colony forming unit/ milliliter

[jake_f]

How do you calculate that?

 

The following results were obtained when a sample of microbes was serially diluted and the diluted samples used for spread plate experiment (0.1 ml per plate). Determine the CFU/ml in the original sample (express your answer to 2 decimal places).

 

Dilution | Plate 1 | plate 2|

10^-4 | 450 | 360 |

10^-5 | 159 | 220 |

10^-6 | 37 | 45 |

10^-7 | 7 | 10 |

 

The steps my lecturer taught me is:

 

-Count the number of CFU between 25 – 250 colonies

-Multiply the number by the reciprocal of the dilution

-Gives an estimate of the number of bacteria/ml

 

What does it means?

[CharonY]

What precisely do you have trouble understanding?

[jake_f]

I don't know how to start, what does it mean by counting the number of CFU between 25 – 250 colonies? Do I take the number, 37 from plate one and 45 from plate 2, divide it by 0.1ml and multiply by 10^6?

[CharonY]

I assume that you are supposed only to count plates which have between 25-250 colonies. If there are more counting them may be considered tricky (though actually quite feasible). The other reason may be that, if you look at the numbers, there was obviously some kind of mistake between 10-4 and 10-5 dilutions.

Yes, you count the colonies for each plate and then calculated back based on the dilution on how many were initially in one ml. In theory you would also want to give average and standard deviation but with two data points it is not very informative.

[jake_f]

My lecturer told me that the answer for that question is 3.00 X 10 ^8. But I still could not get it.

 

I use all the various number that is between 25-250 and calculated back based on dilution but I still couldn't get the answer.

[CharonY]

That is what I get roughly, too. Show the calculations. Also did you consider that you only used a part of each dilution (i.e. that your sample is even more diluted by a factor)?

[jake_f]

I got it finally.

 

I took the number 37 and 159 and multiply to their dilution factor and average it. And did the same for plate 2, 45 and 220. Then I average the answer of the two plates. Thanks for you help.