00:01
So the following dilution were made from a sample of juice from the original sample.
00:05
So you can see that 1 .1 ml was transferred to 9 .9 ml to a sterile dilution and labeled tube a.
00:15
So i'm going to draw the dilution scheme.
00:32
And then from tube a, 3 ml was transferred to 12 ml of sterile diluent and labeled tube b.
00:40
And from tube b, 2 ml was transferred to 2 ml of sterile diluent and labeled as tube c.
00:59
The following table showed the volume plated from the three tubes onto nutrient agar and appear on the plate following incubation.
01:20
So the question obviously asks you about calculating the original concentration of the stock according to these plating.
01:33
So let's say, first of all, let's take a look at the table and find out which one of them are countable plate.
01:45
A countable plate equals a plate that has between 30 to 300 cfu.
01:56
Now when it's more than 300, we consider as too numerous to count.
02:00
When it's less than 300, it's too few to count.
02:03
So in that case, the plate count counts are not valid.
02:07
So you will only count the plate that has colony number within this range.
02:13
So let's take a look at all these numbers.
02:19
A, when you plate volume 0 .5, 320 cfu.
02:34
This is not a countable plate.
02:36
Again, if you take 0 .5 ml from tube a, you can't count too numerous to count.
02:46
Not a countable plate.
02:47
Now, then you take about 0 .25 ml from tube b and you get 63.
02:59
So that is a countable plate.
03:03
Again, you take 0 .5 ml from b and you get 134.
03:11
So that is a countable plate.
03:15
For c, if you take 0 .25 ml, you get 28.
03:26
That's not a countable plate.
03:29
But if you take 0 .5 ml, then you count 66.
03:35
That's a countable plate.
03:37
So we only considered the countable plate.
03:42
Now, once we decide which one of them are countable plate, let's calculate the dilution factor.
03:50
So let's say a.
03:55
For tube a, we know that you go from 0 .1 ml to 9 .9 ml.
04:03
So initial volume 0 .1, final volume 0 .1 plus 9 .9, which is a 10.
04:12
So the volume goes from 0 .1 to 10.
04:18
So this means the volume increase 100 times, times 10, times 10.
04:25
So you get 1 to 100.
04:28
So the volume increase 100 times and the concentration drops 10 times.
04:34
So from the stock to tube a, you did a 1 to 100 dilution.
04:38
From a to b, you go back, you can see that you take 3 out and add into 12.
04:51
Initial volume 3, final volume 3 plus 12.
04:58
So 3 out of 15, it's a 1 to 5 dilution...
