- Place 9cm3 of distilled water into test tubes with bungs.
- Place 1cm3 of the bacterial sample in the first tube and mix.
- Transfer 1cm3 of solution from the first tube into the next tube and mix, repeat for the remaining tubes.
- Transfer 1cm3 of a sample from each tube onto a sterile nutrient agar plate.
- Use a sterile spreader to spread the sample around each plate.
- Repeat this so there is 3 plates per dilution (allows calculation of an average number of colonies)
- Seal with tape but not air tight (around the plate) this prevents growth of anaerobic bacteria.
- Incubate at 25'C for 24hours, don't incubate at 37'C as this is optimum for human pathogenic bacteria.
- Count number of colonies on each plate.
- Multiply the number of colonies by the diultion factor to give the number of bacteria in the original sample.
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Serial diultion = a viable count that assumes a single bacterial cell will reproduce to form a colony that can be seen and counted.
Inaccuracies with experiment:
- Clumping - can lead to an underestimation
- Every dilution decrease accuracy
- Count the lowest dilution with no clumping
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