I have plated some bacteria with the following dilutions: 10^0, 10^-1 and 10^-2. Once I've counted the CFUs on each plate, do I average the numbers to determine the final CFU for that sample? Are some more representative than others, depending on what range they fall in, etc?
Thanks!
Just a comment on that: Do not average the counts of different dilutions. You can only average the counts of several plates of the same dilution (i.e. 5 plates of 10^-1 can be averaged). As Ashraf said, try to take only the plates with colony numbers between 50 and 100 into account. In a final report, I use the highest number cfu/ml I counted of a serial dilution (worst case scenario)...
Cheers
Dear colleague,
I hope I understand your question right. Yes, you have to average the numbers to determine the final CFU. The more plates you have, the better (usually 5 for each dilution). For bacterial viable counts, it is considered that 50 to 100 CFUs per plate is an optimal number to avoid suppression by overcrowding and to give reliable and reproducible counts.
Just a comment on that: Do not average the counts of different dilutions. You can only average the counts of several plates of the same dilution (i.e. 5 plates of 10^-1 can be averaged). As Ashraf said, try to take only the plates with colony numbers between 50 and 100 into account. In a final report, I use the highest number cfu/ml I counted of a serial dilution (worst case scenario)...
Cheers
Do not convert different dilutions and take average as recommended buy Kunal.
The reason is as stated before overcrowding giving less colonies then expected or in case of low amount of colonies were random fluctuations of cell happen to be plated can be too big and not give you proper results.
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