Biochemical determination of bacterial morphology and the geometry of cell division

Abstract

A revised critical size hypothesis states that cells divide when they have integrated the critical types and amounts of new components into their envelopes. The kinetics and geometry of envelope formation are implicated as causative factors in division. Coccoid and bacilliform bacteria are compared because of distinctive biochemical differences in the peptidoglycan of their rigid layers. It is suggested that the spherical shape of some bacteria is permitted by cross-linkings of the peptidoglycan through lysine. The cross-linking in cylindrical walls is more complex and may require participation of the second carboxyl group of diaminopimelic acid. A model is presented for division of spherical bacteria in which the cell wall does not restrict the mode of expansion. The rate of envelope construction per unit of cell mass changes drastically after complete duplication of the appropriate genetic information. The resultant tendency to unbalance the surface area-to-volume ratio demands an alteration of morphology. At this stage the segregation of duplicated chromosomes, or complete nuclei, is sufficient to establish two foci of growth within the cell. The number and locations of genetic centers govern the geometric pattern of growth. A model for division of bacilli depends on the same factors and includes the influence of the rigid layer, which restricts the mode of cell expansion. Morphological transitions within the division cycle are compared to control of cell size and shape at different growth rates. The rapid shift-up of RNA and protein synthesis demands a change to more sphere-like cells, since there is not as early a change in the rate of envelope formation. Similar comparisons are made for filamenting bacilli. Comments are made on insertion of new envelope subunits. The various principles are consolidated into a description of bacterial cell division and its relation to general biological cell division.