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Name: Valerie
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Question:
I'm doing a science project on bacteria. WHat I'm doing is washing forks with different dishwashing liquids, then wiping any remaining bacteria on to Agar petri dishes. Then incubating it and seeing which soap removed the most. My question is what kind of bacteria would be growing? and also do I just count the colonies to compare? and how long and at what temperature should I incubate this bacteria? Thank you very much for your time. I'll be looking forward to your response.



Replies:
The temperature is easy: 37 degrees C is optimal for many bacteria. The medium will determine which bacteria grow best. So if you don't see growth on one medium, but you see growth on another, it tells you that there is a difference in nutrients present in those media that is required for that bacteria. Look at your plates after 24 hr, then put them back in the incubator (keep them sterile) and look at them after 48 hrs--do you see the difference? any slow-growing bacteria visible or did the fast-growing take over the complete plate?

You can count the colonies, but also watch their shape, color and general appearance. If you see different types of colonies ('morphologies') you could pick each different one with a sterile loop and plate it out on a new plate by a three-point streak (ask your teacher how to do this) to see if that colony was pure. Sometimes bacteria grow in one big splash and you have to separate them in single-colony streaks before you can see how many different types there are. The bacteria you can expect depends on what the forks were used for. Was cooked food eaten with it? THen you expect to see the bacteria that live in the mouth of the people that ate with them. Compare it with the growth you get from a streak of saliva on a plate. Was the water used for washing up sterile or not? Use some water before the cutlery was added to see what is growing there, that is your negative control. Before you write your report, make sure you learn more about bacteria at www.bacteriamuseum.org to impress your teacher.
Have fun!

Trudy Wassenaar



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