Bacteria multiply by binary fission. More than 30 genes in E. coli are known to be involved in the process that involves the polar separation of the daughter chromosomes, the formation of the cross-wall and envelope at the point of cell division, and ultimately the separation of the two newly formed cells. In rich medium at 37°C, the entire process is completed in 20 minutes in E. coli and many other pathogenic species. The most astound-ing aspect of this feat is that the replication of the chromosome in these cells takes ap-proximately 40 minutes, largely independently of the nature of the medium. The trick of dividing faster than the chromosome can replicate is accomplished by a mechanism that triggers the start of a new round of replication before an earlier one has been completed. In other words, during rapid growth multiple pairs of replication forks are at work on a given chromosome, and a newborn cell inherits chromosomes that have already been par-tially replicated. Bacteria maintain a constant cell mass:DNA ratio, and because rapidly growing cells have extra DNA (due to the multiple replication forks), cell size obviously is related to growth rate; the faster bacteria grow, the larger is their average size.
Cell division must be precisely coordinated with the completion of a round of DNA replication, or nonviable offspring will be produced. This coordination does not just hap-pen; it requires a special regulatory system. Mutants are known that are defective in this regulation; in some of them, cell division without chromosome replication and segrega-tion leads to the formation of minicells, which are complete cells save for lacking DNA.
The complexity of cell division would lead one to expect that it might be easily dis-rupted by chemotherapeutic agents, and this is the case. Nonlethal concentrations of an-timicrobics that act, even indirectly, on the polymerization or assembly reactions of the cell wall cause the formation of bizarre and distorted cells. Long filaments can result from incomplete cell division in the case of rod-shaped bacteria such as E. coli. Such forms are frequently encountered in direct examination of specimens from patients treated with antimicrobics.