OD is the negative log of transmittance, which is the fraction of the light that is detected when it is passed through a cuvette that contains a sample of the culture. Population density is estimated from the turbidity of the culture and is typically expressed as OD (optical density), typically at a wavelength of 600 nm. Spectrophotometers have been used for over 6 decades as a means of measuring the population density of microbial cultures. Given that relationship OD can be used to calculate cell numbers just as A is used to calculate concentration of a solute. The convenience of measuring cell populations in microtiter plate readers led us to determine the relationship between OD and cells/mL for several microbial species and for plates of different sizes. It can be important to determine cell densities easily and quickly, i.e., when monitoring growth in fermenters to determine when to harvest cells. The classic way was to dry a culture and weigh the cells, a method that does not lend itself to easy measurement of cell densities in small cultures, (to say nothing of the fact that while weighing the dehydrated cells, they absorb moisture from the air and the weight increases even as the balance is watched). It is not a trivial matter to determine the number of cells/mL or the mass of cells in a culture. There is no simple factor equivalent to ϵ that relate number of cells/mL to OD. The relationship between cells/mL and OD is complex and depends on several factors including length of light path, size of the particles (cells), and number of particles. Rather than absorbing light, particles scatter light, which is why we express turbidity as OD (optical density) instead of A (absorbance). It does not apply to suspensions of particulate matter such as microbial cells. However, the Beer-Lambert law applies only to solutions in which molecules of solute are uniformly distributed throughout the solvent. That relationship is what allows us to monitor enzyme reactions by absorbance, to measure protein concentrations by absorbance, and to do enzyme-linked immunoassays (ELISA). Epsilon (ϵ) is given at a specific wavelength and specific light path, usually a 1 cm light path. The Beer-Lambert law relates the molar concentration (C) of a solute to absorbance of light according to the equation C = ϵA where ϵ is the molar extinction coefficient and A is the absorbance. Portia Mira The funders had no role in the study design, data collection and analysis, decision to publish or prepare the manuscript.Ĭompeting interests: The authors have declared that no competing interests exist. Kirschstein National Research Service Award (AI007323) to Dr. This is an open access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.ĭata Availability: All relevant data are within the paper and its Supporting Information files.įunding: We are grateful for funding from a KL2 Fellowship (PJY) through the NIH/National Center for Advancing Translational Science (NCATS) UCLA CTSI Grant Number UL1TR001881, the Presidential Postdoctoral Fellowship and the Ruth L. Received: Accepted: SeptemPublished: October 13, 2022Ĭopyright: © 2022 Mira et al. PLoS ONE 17(10):Įditor: Abdelwahab Omri, Laurentian University, CANADA

Citation: Mira P, Yeh P, Hall BG (2022) Estimating microbial population data from optical density.

Synergy h1 well monitor and plate