may be coated by some other type of an envelope or by a slime layer, at least temporarily.

Lipid composition and lipid metabolism of Spiroplasma citri

“…Along with A. laidlawii and other mycoplasmas, Spiroplasma citri promises to be a useful tool in the elucidation of cell membrane structure and function. It has a filamentous and helical morphology, which is maintained with no rigid bacterial wall components (8,28). It is also capable of screwlike rotational and flexional movement about the long axis of the helix and contractile movements that allow migration through liquid or semisolid media without the need for surface adherence, which is seen in other mycoplasmas capable of motility.…”

“…It is also capable of screwlike rotational and flexional movement about the long axis of the helix and contractile movements that allow migration through liquid or semisolid media without the need for surface adherence, which is seen in other mycoplasmas capable of motility. These movements are most likely the result of some contractile mechanism, since there are no visible axial fibrils or flagella-like appendages (8,14,28).…”

“…In our study of S. citri, the mycoplasma-like organism associated with citrus stubborn disease (7,8,14,15,18,28,38), we have sought to answer the following questions. (i) What is the overall lipid composition of the S. citri mem-brane?…”

An in situ Freeze‐Fracture Study of Spiroplasma citri and the Corn Stunt Spiroplasma

“…Motility and changes in shape during the life cycle of spiroplasmas are phenomena, whose mechanisms are still unexplained. But it is hypothesized that fibrils found in the cytoplasm interact with the plasmalemma and^ therefore, may play an important role in maintaining or inducing a specific cell shape (RAZIN 1978, TOWNSEND et al 1980. Isolated fibrils (average width of 3.5 nm) nearly always remain associated to fragments of the plasmalemma, confirming the hypothesis of interactions between the cytoplasmic fibrils and the plasmalemma (STALHEIM et al 1978, TOWNSEND et al 1980.…”

“…At least two of them up to now are known to be etiological agents of plant diseases, the "Stubborn" disease of Citrus and the corn stunt disease. Spiroplasmas are characterized by their helical morphology during some phases of growth and the ability to move freely (for review see RAZIN 1978 andBREDT 1979).…”