Explain selection by an antibiotic
A resistant fraction can rise even when the absolute resistant count falls.
Extended only (Supplement); Biology 0610, 2026–2028. Use the stated specimen features and evidence. Diagrams are schematic. Written practice is self-checked
Before you begin
- Use supplied invented counts only. Do not infer a real antibiotic response, give treatment advice or use this model as a culturing protocol.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
Which denominator describes resistant bacteria among survivors?
What you will learn
- Compare survival of resistant and susceptible bacteria
- Calculate fractions with the correct before-and-after denominators
Original selection schematic. Population counts and scenarios are supplied teaching models; they are not field observations.
| Stage or record | Meaning |
|---|---|
| Before | 10 resistant + 90 susceptible = 100 |
| Stipulated survivors | 8 resistant + 9 susceptible = 17 |
| Resistant share | 10% before; 8/17 ≈ 47.1% after |
| Absolute resistant count | 10 → 8, so it falls |
Compare survival of resistant and susceptible bacteria
In the natural-selection example, a population contains bacterial variants with different responses to a particular antibiotic. When the drug is present, susceptible bacteria may be killed or have growth inhibited, while resistant bacteria are more likely to survive. The antibiotic is a selection context; it does not give bacteria a goal-directed mutation.
Use original stipulated counts carefully. Suppose a population begins with 10 resistant and 90 susceptible bacteria, 100 total. After a stated exposure, 8 resistant and 9 susceptible bacteria are recorded. The resistant number falls from 10 to 8, yet its share of the recorded survivors rises from 10/100 to 8/17.
Calculate fractions with the correct before-and-after denominators
The calculations describe these invented counts only. The survivor group totals 17; 8/17 is about 47.1%. It is incorrect to say the resistant count increased. The absolute number decreased while the resistant fraction increased because the susceptible count fell more sharply in this example.
Real bacterial survival depends on species, strain, drug and conditions. The syllabus asks for the development of resistant strains as natural selection. Keep units and stages explicit, and do not convert this illustrative table into a treatment recommendation, dose, or practical protocol.
Pause and explain
If resistant bacteria fall from 10 to 8 while susceptible fall from 90 to 9, what happens?
Worked example
For the stipulated counts 10 resistant and 90 susceptible before exposure, then 8 resistant and 9 susceptible survivors, calculate resistant percentage before and after.
Show the worked solution
- Before: 10/(10+90). After: use the 17 survivors as denominator.
- Before = 10%; after = 8/17 × 100 ≈ 47.1%.
Answer The resistant fraction rises from 10% to about 47.1%, while the absolute resistant count falls from 10 to 8. The result follows from the stipulated counts, not a universal prediction.
Try a topic test
Choose Easy, Normal or Difficult. Save your answers and open worked review after finishing.
Test this topic →From guided practice to a new situation
Write your answer and reasoning first. Use a hint only if you are stuck. The model solution is for self-checking; your written work is not automatically marked.
Use 20 resistant and 180 susceptible before exposure; 12 resistant and 18 susceptible survive. Find the resistant percentages at both stages.
Give me a hint
Use 200 before and 30 after.
Compare my reasoning
- 20/200 × 100 = 10%.
- 12/30 × 100 = 40%.
The resistant fraction rises from 10% to 40%; the resistant count falls from 20 to 12 in these stipulated data.
Look for these in your work
- Correct stage denominators
- Count and fraction distinguished
In another invented record, resistant cells change from 6 to 5 and susceptible cells from 94 to 15. Does the resistant count rise? Does its share rise?
Give me a hint
Calculate both the absolute count and the two resistant fractions.
Compare my reasoning
- The count changes from 6 to 5, so it falls.
- The fractions are 6/100 = 6% and 5/20 = 25%.
The absolute resistant count falls, while its share rises from 6% to 25%.
Look for these in your work
- Count correctly compared
- Before/after fractions
A report gives only the after count: 8 resistant bacteria. Explain why a percentage cannot be calculated without another number.
Give me a hint
A proportion needs a total for the same stage.
Compare my reasoning
- The after group may include susceptible bacteria.
- The total after exposure is missing.
The denominator must include all bacteria counted after exposure; without that total, the resistant percentage is unknown.
Look for these in your work
- Stage denominator requirement
- No invented total
Common mistakes
- Using 100 as the after denominator
- Saying the resistant count increased
- Assuming percentages show allele frequencies
What can you explain now?
If resistant bacteria fall from 10 to 8 while susceptible fall from 90 to 9, what happens?
Compare with the explanation
The resistant count falls but its proportion rises
8 is below 10, while 8/17 exceeds 10/100.
After trying it yourself, choose your next review. This is your self-assessment.
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Make it stick
Without your notes, compare survival of resistant and susceptible bacteria and calculate fractions with the correct before-and-after denominators. Explain the mechanism, use the given population or breeding records and state what the evidence does not establish.
If you needed the solution, close it and solve the problem again from a blank page. A correct answer is stronger when you can explain the reason for each step.