A liquid culture of E . coli at a concentration of 2 ×10^8...

A liquid culture of $E .$ coli at a concentration of $2 \times 10^{8}$ cells/ml was diluted serially, as shown in the following diagram, and $0.1 \mathrm{ml}$ of cells from the final two test tubes were spread on agar plates containing rich medium. How many colonies do you expect to grow on each of the two plates?

Key Concepts

Serial Dilution

Serial dilution is a systematic process used to decrease the concentration of a sample step by step. In microbiology, it enables researchers to obtain a dilution level at which microorganisms are countable when plated, ensuring that individual colonies on an agar plate are derived from a single microbial cell or a group of cells.

Dilution Factor

The dilution factor is a measure of how much the original sample has been diluted. It is calculated by considering the volume transferred and the total volume in the dilution process. This factor is essential for back-calculating the concentration of cells in the original sample based on counts obtained from diluted plates.

Spread Plate Method

The spread plate method involves pipetting a known volume of a diluted microbial suspension onto an agar plate and spreading it uniformly across the surface. This technique facilitates the isolation of individual colonies, making it easier to count colony forming units, which represent viable cells in the sample.

Colony Forming Units (CFU)

Colony Forming Units (CFU) represent the count of viable microbial cells that are capable of growing and forming visible colonies on an agar plate. This concept is central to microbial quantification, as converting a colony count from a diluted sample back to the original concentration relies on understanding the concept of CFUs.

Transcript

00:01 All right, so we have an e.

00:01 Coli culture starting at 2 .8 cfu per mil, or two times 10 to the eighth cfu per mil, and we go through a serial dilution, and then we plate 0 .1 mils at the very, the last two dilutions.

00:14 And then we plate them.

00:15 The question is, how many bacteria will we have on these two plates? so first we need to figure out the dilution factor.

00:24 So the first one is 0 .1 mils going into 9 .9 mil, so this is respectively a 1 -2.

00:30 100 dilution.

00:32 We do this one more time, so this is going to be another 1 to 100 dilution.

00:37 And then we go in the third tube, take one mil from the previous tube, go into 9.

00:43 So this is going to be a 1 to 10 dilution.

00:45 And the last one is also a 1 to 10 dilution.

00:48 So the dilution factor for this plate, actually let's go up to here, the dilution factor for this plate is going to be 1 over 100, times 1 over 100 times 1 over 10, which gives us 1 over 10 to the 5th or 10 to the minus 5.

01:18 For this one, the last plate, we take 1 over 10 to the 5th and just multiply by 1 over 10.

01:26 And that gets us to 1 over 10 to the 6th or 10 to the minus 6th.

01:32 Now, remember that we started off with 2 times 10 to the 8th, see if you per mill.

01:41 So, if you look at 10 to the 5th, this dilution, this plate, if we start off with 2 times 10 to the 8th, and we do a dilution, 10 to the minus 5 dilution, oh, one more thing to consider is, of course, volume plated.

02:09 So the volume plated is 0 .1 mils.

02:12 So i guess let's just go through the actual formula.

02:19 Why not? so our starting cfu is 2 times 10 to the 8th cfu per mil.

02:26 And the formula, what we're going to fill in is the number of bacteria, which is going to be in cfu.

02:34 Are dilution factor, so let's do this for the third dilutions in the series.

02:43 So this is going to be dilution factor of 10 to the minus 5, and then the volume plated is 0 .1 mils.

02:58 Okay, so basically we just need to simplify this a little bit better...