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Number of serial dilutions for plating purposes - (Jan/01/2008 )

Hi people,

We're doing plate counts using the spread plate method. Question 1: Is there a standard as to how many dilutions to make? We're doing serial dilutions up to 10^5 but I've read in books that some even dilute up to 9 times.

Q2: I only calculate the number of colonies in the 30-300 range and dilutions having anything outside are excluded. When I get my CFUs for the different "Countable" dilutions, I take the average to arrive at a final CFU/mL. Is there anything wrong with my way of doing things?

Thank you people. Have a nice day!


I think you need to do some optimization to know roughly how many bact cells in your sample that you are plating on plate.

plate with few dilutions (maybe fr 10^5 - 10^9), repeat if you don't get countable colonies. After sometimes you'll have some idea how much to plate and how many dilution to make. we all do that when we need to plate serial dilution.