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# Calculation Bacteria

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### #1 chandch

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Posted 15 November 2012 - 09:35 AM

Hi
I am trying to calculate colony forming units calculation. I cultured bacteria overnight (15ml) and did O.D reading at 0.4 and did serial dilution (10*1 to 10*9) and I got 55 colonies at 10*4 dilution. So I got 5.5 × 10*6 cfu/ml (original, if I am right).  But I need 1 ×10*7 CFU/ml (for my experiment). How can I Increase it it. Please give example calculation.

2) I also cultured bacteria overnight (15ml) and did O.D reading at 0.4 and did serial dilution (10*1 to 10*9) and I got 85 colonies at 10*5 dilution. So I got 8.5 × 10*7 cfu/ml (original, if I am right).  But I need 1 ×10*7 CFU/ml (for my experiment). How I can I reduce it down?

Thank you

### #2 pito

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Posted 15 November 2012 - 11:08 AM

1) 55 colonies at 10^4 dilution, means 550 colonies at 10^3 dilution, 5500 (10^2), 55000 (10) or 550.000 (=5,5x10^5) colonies being not diluted. So I am guessing you missed a factor 10 somewhere?

well, if you need more bacteria.. you need to grow more bacteria.. You cant "make" bacteria yourself, you cant increase the concentration or CFU , all that you can do is dilute...

2) again, check your math.. not sure how you calculate it...
Dilute.. just dilute it...
If you don't know it, then ask it! Better to ask and look foolish to some then not ask and stay stupid.

### #3 chandch

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Posted 15 November 2012 - 12:12 PM

Hi
I am just wondering, how I can Dilute 8.5 × 10*7 cfu/ml  to 1 ×10*7 CFU/ml. I asked everybody in my lab, but they are unsure about this. Could you please give an example.

Thank you

### #4 chandch

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Posted 15 November 2012 - 12:18 PM

I calculated in this way: 85 colonies at 10*5 dilution (100 microlitres). So in 1ml 850.
Then 850 × 10*5 cfu/ml
so 8.5 × 10*7 cfu/ml (original at O.D 0.4)

### #5 pito

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Posted 15 November 2012 - 12:36 PM

chandch, on 15 November 2012 - 12:18 PM, said:

I calculated in this way: 85 colonies at 10*5 dilution (100 microlitres). So in 1ml 850.
Then 850 × 10*5 cfu/ml
so 8.5 × 10*7 cfu/ml (original at O.D 0.4)

Aha, I see where the extra factor 10 comes from.. you used 100µl to plate..

You have to give all the facts...
If you don't know it, then ask it! Better to ask and look foolish to some then not ask and stay stupid.

### #6 pito

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Posted 15 November 2012 - 12:38 PM

chandch, on 15 November 2012 - 12:12 PM, said:

Hi
I am just wondering, how I can Dilute 8.5 × 10*7 cfu/ml  to 1 ×10*7 CFU/ml. I asked everybody in my lab, but they are unsure about this. Could you please give an example.

Thank you

8,5 to 1 , thats just diluting 8,5 times...

You do know how to make dilutions, right?

If you take (for example) 1 ml of the 8,5 "stock" and you add 7,5ml of water..
then you end up with 8,5ml in total, containing 8,5cfu in 8,5 ml.... meaning 1cfu in 1ml
If you don't know it, then ask it! Better to ask and look foolish to some then not ask and stay stupid.

### #7 chandch

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Posted 15 November 2012 - 01:07 PM

Thank you. So If I take 1 ml of 8.5 × 10*7 cfu/ml and  add it to 7.5ml of water then it makes 1 ×10*7 CFU/ml. But, if we take 1ml from that aren't we decreasing it to 8.5 × 10*6 cfu/ml. For example, If I have a stock of 2.0 × 10*8 cfu/ml and if I take a 1 ml from that add to 9ml water, i think it makes 2.0 × 10*7 cfu/ml

Edited by chandch, 15 November 2012 - 01:08 PM.

### #8 pito

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Posted 15 November 2012 - 01:16 PM

chandch, on 15 November 2012 - 01:07 PM, said:

Thank you. So If I take 1 ml of 8.5 × 10*7 cfu/ml and  add it to 7.5ml of water then it makes 1 ×10*7 CFU/ml. But, if we take 1ml from that aren't we decreasing it to 8.5 × 10*6 cfu/ml. For example, If I have a stock of 2.0 × 10*8 cfu/ml and if I take a 1 ml from that add to 9ml water, i think it makes 2.0 × 10*7 cfu/ml

yes, in your example, you diluted it 10 times...

But this "But, if we take 1ml from that aren't we decreasing it to 8.5 × 10*6 cfu/m" , I dont get it.

You mean, if you take 1 ml of the 8.5 × 10*7 cfu/ml and add that to 9ml of water? or?
If you don't know it, then ask it! Better to ask and look foolish to some then not ask and stay stupid.

### #9 chandch

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Posted 15 November 2012 - 01:53 PM

Yes, If I take 1 ml of the 8.5 × 10*7 cfu/ml and add that to 9ml of water, Is it 8.5 × 10*6 cfu/ml?
So to get 1 ×10*7 CFU/ml, do I need to add 1ml of  8.5 × 10*7 cfu/ml of to 7.5 ml of water?

Thank you

### #10 pito

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Posted 16 November 2012 - 10:31 AM

chandch, on 15 November 2012 - 01:53 PM, said:

Yes, If I take 1 ml of the 8.5 × 10*7 cfu/ml and add that to 9ml of water, Is it 8.5 × 10*6 cfu/ml?
So to get 1 ×10*7 CFU/ml, do I need to add 1ml of  8.5 × 10*7 cfu/ml of to 7.5 ml of water?

Thank you

If you take 1ml of the 8.5 × 10*7 cfu/ml and add that to 9ml of water, you will indeed have 8.5 × 10*6 cfu/ml?

So yes, you need to take 1ml of the 8.5 × 10*7 cfu/ml  and add this to 7,5ml of water to get 1 ×10*7 CFU/ml
If you don't know it, then ask it! Better to ask and look foolish to some then not ask and stay stupid.

### #11 chandch

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Posted 20 November 2012 - 11:42 AM

Thank you Pito . Now it is very clearer for me, on how I can calculate bacteria cfu.

Edited by chandch, 20 November 2012 - 11:42 AM.