# Methods to obtain the mass distribution of an ensemble of bacteria?

Are there any mainstream methods to obtain the mass distribution of a bacterial population?

What kind of assumptions to they make?

Could anyone point me to some references?

• Well, careful centrifugation can separate cells based on size. Commented Sep 16, 2022 at 23:49

## 2 Answers

Short Answer: Flow Cytometry or Dynamic Light Scattering (DLS) might be the most "mainstream" methods out there that might give you a size or volume distribution. Then you need to do some reading how to get bacterial mass from size or volume.

### Image Cytometry:

Krößbacher et al. 1998 used electron microscopy (which is not mainstream)

Oldewurtel et al 2021 used Phase microscopy and complex image pipelines (also not mainstream, but definitely cheaper than electron microscopy). They also stress that "surface-to-mass ratio remains nearly constant", which is very nice, since that means that one does not need a weighing scale, but only know the size of bacterial particles.

Therefore, cellular dry-mass density varies systematically with cell shape, both during the cell cycle or after nutrient shifts, while the surface-to-mass ratio remains nearly constant on the generation time scale.

### Flow Cytometry:

Knowing that one can determine bacterial mass from its size, one might just use flow cytometry to get the size distribution (see Felip et al. 2007) and then calculate the mass.

### Dynamic Light Scattering

DLS can also determine size distributions of particles and was used with bacteria (Vargas et al. 2017).

### (Field-Flow Fractionation)

Something fancy I found: Sharma et al 1993

• Thanks, and how do people estimate the density of bacteria? (needed to go from a size distribution to a mass one) Commented Sep 18, 2022 at 17:51
• @Gyromagnetic: Do you really need the exact mass distribution (as opposed to a relative mass distribution)? Either way, you can here use macroscopic measurements, looking at the average volume and mass of a bunch of cells. Commented Sep 18, 2022 at 18:43

In addition to Markur’s answer, Coulter counters suck the cells through a small capillary (in an electrolyte solution), which also serves as a conductor in an electrical circuit. They then estimate the cell volume by the change of conductivity of the capillary.

In my experience this method is more easy and robust than many others (e.g., flow cytometry) and as far as I can tell, the device is rather cheap and low maintenance – on account of being established and mass-produced technology.

Some relevant assumptions that go into it are:

• If you care about relative volumes: All cells are equally affected by the electrolyte solution and have the same conductance properties. If you care about absolute volumes: The cells are not affected or in a known manner.

• Cells do not stick to each other in a way that prevents disentangling double measurements, i.e., two cells entering the capillary at the same time. (If this happens by pure coincidence, you can statistically handle this.)