# Microbiology dilution question

need B and C plates calculations and need all calculations to be shown please

10 grams of soil are placed in 40 ml of water (A). One ml of (A) is placed in 99 ml of water (B) 0.01 of (B) is placed in 9.99 ml of water (C). Dilutions (B) and (C) are plated in triplicates 0.1 ml in each plate.

After 24 hours incubation at 37 degrees Celsius, these are the results:

Plate B-1 counted 102 colonies

Plate C-1 counted 10 colonies

B-2 counted 152 colonies

C-2 counted 12 colonies

B-3 counted 111 colonies

C-3 counted 11 colonies

Calculate the number of bacteria per gram of soil. (Show ALL calculations!)

…. So far what I have

For the above mentioned example, the plate ‘B’ should be used for calculating the number of bacteria taking into consideration that it contains colonies which are in the range of 30 to 300.

The dilution factor for the plate can be calculated as follows :-

DF = initial volume / final volume

= 1ml / 100 ml

= 10-2

Number of bacterial colonies = CFU in plate / (Amount of diluent added) x (Dilution factor)

Plate B1

CFU/ml = 102 / (0.1)(10-2) = 102 x 103

Plate B2

CFU/ml = 152 / (0.1)(10-2)

= 152 x 103

Plate B3

CFU/ml = 111 / (0.1)(10-2)

= 111 x 103

Taking an average, we get (102 + 111 + 152 x 103 / 3 = 121.66 x 103) bacteria or CFU / gram of soil.