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Morphologic Changes During Growth

المؤلف:  Stefan Riedel, Jeffery A. Hobden, Steve Miller, Stephen A. Morse, Timothy A. Mietzner, Barbara Detrick, Thomas G. Mitchell, Judy A. Sakanari, Peter Hotez, Rojelio Mejia

المصدر:  Jawetz, Melnick, & Adelberg’s Medical Microbiology

الجزء والصفحة:  28e , p39-40

2026-04-01

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Cell Division

Most bacteria divide by binary fission into two equal progeny cells. In a growing culture of a rod-shaped bacterium such as E. coli, cells elongate and then form a partition that eventually separates the cell into two daughter cells. The partition is referred to as a septum and is a result of the inward growth of the cytoplasmic membrane and cell wall from opposing directions until the two daughter cells are pinched off. The chromosomes, which have doubled in number preceding the division, are distributed equally to the two daughter cells.

Although bacteria lack a mitotic spindle, the septum is formed in such a way as to separate the two sister chromosomes formed by chromosomal replication. This is accomplished by the attachment of the chromosome to the cell membrane. According to one model, completion of a cycle of DNA replication triggers active membrane synthesis between the sites of attachment of the two sister chromosomes. The chromosomes are then pushed apart by the inward growth of the septum, one copy going to each daughter cell.

Cell Groupings

If the cells remain temporarily attached after division, certain characteristic groupings result. Depending on the plane of division and the number of divisions through which the cells remain attached, the following may occur in the coccal forms: chains (streptococci), pairs (diplococci), cubical bundles (sarcinae), grape-like clusters (staphylococci), or flat plates. Rods may form pairs or chains.

After fission of some bacteria, characteristic postdivision movements occur. For example, a “whipping” motion can bring the cells into parallel positions; repeated division and whipping result in the “palisade” arrangement characteristic of diphtheria bacilli.