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Cell concentration Question - (Sep/27/2009 )

hello all, for my microbio lab, I have some discrepancy for cell concentration calculations obtained from Turbidity readings and my plating experiments... from Turbidity readings I obtained 2.10 x 10^9 cells/mL and from pour plating I obtained 2.81 x 10^9 cells/mL, so quiet a bit of difference

what could be a cause for this discrepancy? is this type of thing normal? any insight would be greatly appreciated.

thank you!


Agree - this is a concern about nothing. The precision of your plate counts is probably not sufficient to separate these, much between techniques. you are also attempting an overly precise interpretation of turbidity data.

-eberthella- on Sep 27 2009, 06:47 AM said:

Your values are within a similar magnitude and so the minor difference might just reflect error inherent in technique. Can you tell us the volumes used during dilution and your colony counts per dilution.

hi, i used
doubling dilutions (1/2, 1/4/ 1/8, 1/16, 1/32) to obtain the turbidity readings. to do this i added 3 mL Luria broth and 3mL of Bacteria successively.

decimal dilutions (10^-6, 10^-7, 10^-8) to pour plate and count the colonies. on to the final plate, i added 1mL of 10^-6 for 10^-6, 0.1mL of 10^-7 and 1mL of 10^-8 to obtain 10^-6, 10^-7, 10^-8 final plate dilutions, respectively. :D

so this kind of discrepancy is normal i assume? as long as they are in the same magnitude it is acceptable?
which method would you guys say is more reliable?

thank you!

-ydogyy1- on Sep 27 2009, 05:11 PM said:

You got it! I would stick with the plate counts because the data really reflects the number of total live bacteria (if that's what is important to you). Good luck!

thank you ;)