Hii everyone, I want to calculate the total number of bacteria cells from OD. Is there any formula or calculation kindly share it with me.
Hi Monika
The generally quoted formula for e coli is that an OD600nm reading of 1.0 = 8x10^8 cells/ml. This is a rough estimate since the individual spectrophotometer used and the strain of bacteria will effect the count. Also consider that if your bacteria are expressing a fluorescent tag it may interfere with the readout.
The ideal way to establish a formula for your own application would be to create a curve measuring the OD against the CFU counts in a plated serial dilution of your bacteria.
This is a protocol for using this company's spectrophotometer but it shows the general idea of how you would measure the OD vs CFU/ml https://tipbiosystems.com/wp-content/uploads/2020/05/AN102-E.coli-Cell-Count_2019_04_25.pdf
Hope that helps you
McFarland standards can be used to visually approximate the concentration of cells in a suspension. The McFarland Scale represents specific concentrations of CFU/mL and is designed to be used for estimating concentrations of gram negative bacteria such as E. coli.
You can find the details here http://www.microbiologynetwork.com/content/JVT_2011_v17n1_Measurement-of-Microbial-Cells-by-Optical-Density.pdf
Please note however that if you are working with a bacteria not similar to E. coli then the values could be significantly different. Smaller cells will give higher cell numbers for the same OD, since OD is how much light is scattered by the cells. Also even for E. coli you will get different numbers depending upon growth condition.
So you would be best to do a growth curve and measure CFU at different times/OD values to generate a plot you can use for your specific organism and growth conditions (if you want to have some reasonable precision).
Standard OD (e.g. 0.0005) X the volume of your broth then divide to the mean of bacteria that you have read (note: in three replicates) the multiply to 1000. This might vary depends on the protocol you're following.
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