Question

Phatogenic bacteria in food are a major problem and especially Staphylococcus aureus' toxins lead to poisoning (at more than> 10 (5) CFU / g the food is not recommended). Your best friend has had a glass of nutella® standing open, at the Roskilde Festival, and asks you if it is safe to still eat the Nutella®. You perform the following experiment: Weigh 43g of the Nutella® and put it in 350 ml of sterile water and mix well. From here you make a 1/6 dilution, and out of it (ie your 1/6 dilution), you make a 1/90 dilution. In your last dilution, take 1 ml of it (ie your 1/90 dilution) and transfer it into 60 ml of sterilized water. Finally, take 0.1 ml of your last dilution and flatten it on one agar plate and incubate overnight at 37oC. The next day you can count 143 colonies (CFU).

Calculate the CFU / g and briefly explain whether your friend should be advised not to take the Nutella®.

Answer 1

Dilution factor in this experiment

1) First dilution: 43 g / 350 mL

2) second dilution: 1/ 6

3) Third Dilution: 1/90

4) 1 mL tranferred to 60 mL of water i.e. fourth dilution 1/61

5) 0.1 mL last dilution was plated

Formula no of CFU in 43 g is = CFU count in 0.1 mL / Dilution factor * 0.1 = 143/ (8.7 X 10-8) X 0.1 = 16486470000

43 g contains 16486470000 no of colonies , so 1g contains 383406279 no CFUs

Allowed limit is not more than 10 (5) CFU / g,

Hence Nutella contains more than allowed limit of Staphylococcus aureus CFUs, hence he should be advised not to take Nutella