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Two Flasks in the Hospital Lab: How Resistant Bacteria Take Over

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Two Flasks in the Hospital Lab: How Resistant Bacteria Take Over

Use daily counts and a simple model to explain how an antibiotic changes the share of resistant bacteria.

Part 1: Six days in two flasks

Read the lab story and Table 1, then answer questions 1–5.

Career Day in the Microbiology Lab

1Last winter, eighth grader Ximena Kovacs had an ear infection. Her doctor prescribed an antibiotic, a medicine that kills bacteria or stops them from growing, and told her to take it exactly as directed. Ximena had heard that some bacteria are no longer harmed by certain antibiotics, and she wondered how that happens. This spring, on her school's career day, she spent a morning in the microbiology lab of the city hospital with lab scientist Darnell Asante.

2Mr. Asante explained that bacteria reproduce by splitting in two. In warm broth at 37 °C, many kinds can divide about every 20 to 30 minutes. In a large group of bacteria, some cells may carry gene versions that let them survive an antibiotic. These cells are called resistant, and when a resistant cell splits, both new cells are resistant too. The other cells are called sensitive.

3To show what happens over many generations, the lab keeps a demonstration that uses a harmless lab strain of bacteria. Mr. Asante started two flasks of broth with the same mix of cells: about 2 of every 1,000 cells came from a resistant line of the strain, and the rest were sensitive. Flask A got a low dose of an antibiotic, and Flask B got none. Both flasks stayed at 37 °C. Every 24 hours he moved a small drop from each flask into fresh broth, adding the same low dose to Flask A each time, so the bacteria kept growing. Each day, the bacteria in each flask went through about 7 generations.

4Each day he also spread a small sample from each flask on two kinds of plates, one with the antibiotic and one without. Only resistant cells grow into colonies on the antibiotic plates, so comparing the colony counts told him how many of every 1,000 cells in the flask were resistant. Ximena watched from behind the safety glass while he worked.

5The low dose in Flask A killed many sensitive cells each day, but it did not kill all of them. Mr. Asante said that the same kind of change can happen, more slowly, in bacteria that live in people and animals.

Table 1. Resistant cells per 1,000 cells
Day	Flask A (antibiotic)	Flask B (none)
0	2	2
1	10	2
2	48	3
3	200	2
4	556	2
5	862	3
6	969	2

1.

Use Table 1. Write each answer as a percent of all the cells in Flask A.

On day 0, % of the cells in Flask A were resistant. On day 6, % were resistant.

2.

Use the Flask A column of Table 1.

From day 0 to day 1, the number of resistant cells per 1,000 became times as large.

3.

In Flask A, the count rose by 356 per 1,000 from day 3 to day 4 but by only 107 per 1,000 from day 5 to day 6. Which explanation fits best?

A. The antibiotic stopped working after day 4, so the resistant cells lost their advantage.B. The bacteria divided fewer times on days 5 and 6 than on days 3 and 4.C. By day 5, few sensitive cells were left, so there was little room for the share to keep rising fast.D. The resistant cells began to turn back into sensitive cells after day 5.

4.

What do the Flask B results show?

A. Without the antibiotic, resistant cells slowly took over, only more slowly than in Flask A.B. Flask B had no resistant cells in it, so the antibiotic test plates found none there.C. The bacteria in Flask B stopped dividing after day 0, so the counts could not change.D. Without the antibiotic, resistant cells had no clear advantage, so their share stayed near 2 per 1,000.

5.

Use paragraph 3 and Table 1.

From day 0 to day 6, the bacteria in Flask A went through about generations.

Part 2: A step-by-step model

Use this simple model for questions 6–8. A flask holds 10,000 cells, and 500 of them are resistant. Each day, the antibiotic kills 90% of the sensitive cells and none of the resistant cells. Then every surviving cell divides once.

6.

Day 1, after the antibiotic acts but before any cell divides. Round the percent to the nearest whole number.

Cells that survive: cells
Percent of the survivors that are resistant: %

7.

Next, every survivor from question 6 divides once. What percent of the cells are resistant after they divide?

A. About 68%, because the number of cells doubledB. About 34%, because both kinds of cells doubledC. About 17%, because the new cells are sensitiveD. 100%, because dividing makes every cell resistant

8.

Day 2 starts with the cells from question 7: 1,000 resistant cells and 1,900 sensitive cells. The antibiotic acts again. Round to the nearest whole percent.

After the antibiotic acts on day 2, % of the surviving cells are resistant.

9.

Sort each statement by whether it helps explain the change in Flask A or does not fit the page.

10.

Suppose that from day 6 on, Mr. Asante moves drops from Flask A into broth with no antibiotic. Use the Flask B results. What is the most likely result over the next few days?

A. The share would likely stay high, since without the antibiotic neither kind of cell gets an advantage.B. The cells would stop dividing without the antibiotic, so the flask would soon have no living cells.C. The share would drop back to about 2 per 1,000 the next day, since the cells forget their resistance.D. The share would jump to 1,000 per 1,000 the next day, since the cells no longer need to fight the drug.

11.

For her career-day report, Ximena wants a paragraph that answers: "Why did resistant bacteria take over Flask A but not Flask B?" Write a claim, use at least two calculations from this page as evidence, and explain your reasoning about survival and reproduction.