Small but Mighty: Cell Size and Bacteria

Abstract

Our view of bacteria is overwhelmingly shaped by their diminutive nature. The most ancient of organisms, their very presence was not appreciated until the 17th century with the invention of the microscope. Initially, viewed as "bags of enzymes," recent advances in imaging, molecular phylogeny, and, most recently, genomics have revealed incredible diversity within this previously invisible realm of life. Here, we review the impact of size on bacterial evolution, physiology, and morphogenesis.

Figure 1.

Giant bacteria. On the left is a chain of Thiomargarita namibiensis cells. In this bright-field image, sulfur granules can be seen in the cytoplasm. The panel on the right shows an exceptionally large Epulopiscium cell with two large internal offspring. Scale bars, 100 µm.

Figure 2.

DNA in Epulopiscium sp. type B. This cluster of Epulopiscium cells is stained with the DNA dye DAPI. Each of the two large mother cells at the center of this field contains two large offspring. DNA is located at the periphery of the cytoplasm in mother cells and offspring. At this late stage of development, the mother-cell DNA is difficult to see because much of it has degraded. Offspring cells contain brightly stained polar structures; these are the start of “granddaughter cells.” Scale bar, 100 µm.

Figure 3.

Histogram of Escherichia coli cell size during growth in nutrient-rich medium. The size of wild-type (black) cells is restricted within a narrow twofold range. The graph is biased slightly toward the left owing to the higher number of newborn cells, the consequence of binary fission. UDP-glucose biosynthesis mutants (pgm::kan) (hashed lines) are smaller than wild-type on average; however, the size of these cells is similarly constrained within a twofold range. (Data courtesy of Norbert Hill.)

Figure 4.

Circular depiction of the cell cycle in rapidly growing (mass doubling time <60 min) bacterial cells. Note that DNA replication and growth are more or less constant—a consequence of multifork replication. A new round of replication is initiated only once per cycle. At least one round of replication must be terminated before division. Although the division machinery is assembled well before cytokinesis (the FtsZ ring is extant for ∼90% of mass doubling time in cells cultured in very rich medium), constriction itself takes only a few minutes.