Dilution plating gives a viable count: it counts only cells that are alive and able to divide. A culture usually contains far too many cells to count on one plate, so it is diluted first by serial dilution.Method:
1. Using aseptic technique, transfer 1 cm3 of culture into 9 cm3 of sterile water and mix. This is a 1 in 10 (10−1) dilution.
2. Transfer 1 cm3 of this into another 9 cm3 of sterile water to give 10−2, and so on, using a fresh sterile pipette each time.
3. Spread 0.1 cm3 of each dilution evenly over an agar plate with a sterile spreader.
4. Incubate, then count the colonies.
cells per cm3 = colonies × (1 ÷ volume plated) × dilution factor The dilution factor is 105 for a 10−5 dilution.Worked example: the 10−3 plate has 45 colonies from 0.1 cm3.45 × 10 × 103 = 4.5 × 105 cells per cm3 The result may be an underestimate, because two cells lying close together can produce a single colony.
1. Using aseptic technique, transfer 1 cm3 of culture into 9 cm3 of sterile water and mix. This is a 1 in 10 (10−1) dilution.
2. Transfer 1 cm3 of this into another 9 cm3 of sterile water to give 10−2, and so on, using a fresh sterile pipette each time.
3. Spread 0.1 cm3 of each dilution evenly over an agar plate with a sterile spreader.
4. Incubate, then count the colonies.