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Name: Maureen H.
Status: Educator
Age: 40s
Location: N/A
Country: N/A
Date: 2002

My 12 year old son would like to complete a science project on food irradiation. How would we go about irradiating hamburger to test for E.coli? I am professor at Broome Community College in Dental Hygiene and will supervise the experiment irradiating the food. Also at the college there is a microbiologist who will help test for the bacteria. Any suggestions on the procedures?

I would suggest a gamma radiation source that is used in food irradiation. I would do a classic "dose -response" increasing from no radiation to at least twice sterilization levels. To test for the bacteria I would also monitor for any presence every 48 hours after irradiation.

Peter Faletra Ph.D.
Assistant Director
Science Education
Office of Science
Department of Energy

Dear Michael:

This is an interesting experiment. Here is how I would do it. Get a non-pathogenic E. coli strain from your microbiologist friend (meaning it cannot cause disease so it will not be problematic in untrained hands) and grow this on an agar plate (which teaches the basics of sterile working). Scrape off the growth, do a Gram stain if facilities allow (to 'confirm' the identity of the growth) and suspend in a small volume of sterile saline (0.9% NaCl) water. that is your starting material.

I would split this amount in 2 portions. One is to be diluted serially (1:10 in sterile saline, mix, take from this the next dilution 1:10 and so on untill, say, 6 dilutions). Of these dilutions equal aliquots (for example 1 loopful or 1 drop from a pipette) are plated out on agar plates, to count how many colony forming units (CFU) were present in your starting material. You will be able to count the colonies on 2, maybe 3 plates only. The others are either empty or too full.

The second portion you sprinkle on the meat of a raw hamburger and let it stand in the fridge for 3 hrs or longer, to mimic the situation in a fast-food restaurant. Then you take a sample of the meat (this should be quantitative, so best would be to weigh it accurately), put in sterile saline water and mix well. Of this 'broth' you make another series of serial dilutions (with sterile saline as before). These are also plated on agar plates and these CFU counts will tell you how many bacteria were present on the so many grams of meat before irradiation.

The actual experiment is to irradiate your hamburger in a microwave, at defrost mode, with intervals to prevent the meat from getting warm. It should not decolor (a sign of heat). Depending on your culture resources, you could take samples of the meat in between the doses of irradiation, or just do one sample after what you think is a reasonable dose. Do the serial dilutions, the plating, and at the end of the culture time (1 or 2 days at 37 C) count the colonies.

This is what you should pay attention to when working out the results:

1. Count the colonies on the best dilution agar plate of each series and calculate back how many bacteria were in your starter culture (per ml) and on the meat (per gram) before and after irradiation.
2. did the amount of coli's increase/decrease/ stay constant during the time they were on the meat in the fridge (compare counts from starter material with the meat sample before irradiation). Can you explain the observation?
3. did the amount of coli's decrease/increase/ stay constant during irradiation?
4. do you observe any other growth on the hamburger samples that didn't show up in your starter material? if so, can you explain where these came from?

You will need some supervision with the bacterial handling and wash hands with soap after the experiments but if you have an incubator, agar plates, sterile saline and a culture of E. coli, it is doable.

Have fun!

Trudy Wassenaar

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