Correct Equation for Calculating

**CFU/ml =
(No. of colonies x Total dilution factor)/Volume of culture plated in ml**

**CFU **is the
**C**olony **F**orming **U**nit

Lets workout a question to understand the concept

This is the summarized video of this topic

**Question: What is the number of CFU per ml of original stock? the following figure.**

**Step 1: Find out the plate with countable no. of colonies**

Countable no. of colonies is a plate with colony count between 30-300.
This number is experimentally and statistically taken as per standard lab
manuals in microbiology.

**In the Question given select a plate with number of colonies between 30-300.**

In plate 1 colony count is 443; it is considered as “**T**oo **M**any **T**o **C**ount” **(TMTC)**; that is overcrowded therefore chance of error is high.

**In
plate 2 colony count is 63; between 30-300; this plate is to be taken for
further calculation.**

In plate 3 colony count is 3; it is considered as “**T**oo **F**ew **T**o **C**ount” **(TFTC)**; that is small sample size; therefore chance of error is
high.

So we have taken plate 2 with 63 numbers of colonies

**Step 2: Find out
the Total Dilution Factor (TDF)**** **

**Dilution
Factor (DF) =Vf/Vi **

**Where Vf = Volume of diluent +Stock volume**

**
Vi=Volume of stock transferred **

DF=Vf/Vi=10/1=10 or**10 ^{1}**

For the first testtube DF and TDF is the same as we have
diluted only once that is **10 ^{1}**

For Test tube 2

DF=Vf/Vi=10/1=10 or**10 ^{1}**

**Total Dilution Factor (TDF)=DF1xDF2XDF3**

**B tube=10 ^{1}x10^{1}=10^{2}**

**C tube=10 ^{1}x10^{1}x10^{1}=10^{3}**

As a serial dilution procedure; it is very easy; ten
times dilution at each step

So find out the TDF of the culture tube from which we
cultured to get the plate with colony count 63

The TDF of plate with 63 colonies is **10 ^{5 }**

This is to be taken as TDF for further Calculation

**Step 3: Substitute
the Values in the Equation**** **

No. of colonies=63

Total dilution factor=10^{5}

*Vol. plated=0.1 ml

*Volume plated is 0.1ml that should be considered.

**CFU/ml =
(No. of colonies x Total dilution factor)/Volume of culture plated in ml**

CFU/ml
in original stock= 63x10^{5 }/0.1
ml =63x10^{6 } =63,000,000^{ } CFU/ml

=**6.3 × 10 ^{7}
cfu/ml**

^{ }

Here TDF is 10^{6 }. From the culture tube with
TDF 10^{5 }, we have taken only 0.1 ml of 1 ml, that is further diluted
to 10 times to become 10^{6 }total dilution or TDF.

Related Topic: Serial Dilution Protocol

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