I want to draw a growth curve for an urease active bacteria cultivated in a urea contained medium, but the color of culture solution changes by decomposition of urea due to bacterial activity. How can I measure the OD which has resulted from cell growth in the medium only (without using the solution color changes)?
You could clear a part of your sample by centrifugation and sterile filtration and use this as a blank.
You could measure at a wavelength that does not absorb at the initial and the final colour. Check the colour change against the spectrum of light...
Is your culture pure culture? If yes and you are working only with one culture you can first check its urease activity in Christensen's Urea medium. This will confirm whether the culture is urease positive. Secondly after confirming that the culture is positive, you can make the medium without the indicator dye as the dye is not required for the growth curve experiment it is required only to confirm whether the culture is urease positive. Therefore, you can make the medium without indicator dye that is phenol red and then use the medium for the inoculation and measure the OD.
I hope this will solve your problem.
You could clear a part of your sample by centrifugation and sterile filtration and use this as a blank.
I Echo Stefan. Centrifuge and wash your cell pellet with ID water and re-suspend in DI water. Use DI water as blank in spec. Alternatively, you may also use cell free supernatant as blank every time you read a sample. Best
If your organism picks up the colour of the solution, centrifugation and washing might not remove the colour completely. For such studies, it is better to monitor growth using dehydrogensae assay.
It is better that at each interval time, you withdraw a few ml of your culture medium, then centrifuge a part of that to remove the cells. Use this clear solution as a blank.
An alternative is to spin down bacterial cells and resuspend cells in the fresh medium and then do OD reading.
Given that you are doing a curve and the background "color" will be changing with time, I agree with Bastiaan that the best approach is to find a wavelength where neither the starting nor the ending media color absorb. Otherwise you start getting a very complicated interplay between absorbance and transmission/reflection that is not likely to be linear throughout the range you are working at.
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