» How big is an E. coli cell and what is its mass?
Table 1: Relation between bacterial mass and division time. The dry mass per cell is given as a function of the generation (doubling) time. Mass is suggested to increase roughly exponentially with growth rate as originally observed by M. Schaechter et al J. Gen. Microbiol., 19:592, 1958. The cell dry weight was calculated using a value of 173 µg per OD460 unit of one mL (BNID 106437). Strain used is B/r, a strain commonly used in early bacterial physiology studies. Values taken from F. C. Neidhardt, “Escherichia coli and Salmonella: Cellular and Molecular Biology”, Vol. 1., Chapter 3, ASM Press, 1996.
Table 1: Relation between bacterial mass and division time. The dry mass per cell is given as a function of the generation (doubling) time. Mass is suggested to increase roughly exponentially with growth rate as originally observed by M. Schaechter et al J. Gen. Microbiol., 19:592, 1958. The cell dry weight was calculated using a value of 173 µg per OD460 unit of one mL (BNID 106437). Strain used is B/r, a strain commonly used in early bacterial physiology studies. Values taken from F. C. Neidhardt, “Escherichia coli and Salmonella: Cellular and Molecular Biology”, Vol. 1., Chapter 3, ASM Press, 1996.
The size of a typical bacterium such as E. coli serves as a convenient standard ruler for characterizing length scales in molecular and cell biology. A “rule of thumb” based upon generations of light and electron microscopy measurements for the dimensions of an E. coli cell is to assign it a diameter of about ≈1µm, a length of ≈2µm, and a volume of ≈1µm3 (1 fL) (BNID 101788). The shape can be approximated as a spherocylinder, i.e. a cylinder with hemispherical caps. Given the quoted diameter and length we can compute a more refined estimate for the volume of ≈1.3 µm3 (5π/12 to be accurate). The difference between this value and the rule of thumb value quoted above shows the level of inconsistency we live with comfortably when using rules of thumb. One of the simplest routes to an estimate of the mass of a bacterium is to exploit the ≈1 µm3 volume of an E. coli cell and to assume it has the same density as water. This naïve estimate results in another standard value, namely, that a bacterium such as E. coli has a mass of ≈1 pg (pico=10-12). Because most cells are about 2/3rd water (BNID 100044, 105482) and the other components, like proteins, have a characteristic density of about 1.3 times the density of water (BNID 101502, 104272) the conversion from cellular volume to mass is accurate to about 10%.
