3. (5 pts.) You are asked to determine how many colony forming units are found i
ID: 150818 • Letter: 3
Question
3. (5 pts.) You are asked to determine how many colony forming units are found in 100 grams of soil. You begin by mixing 1 gram of the soil into water, totaling 100 ml in volume. Then you remove 5 ml of this solution and mix it in 95 ml of sterile water. After repeating this serial dilution 2 more times, you plate (in triplicate) o.1 ml of the final dilution. After incubation, you observe 142, 173, and 180 colonies on the three different plates. How many CFU are present in the 100 grams of soil?Explanation / Answer
3. 1 g of soil is dissolved in 100 ml of water. 1: 10 dilution of soil requires dissolving 1 g of soil in 10 ml. Hence, this is a 1: 100 dilution of soil sample and is a 10-1 dilution.
5 mL of this solution is added to 95 mL distilled water. This is a 1: 20 dilution of the sample. This is 10-2 dilution. The next two dilutions will be 10-3 and 10-4 dilutions
Dilution = Multiply all dilutions= 1/100* 1/20*1/20*1/20= 1/800000
Dilution factor =800000
Average number of colonies= (142+173+180)/3= 495/3= 165
Number of colonies in 1 g of soil= Average Number of colonies *Dilution factor /Volume plate
= 165/800000* 0.1= 165*8*103/10-1= 1320 X 104 CFU/mL= 1.320*107 CFU/g
Number of colonies in 100 g of soil= 1.320*107*100= 1.320 X 109 CFU/100g
4. Sample count= 3.7X 109 CFU/mL
Hence, 1 ml will contain 3.7 X 109 colony forming unit (bacteria). Number of colonies that can be counted easily will be 30-300 colonies.
10-1 dilution= 1 mL of 3.7 X 109 CFU/mL dilution + 9 mL of distilled water= 3.7 X 109 CFU/mL/10= 3.7 X 108 CFU/mL
10-2 dilution= 1mL of 10-1 dilution + 9 mL of distilled water= 3.7 X 108 CFU/mL/10= 3.7 X 107 CFU/mL
10-3 dilution= 1mL of 10-2 dilution + 9 mL of distilled water=3.7 X 107 CFU/mL/10= 3.7 X 106 CFU/mL
10-4 dilution= 1mL of 10-3 dilution + 9 mL of distilled water=3.7 X 106 CFU/mL/10= 3.7 X 105 CFU/mL
10-5 dilution= 1mL of 10-4 dilution + 9 mL of distilled water=3.7 X 105 CFU/mL/10= 3.7 X 104 CFU/mL
10-6 dilution= 1mL of 10-5 dilution + 9 mL of distilled water=3.7 X 104 CFU/mL/10= 3.7 X 103 CFU/mL
10-7 dilution= 1mL of 10-6 dilution + 9 mL of distilled water= =3.7 X 103 CFU/mL/10= 3.7 X 102 CFU/mL= 370 colonies/mL
10-8 dilution= 1mL of 10-7 dilution + 9 mL of distilled water= =370 colonies/10= 37 colonies/1 mL
Hence, 0.1 mL of colonies from 10-7 dilution that are spread plated on the two plates will have 370 colonies/mL *0.1 ml= 37 colonies
0.1 mL of 10-7 dilution is spread plated on the two plates to obtain measurable number of colonies.
If pour plate technique is used, 1 ml of sample is used. Hence, 1 ml from 10-8 dilution can be used for the two plates each.
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