You are testing unpasteurized milk for the presence of...

You are testing unpasteurized milk for the presence of bacterial contamination. Starting from the undiluted milk, you do serial dilutions as shown below, and plate $1.0 \mathrm{ml}$ of each dilution on agar. If the undiluted milk contains $5 \times 10^{6}$ bacteria/ml, how many colonies would you expect to see on each plate?

Key Concepts

Dilution Factor

The dilution factor is the ratio by which the concentration of the original sample is reduced at each step of a serial dilution. This factor is crucial for calculating the expected number of bacteria in the diluted samples and ultimately determining the microbial load in the original sample based on the number of colonies observed.

Colony Forming Units (CFUs)

Colony forming units (CFUs) are a measure of viable microorganisms capable of growing into colonies on an agar plate. Each colony is assumed to arise from a single viable bacterium, allowing the enumeration of CFUs to provide an estimate of the number of living bacteria present in a given volume of the original sample.

Serial Dilution

Serial dilution is a fundamental microbiological technique involving the step-wise dilution of a concentrated sample. It is used to obtain a range of bacterial concentrations such that the number of colonies on a plate becomes countable, facilitating the estimation of microbial density in the original sample.

Transcript

00:01 To determine the number of colonies you would expect to see on each plate after performing serial dilutions of the milk with an initial bacterial concentration of five times 10 to the sixth bacteria per milliliters, you're gonna have to calculate it using the dilution factor.

00:16 So the number of colonies on each plate is gonna be inversely proportional to the dilution factor.

00:23 So let's assume that you performed several serial dilutions and each dilution involves taking a portion of that previous solution and diluting it by a known factor.

00:33 So let's say we have a series of dilutions that looks like this.

00:36 Let's say the first, we have our undiluted milk, which we see is five times 10 to the sixth bacteria milliliters.

00:54 So then second, let's say it's one to 10 dilution.

01:02 So one part milk plus nine parts of the diluent.

01:15 Okay, and then it'll just kind of continue that trend.

01:18 We have one to 10 of the previous solution, our fourth, same thing, one to 10 previous and our fifth, same thing, one to 10 of the previous.

01:36 So now we can calculate the dilution factor for each plate.

01:41 So looking at plate number one, and i'm gonna change to blue.

01:44 Looking at plate number one, there's no dilution, right? so our dilution factor, i'm gonna abrogate that as df.

01:51 Our dilution factor is going to be one.

01:54 For plate number two, our dilution factor is going to be one 10th, right? because we have one part milk and nine parts of that diluent.

02:06 Now, when we go down here to plate number three, we are going to see, you're gonna have your one 10th and you're going to need to multiply that by another 10th because again, you're taking one to 10 of the previous solution, not one to 10 of the diluted milk, you're taking it from plate number two...

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