Gimenez staining of presumably axenic Acanthamoeba sp., strain HN-3, showed rod-shaped cytoplasmic inclusions. Electron microscopy of thin sections of the amebae showed these to be bacilli which measured 1.3 to 3.3 microns by 0.22 to 0.33 micron. Their cell envelopes were those typical of gram-negative bacteria, surrounded by an electron-translucent area that stains with ruthenium red, suggesting the presence of a capsule. The bacilli grew and reproduced in the cytoplasm of both trophozoites and cysts of Acanthamoeba sp. There was no evidence of a surrounding phagosomal or phagolysosomal membrane. They were retained by the ameba both during encystment and excystment. All attempts to isolate the endosymbionts in embryonated eggs and/or standard bacteriological media failed; and they persisted within the amebae for 1 to 6 mo despite temperature shocking or constant treatment of cultures with penicillin, streptomycin, chloramphenicol, tetracycline, erythromycin, polymyxin B, ampicillin, isoniazid, rifampicin, or gentamycin at concentrations of 10(-5) to 10(-3) M.
This investigation concerns the nature of the structural changes in Myxococcus xanthus during cellular morphogenesis. These changes have been investigated by means of electronmicrographs of thin sections of cells taken during various stages of the life cycle. The conversion of vegetative cells to microcysts involves the formation of a capsule but no drastic reorganization of the limiting cell membranes. Vacuoles appear in the cell during microcyst formation and germination. Microcyst germination involves a separation of the inner cell and the outer sheath, followed by the dissolution of a segment of the outer sheath and the emergence of the cell. Dense bodies within the cytoplasm and peripheral bodies between the two limiting membranes have been observed.
SUMMARYAn investigation has been made of the sequence of morphological events involved in the proccsses of microcyst formation and germination in the fruiting myxobacterium A4yxococcus xanthus. By using time-lapse photomicrographs and phase-contrast microscopy of living cells, microcyst formation is shown to involve a shortening and thickening of the entire vegetative cell with a subsequent increase of refractility. Germination is preceded by the casting off of a sheath followed by the gradual elongation and loss of refractility of the cell. Vegetative rods will readily form spheroplasts when exposed to a variety of conditions including sulphydryl compounds. The necessity for distinguishing between spheroplasts and microcysts is pointed out.
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