The Effect of Cooking Temperature on the Amount of Bacteria in Ground

Beef(8/13/2009)

The results are exactly what you can expect to happen in class. I encourage you to have some great discussion with you students about the results.

I love drawing conclusions from results; it is the thinking part of the investigation. Please don’t be offended by my notes. As scientists, we need to be

critical of the experimental procedure and the results obtained. We are the checks and balances of the scientific method. Without our ability to think and

reason, the experiment/data mean very little.Remember, the interpretation of results may lead to modification of the

experiment, not necessarily answers!!

What happened here?Teachable moment!

I love it when this happens!!!!!!

Notice the lack of uniformity in the plating process. This results in differing number of colonies due to procedure, not bacteria present.

Nice results!!!!!However, notice the writing

covers some results.Again, the plating/swabbing is

not uniform on all plates.

These guys had a great time!Observations were recorded directly on the plates…probably would be better to

record in lab journal.

Explain the lack of difference between 120 and 140 o F…we

need more trials don’t we.

Quantifying Data: Counting Colonies

• From the back/bottom of the sealed plate, count each colony (spec) of bacteria

• As you count mark the colony with permanent marker

• I count the first 100 colonies, then I let my partner count the next 100. We trade off until all colonies are counted.

The black dots are the colonies I counted. Using the marker is essential to keep track of counted colonies.

Note: It is important to remind students that not all of these bacteria here are pathogenic. If you purchase meat at your local supermarket, the facility where the meat was processed and the supermarket are inspected for pathogens and/or bad practices. Sometimes the meat even goes through an irradiation process to kill the bacteria. However, we all know that finding E coli 0:157 is like finding a needle in a haystack. It could be in your burger.

1 bacterium after 24 hours of incubation at 37oC = 1 colony of bacteria

Results: 0 colonies (interpretation: no bacteria on TSA plate)

Results: 500 colonies (interpretation: WOW, there’s a lot of bacteria in ground beef. I hope none of them are pathogenic!) note: you’re right, I didn’t really count them ;-)

Results: 400 colonies (interpretation: 120oF is not very effective at killing bacteria) (Practical application: If any of these bacteria are pathogenic, I am at risk of ingesting a food-born pathogen when I eat this rare burger.)

Results: 70 colonies (interpretation: 140oF is relatively more effective at killing bacteria when compared to 120oF.) (Practical application: Nick Adams, colleague here at West, says he would eat this burger. There is still the potential for food-born illness if the bacteria are pathogenic…Salmonella or E coli 0:157.

160oF

Results: 0 colonies (interpretation: ????)(Practical application: ????)

Have a great year of discovery, intrigue and enlightenment

through scientific investigation.