Question

Your milk is getting close to its expiration date in the fridge and you're not sure if you want to actually drink it. Instead of smelling or tasting it as a normal person might do, you decide to calculate how many bacteria are growing in it. You remove 1 ml and plate it onto a general all-purpose media. (Undiluted) To make sure you'll have an accurate range of colonies to count, you also do a few dilutions and plate them as well. You take another 1 ml of milk and dilute it in 8ml of sterile broth (Dilution 1). After mixing, you dilute another time using 1ml into 9mls of broth (Dilution 2). After mixing, you remove 1ml for the final dilution into 10mls of broth (Dilution 3). From each of these dilutions you plate 1ml of liquid (twice, for accuracy) onto a general all-purpose media. The results of how many colonies that grow the next day are listed below. What was the original concentration of bacteria in the milk (in CFUs/ml)?

Sample	Plate 1	Plate 2
Undiluted	Lawn	Lawn
Dilution 1	Lawn	Lawn
Diltuion 2	137	142
Dilution 3	12	8

Answer 1

CFU/ml = number of colonies / Dilution factor.

Dilution factor = volume of inoculum / total volume

Dilution factor for dilution 1 = volume of milk/volume of milk + volume of sterile broth.

1 / 1+8 = 1/9 or 0.11

So dilution is 1/9 or 0.11

Dilution factor for dilution 2 = volume from previous dilution/volume from previous dilution +volume of sterile broth.

1 / 1+9 = 1/10 or 0.1

Overall dilution = 1/9*1/00 = 1/90.

We generally take the dilution that gave us colonies between 30-300.

In the given case, dilution 2 gave colonies between 30-300.

Average of colonies given by dilution 2 is (137+ 142) / 2 = 279/2 = 139.

CFU/ml = 139 / 1/90

= 139 * 90

= 12510 or 1.251 *104 CFU/ml

the original concentration of bacteria in the milk (in CFUs/ml) is 1.251 *104 CFU/ml.

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