Ask A Scientist Molecular Biology Archive

name         Maureen H.
status       educator
age          40s

Question -
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? 
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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
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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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