How to weigh a bacterium? / VAAM - Vereinigung für Allgemeine und Angewandte Mikrobiologie e.V.

How to weigh a bacterium?

It is not so easy to weigh a single bacterial cell, although there is actually a very accurate measuring device for this. But to find out approximately how heavy a bacterium is, you don't necessarily have to put it on a scale. The simplest and quickest way to estimate the weight of a microbe is to measure its size.

A typical specimen of the intestinal bacterium Escherichia coli (E. coli) has a length of about 2.5 µm and a diameter of 0.75 µm. The ends of the rod-shaped cells resemble hemispheres and the body in between resembles a cylinder. The cell volume can therefore be determined approximately with that of a sphere and a cylinder of the same diameter and provides the value 1.0 µm3 for our example. The specific density of a cell is used to determine its weight. Assuming that E. coli consists only of water with a density of 1 g/cm3 (10-12 g/µm3), 10-12 g (one millionth of a microgram or one picogram) is already a reasonable estimate. Of course, the density and weight of the cell increase slightly due to its content of proteins, sugars, nucleic acids, lipids and other molecules.

If you want to know the mass of a cell more precisely, you have to determine its specific density. An elegant method is density gradient centrifugation. The cells are placed on a liquid that becomes denser from top to bottom in small steps or continuously. A solution of the sugar sucrose with an increasing concentration gradient is suitable for this purpose (or, better still, polymeric materials that do not simultaneously change the osmotic pressure of the solution). If the cells are now allowed to sink in a centrifuge at a high number of revolutions, they collect in the density medium at the position corresponding to their own density and "float" there. Values of 1.08 to 1.10 g/cm3 were found for E. coli. Our standard cell therefore has a mass of approximately 1.1-10-12 g.

How do you weigh microbes directly?
With a special scanning probe microscope, you can actually determine the pure biomass of individual cells directly. The measuring probe of an atomic force microscope consists of a fine leaf spring (cantilever) that bends under the smallest of forces. The bending can be tracked very precisely using a reflected laser beam.
If the spring is stimulated to vibrate, a characteristic vibration frequency (natural frequency) is produced, which can be measured with the laser beam. The frequency reacts very sensitively to forces acting on the vibrating spring. An aqueous medium flows through a small U-shaped channel inside the spring. As soon as a bacterium enters the spring with the liquid flow, its natural frequency shifts slightly. The cell has a slightly higher density than the surrounding medium and changes the total mass and thus the oscillation characteristics of the spring. The difference in frequency gives the pure biomass of the bacterium and, with a series of measurements, the mass distribution. Its shape is not exactly symmetrical to the most frequent value and is similar to that of the size and volume distribution of the microbes. According to this, E. coli has a typical biomass (without cell water) of 0.11-10-12 ± 0.03-10-12 g. If the measurements are repeated with media of different densities, the specific density of the cells can also be determined. The value found for E. coli was 1.16 g/cm3.

If the leaf spring of a scanning probe microscope is coated with proteins that bind to the surface of cells, individual cells can be picked out of a culture. This is possible with human cells, which are considerably larger and around 2000 times heavier than E. coli. Using this method, it is now possible to measure the cell mass together with the cell water and even track it over a longer period of time. Periodic fluctuations in cell weight were observed for the first time.

Microbes change their size (and therefore mass) significantly during growth and division. Their density varies with different nutrient supplies and with the storage of nutrients. Differences in size and mass by a factor of two or more in cells of one species are therefore not unusual. Incidentally, different microbial species sometimes differ considerably in size and mass.

F. C. Neidhardt (1987) Escherichia coli and Salmonella typhimurium. Cellular and
Molecular Biology. Vol. 1, American Society for Microbiology, Washington, S. 3-6

T. P. Burg et al., Weighing of Biomolecules, Single Cells, and Single Nanoparticles in Fluid (2007) Nature 446, 1066-106 9

M. Godin et al. (2007) Applied Physics Letters 91, 123121

D. Martinez-Martín et al. (2017) Nature 550, 500-505

© Text and figure Harald Engelhardt / VAAM, engelhar[at]biochem.mpg.de, Use according to CC 4.0