**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.

__Calculate CFU/mL in Excel__

**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?

**Answer:**

**10^5**

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

**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

Got something to say about this post? Leave a comment…your comments are valuable for improving the posts.

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

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.

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!

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.

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

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?

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.

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.