You performed a soil extraction (1 mL peptone solution for 1 g soil) as the extr
ID: 255997 • Letter: Y
Question
You performed a soil extraction (1 mL peptone solution for 1 g soil) as the extracting solution. A dilution and plating technique is performed using the spread plate method by spreading 0.1 mL of diluted solution in agar plates. The following results are obtained:
a. 150 colonies are counted on an R2A plate at the 105 dilution; b. 30 colonies are counted on a nutrient (nutrient rich) agar plate at 104 dilution.
Calculate the number of viable bacteria per gram of soil for R2A and nutrient agar. Why are the counts for nutrient agar lower than for the R2A media?
Explanation / Answer
a. 150 colonies are counted on an R2A plate at the 10^5 dilution
Original CFU/ml = ?
Dilution factor = 10^-5
Number of colonies= 150
Volume of sample =0.1 mL to the plate
cfu/ml = (no. of colonies)/( dilution factor x volume of culture plate)
cfu/ml = (150)/ (10^-5 x 0.1)
cfu/ml = 1.5 x10^8 cfu/mL
number of viable bacteria per gram of soil for R2A 1.5 x10^8 cfu/mL
b. 30 colonies are counted on a nutrient (nutrient rich) agar plate at 10^4 dilution.
Original CFU/ml = ?
Dilution factor = 10^-4
Number of colonies= 30
Volume of sample =0.1 mL to the plate
cfu/ml = (no. of colonies)/( dilution factor x volume of culture plate)
cfu/ml = (30)/ (10^-4 x 0.1)
cfu/ml = 3 x10^6 cfu/mL
number of viable bacteria per gram of soil for nutrient (nutrient rich) agar is 3 x10^6 cfu/mL
R2A Agar is a low nutrient medium, and in combination with a lower incubation temperature and longer incubation time, stimulates the growth of stressed and chlorine-tolerant bacteria. Nutrient agar being rich in nutrients might suppress slow growing or stressed bacteria.
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