Antibiotic resistance in the lab … with actual bacteria!

Apparently my post on the antibiotic resistance activity using green, yellow, and red beads was a big hit with instructors, because lots of people want a copy of the worksheet. I have been happy to oblige, and now I want to follow up by calling your attention to a potential companion activity in the March 2018 edition of The American Biology Teacher.

I hasten to add the disclaimer that I have not tried this lab in my own class; I only just read about it this week. But the title certainly seems promising: Modeling the Emergence of Antibiotic Resistance in Bacterial Populations.

The authors outline a 5-day procedure in which students expose a bacterial population to antibiotics at two different times. On day 1, students select a single colony of bacteria from an agar plate and suspend the cells in a nutrient broth. The bacteria-laden broth is then swabbed across the surface of an agar plate. Students then place paper disks with and without antibiotics on the plate. The bacteria on the plate are allowed to grow overnight in an incubator and are then placed in the refrigerator until day 5. Meanwhile, the original broth also incubates overnight. On day 2, the instructor transfers bacteria from the broth into fresh broth and again allows the cells to multiply overnight in the incubator. On day 3, each student group swabs the resulting inoculum onto a fresh agar plate and adds disks with and without antibiotics. Like the plate from day 1, this plate is allowed to incubate overnight and is subsequently stored in the fridge until day 5.

On day 5, it’s time to collect the data. Students examine their plates, measuring the zone of inhibition around each antibiotic-infused paper disk. They use the data they collect to test two models of antibiotic resistance. According to the article, “Model 1 illustrates need-based acquisition of antibiotic resistance. Here, the bacteria rapidly gain antibiotic resistance only when exposed to the antibiotic.” If model 1 is correct, then the number of resistant colonies arising from the “day 1” swab should be equal to the number of resistant colonies arising from the “day 3” swab. In contrast, “Model 2 illustrates acquisition of antibiotic resistance through mutation. Here, the bacteria acquire mutations over time that increase their level of resistance to the antibiotic independent of antibiotic exposure.” If model 2 is correct, then the bacteria plated on day 3 should have more mutations — and more resistance — than those plated on day 1.

The article contains great illustrations, explores the molecular mechanisms of antibiotic resistance, introduces the idea of intermediate resistance, and has lots more information about how to implement this activity. The authors also offer student handouts and even a PowerPoint presentation. I can’t endorse it because I haven’t tried it, but heck, if you’re looking for antibiotic resistance activities that actually involve growing bacteria, you might want to check it out.

Reference:

Michelle A. Williams, Patricia J. Friedrichsen, Troy D. Sadler, Pamela J. B. Brown. Modeling the Emergence of Antibiotic Resistance in Bacterial Populations.
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