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L02 — The Superbug

Module: M07 — The Arms Race Periods: 1 NGSS: HS-LS4-2, HS-LS4-5 Lesson Type: Explore / Explain

Learning Objective

  • By the end of this lesson, students will be able to explain how antibiotic resistance emerges in bacterial populations through natural selection by analyzing data from a virtual simulation and connecting it to the four conditions from L01.

Phenomenon / Hook

MRSA Story, Part 2: “When Addie arrived at the hospital, doctors ran a culture of the bacteria in her blood. The results came back: Staphylococcus aureus — resistant to methicillin, oxacillin, amoxicillin, and three other antibiotics. This wasn’t one mutant bacterium. This was an entire population of resistant bacteria. How did that happen?”

Show an image of an antibiotic susceptibility test (zone of inhibition plate). “The clear circles are where antibiotics killed the bacteria. The cloudy areas? The bacteria survived.”

Materials

Agenda

Opening (7 min)

  • Show Addie Part 2 story + antibiotic susceptibility plate image (3 min)
  • Think-Pair-Share: “Look at this plate. How do you think doctors figured out which antibiotics might still work? What does it mean that some bacteria survived in the cloudy zones?” (3 min)
  • Quick review: four conditions of natural selection from L01 — students state them from memory (1 min)

Explore (18 min)

  • Part A: MEGA-Plate Video (whole class, 5 min)
    • Show the HHMI BioInteractive “Evolution At Warp Speed” MEGA-Plate experiment: bacteria evolving resistance in real time across a giant agar plate with increasing antibiotic concentrations
    • Students observe and note: Where do bacteria stop growing? What happens over time? How do they “escape” each antibiotic zone?
  • Part B: Cave Bacteria Data Analysis (pairs on Chromebooks, 13 min)
    • Students use HHMI BioInteractive: Origins of Antibiotic Resistance
    • Analyze a published figure showing resistance percentages in bacteria from Lechuguilla Cave — a location never exposed to human antibiotics
    • Guiding questions on handout:
      1. What percentage of these cave bacteria were resistant to at least one antibiotic? Why is this surprising?
      2. If these bacteria were never exposed to humans, where did the resistance genes come from?
      3. Map the four conditions of natural selection onto the MEGA-Plate:
        • Variation? (some bacteria were naturally more resistant)
        • Inheritance? (resistant bacteria passed on resistance genes)
        • Selection pressure? (antibiotic concentration zones)
        • Differential reproduction? (survivors reproduced and colonized new zones)
      4. Did the antibiotics cause resistance, or was it already there? What evidence supports your answer?

Explain (10 min)

  • Class discussion on cave bacteria data:
    • Key insight: Resistance genes existed in bacteria before antibiotics were ever used by humans. The antibiotic didn’t cause resistance — it selected for bacteria that already had it. Variation existed before the pressure was applied.
    • MEGA-Plate connection: The video showed exactly this — bacteria with pre-existing resistance survived each zone and expanded into the next.
    • MRSA connection: This is exactly what happened in Addie’s body. Hospitals are hotbeds of selection pressure because antibiotics are used heavily.
    • Superbug evolution timeline: Methicillin introduced 1959 → MRSA detected 1961 → Now resistant to most antibiotics. Evolution in 2 years.
  • Students annotate data sheet with the “already resistant” vs. “became resistant” distinction

Elaborate (8 min)

  • Hospital Scenario Challenge: Groups receive a scenario card:
    • “You’re a hospital administrator. MRSA infections in your hospital have increased 300% in 5 years. You have a limited budget. Which 2 of these 4 strategies would you prioritize?”
    • Options: (A) More hand sanitizer stations, (B) Reduce antibiotic prescriptions by 50%, (C) Isolate all patients with resistant bacteria, (D) Invest in new antibiotic research
    • Groups write a 2-sentence justification for each chosen strategy
    • Quick share-out: compare choices and reasoning

Closing (5 min)

  • Exit Ticket: [[exit-tickets/L02-exit-ticket]]
    1. Explain why the statement “antibiotics cause bacteria to become resistant” is incorrect. Use the four conditions of natural selection in your answer.
    2. A doctor prescribes antibiotics for a viral infection (like a cold). Why is this a problem from an evolution standpoint?

Differentiation

SupportExtension
Provide a partially completed data table — students fill in counts and calculate percentagesAsk: Research one real strategy hospitals use to combat MRSA. How does it reduce selection pressure? Present to class.
Pair struggling students with a stronger partner who handles the simulation controls while they focus on recordingChallenge: If resistance costs bacteria energy (it’s a trade-off), what happens to resistant bacteria when antibiotics are removed? Predict and explain.

Assessment

  • Formative: Check data sheets during simulation — are students accurately recording and interpreting trends?
  • Exit Ticket: Misconception correction + real-world application

Teacher Notes

  • Simulation backup: If BioInteractive is down or slow, use a paper alternative: give each pair a grid of 50 dots (bacteria). Circle 5 as “resistant” (different color). Remove non-resistant dots. Remaining reproduce. Repeat on paper for 5 generations.
  • The “already resistant” point is the single most important concept in this lesson. Hammer it. Students almost always think the antibiotic causes the mutation. It doesn’t — it selects for pre-existing variation.
  • Connect back to L01: “Remember our colored tokens? The antibiotic is the blue background. Resistant bacteria are the blue tokens. Same mechanism, real consequences.”
  • The hospital scenario is intentionally debatable — there’s no single right answer. Use it to show that evolution knowledge informs real-world decisions.