Question

Hey, help me with these problems if you can please.

Answer 1

Allright i love microbiology :)
Do you know what CFU/mL is and how you calculate it?

Answer 2

No, we've never discussed this before.. She just assigned it and since it's a summer course were left to our own devices.

Answer 3

oke then i'll explain it :D
CFU = colony forming units
This is just a single viable cell capable of forming a colony
/mL is how many of these cfu's are in 1 milliliter of sample

So to calculate CFU/mL you apply this formula:

\(\sf\Large\frac{number~of~colonies~*~dilution~factor}{plated~volume}\)

In your case, there are more number of colonies. So you just calculate the CFU/mL for every dilution. If the plating and diluting was done right, you'd suspect every next \(\sf\large\frac{1}{10}\) delution to have about 10 times less colonies.
You can already see that's the case in your example. the second one is 730 and the third one is 67. If you multiply 67 by 10 you get 670 which is close to 730.

Now the calculation: for the second \(\sf\large\frac{1}{10}\)delution you would have a dilution factor of 100 because 10*10=100. A 1 in 10 dilution is the same as 10x dilution factor.

you'd get: \(\sf\Large\frac{730~*~100}{0.1}= \normalsize{730,000}\)

Does this make sense?
Could you do the same for the third \(\sf\large\frac{1}{10}\)delution?

Answer 4

Hey man! I am sorry I didn't respond back then! I went through the problems and I was absolutely confident that I had them right but apparently I was wrong on the first 2 problems

Answer 5

it's okay :)
do you understand it now?

Answer 6

I think I understand, I was just wondering if I could run my revised answers by you and see if you agree

Answer 7

sure, post your answers and i'll see if they're correct

Answer 8

So originally my answers for 1 & 2 were
1) 6.7 X 10^5
2) 9.63 X 10^8

But she said those were wrong & that we forgot the first dilution so we redid them and raised the exponent on each of them.

New answers are
1) 6.7 X 10^6
2) 9.63 X 10^9

Answer 9

Hey man, whatcha thinking?

Answer 10

sorry i was busy, let me see...

Answer 11

first one: (when you calculate from the second 1/10)
730*10 = first 1/10 (7,300 )
original sample = 7300 * 10 = 73,000
last step is another multiplications with 10 (because you plated 0.1ml) so you end with 730,000 cfu (7.3*10^5)

when you calculate from the third 1/10 you'll end up with 6.7*10^5

Answer 12

She said that was wrong. That was the original answer I turned in. :(

Answer 13

second one:
289*10*10*10*10*100=2.89*10^8
3g soil in 27 ml water -> 3/27 = 0.11 g/ml
\(\Large\frac{2.89*10^8}{0.11}\)= 2.6*10^9 CFU/g soil

Answer 14

I don't know who told you it's wrong but the way i calculated it, you're correct :/

Answer 15

Thank you for your help, I guess I will just have to talk to my teacher about it because who knows. I got the same answers as you originally. I just divided number 2 by 3 after I got the final answer because it words the question asking for cfu/g but it gave it to us in 3 grams at first.