This image shows the results from a past semester's "Evolution of Antibiotic Resistance" experiment.
As described in your Lab Manual (pp. 49 - 53), the Petri dishes were each inoculated with a strain of the bacteria, E. coli. There were three types of plates--two control plates, and one experimental plate:
- Three replicate Petri dishes of sterilized Luria Bertani nutrient medium (labeled "LB") inoculated with E. coli (= control)
- Three replicate Petri dishes of sterilized LB nutrient medium inoculated with E. coli into which streptomycin (labeled "LB+STR") was added (= control)
- Three replicate Petri dishes of sterilized LB nutrient medium inoculated with UV irradiated E. coli into which streptomycin (also labeled "LB+STR") was added (plates marked in blue ink)
Zoom-in on the above photograph so that you can see the Petri dishes, their labels, and their contents really well.
Each line (drawn with a marker pen) on each Petri dish indicates where a linear inoculation streak of E. coli occurred -- one streak per team of students per Petri dish. The Petri dishes were streaked by 3 or 4 teams. So, they have 3 or 4 lines. The numbers written on the Petri dishes identify the student teams who did each streak. Areas where streaks produced living E. coli colonies look like linear beige-colored smears. Locations of streaks that did not generate living bacterial colonies remained clear. Each streak is a replicate.
QUESTIONS: (Your answers should be 1 to 3 sentences per question)
In the photo above, can you identify the Petri dish(es) containing mutated streptomycin-resistant E. coli colonies that descended from UV-exposed cultures? What evidence are you using to make this determination?
What percentage of replicate streaks of E. coli evolved antibiotic resistance? What percentage did not? Why was it that not all replicate streaks gave the same results?
Briefly explain what our controls were. What other treatments or experiments would improve the study (though be more costly in terms of plates and time)?
Why should we care about the results of this experiment? (in other words, what is its broader relevance?