As part of the unit on Natural Selection and Evolution, I use antibiotic-resistant bacteria as an example of evolution that we have been able to see and measure in real time. At this point in the year, we haven’t talked much about bacteria, so my students don’t have a lot of background knowledge about them. The past couple of years, my bio colleagues and I have used the Antibiotic Sensitivity lab to give students a hands-on experience of working with bacteria, agar plates, and antibiotic disks. (If you haven’t done this lab before, both Flinn Scientific and Carolina have kits that give you everything you need.)
Students read the results of their lab about 24 hours after they plate the bacteria. They have to look for and measure any zone of inhibition around the antibiotic disks that they placed on the agar. From this, they have to conclude whether the antibiotic is effective against the bacteria.
I did not have students prepare a formal lab report for this assignment. Instead, there were a few targeted questions asking about how they know if the bacteria is resistant to an antibiotic, and how they would determine which antibiotic is most effective against the bacteria. I was looking for evidence that students understood that if an antibiotic is effective against bacteria, there will be a clear zone of inhibition around the disk where the bacteria were killed. If the bacteria was resistant to the antibiotic, then there would be no zone of inhibition around the disk.
Within the first few assignments I started grading, I quickly saw that students were reversing the two results. It was one of those “ooooh dear” moments, so I started spot-checking answers across all of my classes to see if it was a fluke (I was crossing my fingers!), or if there was a real pattern emerging. Sure enough, a significant number of students thought that if there was a zone of inhibition, that meant the bacteria were “resisting” that antibiotic.
Ooooooops. Double oops, when I considered the fact that there were questions on the upcoming test that assessed this concept. But I was also running out of time – there was only one class period before the test. At first I thought that I would reteach the concept to the whole class, although that would take away time that I had planned to give them for review. Thankfully, the lightbulb went on and I realized I could do a quick spot check to figure out which students did not understand. Then I could pull them aside for some one-on-one review.
I teach at a 1:1 school, so I knew I could use a Google form for quick answers. I started with a photo of a bacterial plate with five antibiotic disks on it, showing a range of zones of inhibition. I edited the photo to number each disk. I wrote four brief questions: (1) which antibiotic is the bacteria resistant to (they could choose as many as they wanted); (2) if the bacteria are resistant to an antibiotic, what will you see on the petri dish (short answer); (3) which antibiotic is the most effective against the bacteria (multiple choice – they could only choose one); and (4) explain how you know which antibiotic is most effective against the bacteria (short answer).
I had students fill out the Google form at the very beginning of class. After everybody had submitted their responses, I could quickly see who understood and who did not.
I also exported the data to a Google sheet so I could identify which students answered incorrectly. (I set the form to automatically collect their email address.) For those students, I had a paper copy of the image I used. I went to each student individually and went over the answers to make sure they had the correct information. They got an annotated copy of the correct answers to put in their notebooks so they can refer to it when they are studying for their test.
The best things about this were that I could find out exactly who needed help, and that it only took a few minutes of class time to find out the information!