Last week our 8th grade Bacteria Labs went all wrong. Student teams went all around the school swabbing different places to see where they would find the most bacteria. What happened was something I didn’t expect, not one single petri dish showed any growth. No bacteria, no mold, nothing. We discussed possible sources of error and both classes conceded that although it could be possible that every team swabbed poorly it was very unlikely that was the cause. Everyone also agreed that we didn’t have evidence that our school was bacteria free! So it came down to two possible sources, the incubator and the agar I had made.
Strangely enough the incubator was running hot, 9˚C hotter than it was indicating. I emailed the company and a guy called trying to problem solve this discrepancy. He said that if I put too many petri dishes on the very bottom it would cause the unit to keep heating to compensate. Since I didn’t have enough room to put all the petri dishes I just set the temp to 28˚C to get it to incubate at 37˚C. 🙂
Even with that nine degree difference bacteria should have grown. So I emailed our high school biology teacher. She noticed that if I used agar alone and not nutrient agar the bacteria would have no food! The Agar I used is probably one purchased in the 60’s! The jar, pictured here, had no information. What I did find was that my Foss Diversity of Life kit came with, wait for it, nutrient agar. Long story. So I heated the nutrient agar vials and prepared another set of petri dishes.
My students already had problem questions, hypotheses, and procedures so it took them no time to re-swab all the places they selected before. This time within less than 24 hours we got what I expected.
Here are some photos of what students got. So they got to record their newly found data and rewrite their conclusions. Here are their blogs where they will be sharing their conclusions.
In my defense, I followed a recipe I found online, which was good, and I followed the directions for preparing the Agar. I just didn’t know that it had not nutrients in it! A noob mistake.