Are we really monitoring the concentration of the bacteria or bacterial growth.
In theory, higher the concentration greater the chance of spectrophotometric errors. Overnight cultures are highly concentrated. At that concentration the OD values are not that reliable, and hence you get a wide range of OD values. I personally don't trust OD values above 2.0. Besides, the concentration also varies from sample to sample as it depends on volume of initial inoculation which could easily vary. If you need to check the bacterial concentration you should dilute the culture first.
N.B.: Whether you need to check OD or what concentration of agrobacterium you need for transformation varies for different protocol and plant species.
Finding OD of the bacterial culture will let us know the optimum concentration of the bacterial cells required for transformation. Accordingly the culture can be diluted.
Follow this link please:
https://bitesizebio.com/1005/what-you-need-to-know-about-od600/
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