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average microbial load

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20. One-hundred grams of fresh tomatoes were placed in a stomacher and blended for 45 seconds. A 1 mL sample of blended tomatoes was placed in tube containing 9 mL of Trypticase Soy Broth, and incubated at 37C for 24 hours. After the incubation period, 1 L and 10 L loops were used to streak two separate TSA plates in order to enumerate the microbial load of tomatoes. The inoculated plates were incubated for 24 hours at 37C. After the second incubation period, a microbiologist manually counted 17 and 210 colonies, respectively. Calculate the average microbial load in fresh tomatoes. Report the microbial load as cfu/g. Show your work.

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This question is a good example of using a standard plate count method to determine the microbial load in fresh food. The tricky parts of this type of question are keeping track of the dilutions made and converting the volumes so that the answer is expressed using the desired units, in this case cfu (colony forming units) per gram of original food sample. I will start with a quick review of the standard plate count method then demonstrate the calculation.

In this method, a small amount of a homogenized (blended) food sample is diluted and plated onto a solid growth medium in a petri plate. After the incubation period, bacteria appear as individual colonies on the surface of the growth medium. The assumption with this method is that each colony has arisen from a single cell, an assumption that has been rightfully challenged many times, since bacteria in solution tend to aggregate. That is why we express a numerical value in colony forming units, which is likely a more accurate assumption that each colony arises from a cell or small group of cells attached to each other. Once we count the number of colonies on each plate, this can be used to calculate back to an estimate of bacteria in the original ...

Solution Summary

Calculate the average microbial load in fresh tomatoes