The method mentions that the 24h broth culture should be diluted to get 1-2 x 10^5 E-coli cells. My question is how to figure out the relation between dilutions and number of cells? I am sure we have to use Optical density. However, I have not come across a resource which explains the methodology.
I assume what you are looking for is the McFarland standard. You can measure the turbidity of a cell suspension to estimate the number of bacterial cells in the suspension. A McFarland standard of 0.5 is appr. 1-2 10E8/mL E. coli cells. If you want to go more precisely, you need to measure the turbidity and to plate out tenfold dilutions.
Appart from what has been said already (McFarland standard) you can also prepare a standard curve of the organism by ploting a graph of optical density against log 10 number of cell in a broth suspension plated out on a suitable solid growth medium. Then draw a line of best fit through the points (regression line). With this you can estimate the number of cells when you measure the optical density of the cell suspension.
Thank Oliver, Ngule and Chika all for your time and efforts in explaining the method.
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