Comparative morphology of the honeybee spiroplasma, Spiroplasma melliferum, corn stunt spiroplasma S. kunkelii, citrus stubborn disease spiroplasma S. citri and the flower spiroplasma S. floricola was studied by scanning and transmission (TEM) electron microscopy. In thin sections and negatively stained TEM preparations, the tapered ends (tips) of spiroplasma helices were found to be structurally different than the rest of the helix, exhibiting a conical/rod shaped electron dense core with no apparent ribosomes or nuclear material. In the midgut of the leafhopper Dalbulus elimatus, vector of S. kunkelii, these spiroplasma tips were usually aligned between the midgut microvilli, and were observed frequently attached to the plasma membrane of the midgut epithelial cells, some of which were apparently infected by spiroplasma. These findings suggest that the tapered ends of spiroplasmas may be functionally comparable to the “attachment organelle” in Mycoplasmas infecting animals and humans, in which the tip structure is the primary site of mycoplasmal attachment to host cells. Additionally, fimbriae-like structures were observed by negative staining TEM on the surface of S. melliferum and in thin sections of S. kunkelii infecting the leafhopper vectors D. gelbus and D. maidis.
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