Frank (10) have postulated the existence of mo- tile granules which occur in cells of a ..... Komagata, K., K. Yamada, and H. Ogawa. 1969. Taxonomic studies.
JOURNAL OF BACTERIOLOGY, Jan. 1971, p. 408-412 Copyright 1971 American Society for Microbiology
Vol. 105, No. I Printed in U.S.A.
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Ultrastructural Explanation for Snapping Postfission Movements in A rthrobacter crystallopoietes TERRY A. KRULWICH' AND JACK L. PATE Department of Bacteriology, University of Wisconsin, Madison, Wisconsin 53706
Received for publication 2 September 1970
The ultrastructure of dividing rod-stage cells of Arthrobacter crystallopoietes was examined by electron microscopy. The cell walls consist of two layers. During cell division, the inner layer invaginates to form the septum. The outer layer does not participate in septum formation. After septum formation is completed, the two daughter cells remain attached by the outer layer of the cell wall. It appears that localized rupture of the outer layer during further wall growth is responsible for the phenomenon known as "snapping division" or "snapping postfission movement." The term "snapping division" describes a kind of postfission movement peculiar to certain coryneform bacteria. Such postfission movements were first described by Kurth (13) and have since been reported by many other investigators (6, 8, 9, 11, 15, 20). Upon completion of cell division, one or both of the two daughter cells suddenly swing around, bringing their distal ends closer together while still remaining attached by a small region at their proximal ends. This kind of postfission movement results in the V-forms commonly observed in cultures of coryneform bacteria. Hill and Rickards (9) in 1903 concluded that snapping division could be used as a taxonomic criterion, since it occurs regularly among certain strains of bacilli and not at all among others. Bisset (1) claimed that all so-called postfission movements were nothing but artifacts due to mechanical stress on the dividing cells (e.g., cells growing between solid agar and a cover slip) and would not occur if the same cells were grown in liquid cultures. Sguros (19) suggested that Vforms resulted from "germ tube extrusions" from each of a pair of attached arthrospores and were not due to postfission movements. More recent studies have demonstrated that snapping division is a true characteristic of certain strains of bacteria and is not due to mechanical stress (11, 20). Komagata et al. (11) examined the mode of cell division among 37 strains of bacteria. They separated these strains into three groups on the basis of their mode of cell division
and reported that snapping division occurred regularly among certain bacteria, which they placed in one group. Snapping division was never observed among members of the other two groups. Starr and Kuhn (20) investigated the origin of Vforms in cultures of Arthrobacter atrocyaneus. They concluded that V-forms in this organism could arise by any of three methods: (i) germination of adjacent coccoid elements, (ii) subpolar germination ("budding") of rods, and (iii) snapping postfission movements. Their observations demonstrate that snapping division is unquestionably a characteristic of certain bacteria and is not an artifact due to cultural conditions. Although various hypotheses have been advanced to explain the mechanisms responsible for snapping postfission movements, an adequate demonstration of the mechanisms responsible has not been achieved so far. Graham-Smith (6) has suggested that postfission movements are due to the partial rupture of the cell capsule at the time of division of the mother cell. Hoffman and Frank (10) have postulated the existence of motile granules which occur in cells of a wide variety of bacteria. According to these authors, the motile granules continuously impinge upon the inner walls of the bacterial cells. They report that one of these granules may finally break through the cell wall, pulling some of the cell's contents out and dragging the cell along with it for a short distance, thus bringing about postfission movements. The present report presents evidence suggesting that snapping division in cells of A. crystallopoietes is due to their peculiar pattern of cell wall
' Present address: Department of Biochemistry, Mount Sinai School of Medicine of the City University of New York, New York, N.Y. 10029.
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VOL. 105, 1971
SNAPPING POSTFISSION MOVEMENTS
synthesis followed by a sudden localized rupture of one layer of the cell wall. A model for the mechanism of postfission snapping is proposed. MATERIALS AND METHODS Growth of cells. Cells of A. crystallopoietes ATCC 15481 (3) were grown as spheres in glucose-salts medium as previously described (12). Rod-stage cells were obtained by adding peptone (0.5% final concentration, Difco) to the cells growing as spheres. The cells shown in this report were prepared for sectioning 6.5 hr after the addition of peptone. At this time the cells were growing and dividing as rods. Phase microscopy. Cells grown for 6 hr in 0.5% peptone broth were immobilized on a thin layer of 1% Ionagar on a glass microscope slide. The preparation was then examined with a Zeiss phase-contrast photomicroicope and recorded on Kodak high-contrast film. Electron microscopy. The cells were fixed by the method of Kellenberger, Ryter, and Sechaud (18). They were then dehydrated in a graded series of alcohols and embedded in Epon 812 (14). Sections were cut on a Porter-Blum MT-2 ultramicrotome (Ivan Sorvall, Inc.) with a diamond knife. Sections were picked up on 200mesh copper grids with carbon-coated parlodion films, stained with lead citrate (17), and examined in a Hitachi HU-IIE electron microscope at an accelerating voltage of 50 kv with a 50 ,um objective aperture.
RESULTS The phase-contrast photomicrograph shown in Fig. I illustrates the typical V-forms produced by postfission movements of cells of A. crystallopoietes.
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