## Monday, November 21, 2011

### CFU: Colony Forming Unit & Calculation

colony-forming unit (CFU or cfu) is a measure of viable bacterial or fungal cells. In direct microscopic counts (cell counting using haemocytometer) where all cells, dead and living, are counted,but CFU measures only viable cells. For convenience the results are given as CFU/mL (colony-forming units per milliliter) for liquids, and CFU/g (colony-forming units per gram) for solids. CFU can be calculated using miles and misra method, it is useful to determine the microbiological load and magnitude of infection in blood  and other samples.

Example:
Calculate the number of bacteria (CFU) per milliliter or gram of sample by dividing the number of colonies by the dilution factor The number of colonies per ml reported should reflect the precision of the method and should not include more than two significant figures.

Serial Dilution of Bacterial Culture

The CFU/ml can be calculated using the formula:

cfu/ml = (no. of colonies x dilution factor) / volume of culture plate

For example, suppose the plate of the 10^6 dilution yielded a count of 130 colonies. Then, the number of bacteria in 1 ml of the original sample can be calculated as follows:

Bacteria/ml = (130) x (10^6) = 1.3 × 10^8 or 130,000,000.

CFU/mL Practice Problems - CFU/mL Calculation Examples

Problem 1:
Five ml of Bacterial Culture is added to 45 ml of sterile diluent. From this suspension, two serial, 1/100 dilutions are made, and 0.1 ml is plated onto Plate Count Agar from the last dilution. After incubation, 137 colonies are counted on the plate. Calculate CFU/mL of the original Sample?

First thing we need to know is the Dilution Factor, or how much the original sample is diluted:
here Initially 5mL in 45mL = Final Volume / Sample volume = 50/5 = 10.

Then two serial dilutions of 1/100.

Total Dilution Factor = 10 * 100 *100 = 10^5

CFU/mL = cfu/ml = (no. of colonies x dilution factor) / volume of culture plate

= (137 * 10^5)/0.1
=1.37*10^8

So Total colony forming units = 1.37*10^8 CFU/mL

Converting CFU/mL to Log value

For example,

Total colony forming units = 1.37*10^8 CFU/mL and you want to convert it into Log value,

Just take Log(CFU/mL)

Here log (1.37*10^8) = 8.13924922.﻿

Useful for expressing log reduction of microbes / biologic log reduction.

References
Laboratory Exercises in Microbiology 1. how come when we dilute a solution its bacterial count start increasing by putting the values in formula, i think so it should decrease, as solution is diluted

2. hi,
Bacterial count per ml of the original sample is calculated here in that example. Please go through it once again. Bacterial count will be higher in the original sample than the diluted one.

3. I just had a bonus question that was the reverse and it threw me off. Question was "you have ordered a solution you expect to be 1 x 10(*6) (sorry, texting). You plate 1 ml each time. How many dilutions would you expect to do to reach a usable number of plated cfu's? We use 20-200 as our range, so I was thinking the desired plate would be 100cfu/ml so I said 4 dilutions but that was a wild guess based on very hesitant reverse math- not my strength. What's the answer and how do we do this in reverse? Thanks!

1. Hi,
Just substitute the values in the below equation

(no.of colonies * dilution factor)/volume plated = cfu/ml

from the data you provided,

CFU/ml = 1*10^6 ie, 10^6
No of colonies = 100
Volume plated = 1ml
Dilution Factor = ?

substituting the values in the equation:

(100 * DF)/1 = (10^6)/1

DF = (10^6)/100
= 10^4

Dilution Factor is 10^4.
you need to do 10000 times dilution, ie 10x dilutions 4 times.

Hope you understood.

4. Hi,
Just substitute the values in the below equation

(no.of colonies * dilution factor)/volume plated = cfu/ml

from the data you provided,

CFU/ml = 1*10^6 ie, 10^6
No of colonies = 100
Volume plated = 1ml
Dilution Factor = ?

substituting the values in the equation:

(100 * DF)/1 = (10^6)/1

DF = (10^6)/100
= 10^4

Dilution Factor is 10^4.
you need to do 10000 times dilution, ie 10x dilutions 4 times.

Hope you understood.

5. 1. microbial plate count =10 to the power of 5/grams = means what value please help me and also let me know

2.cfu= how many grams

6. Bacteria/ml = (130) x (10^6) = 1.3 × 10^8 or 130,000,000.

should have divided to the "volume plated" too unless its 1mL, right?

1. Yes, you should divide it with the volume plated. if its 250 uL then divide by 0.25.I considered plating volume here as 1mL.