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# CFU/cm2 ??? HOW

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### #1 maryjo

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Posted 30 September 2010 - 12:25 PM

How is the number of CFU/ cm2 calculated?
A plate left in  a room (6m2) in the open air, grew on a 6mm agar plate 16 colonies after 24 hr incubation.

What is the CFU/cm2 or CFU/m2

### #2 protolder

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Posted 30 September 2010 - 10:29 PM

maryjo, on 30 September 2010 - 12:25 PM, said:

How is the number of CFU/ cm2 calculated?
A plate left in  a room (6m2) in the open air, grew on a 6mm agar plate 16 colonies after 24 hr incubation.

What is the CFU/cm2 or CFU/m2
Hola, I think that you have a mistake in the measure of the plate, 6mm is very small.Well, supose that the plate is of 6 cm of diameter => 3.14x3x3= 28cm2. 6m2= 60000cm2, 60000/28=2.143 times the surface of a plate in the total room surface. 16colonies x 2143= 34,288 col in 6 m2 <> 5714cfu/m2 <> 57.1/dm2 <>0.571/cm2.
If the plate was different repeat he same procedure. Good luck

### #3 philman

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Posted 01 October 2010 - 05:50 AM

maryjo, on 30 September 2010 - 12:25 PM, said:

How is the number of CFU/ cm2 calculated?
A plate left in  a room (6m2) in the open air, grew on a 6mm agar plate 16 colonies after 24 hr incubation.

What is the CFU/cm2 or CFU/m2

CFU/ cm2 is the number of colonies formed per cm2 of surface, so first work out the surface area of your plate.

I will assume you meant 6cm diameter plate rather than 6mm diameter, as 6mm is very small.

radius is 1/2 of the diameter, so pi*32 which is about 28cm2

if you had 16 colonies on that, then the CFU is 16 colonies per 28cm2

CFU/cm2 would me 16/28 = 0.57CFU/cm2

I am not sure what the size of the room has to do with anything, but I suppose you could calculate the total number of CFU in the whole room if you wanted to.

### #4 maryjo

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Posted 01 October 2010 - 09:45 AM

Thanks "protolder" and "philman"   it makes alot more sense now