Interpret selection and reproduction in bacterial count data
Use proportions and absolute counts to separate the two stages.
Exam scope and assessment notes
Mapped to the named 0973 section using teaching from the corresponding single-science course. A 0973-specific review of every Core and Supplement objective is still in progress. Written practice is self-checked; it does not demonstrate supervised laboratory competence or award an official Cambridge grade. This qualification reports two grades on the 9–1 scale; confirm the entry route for your zone and exam series.
Before you begin
- Distinguish bacteria from viruses; recall enzymes, chemical reactions, pathogens and reproduction of bacteria.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
Why does resistance frequency rise immediately after the stated exposure?
What you will learn
- Calculate resistance frequency before and after exposure
- Describe reproduction without inventing individual conversion
Interpret selection and reproduction in bacterial count data
| Stage | Susceptible | Resistant |
|---|---|---|
| Before exposure | 900 | 100 |
| After exposure | 0 | 100 |
| After three later doublings | 0 | 800 |
| Feature | Meaning |
|---|---|
| Initial | Total 1000; resistant 100 = 10% |
| After exposure | Total 100; resistant 100 = 100% |
| After reproduction | 100→200→400→800 resistant; eightfold increase |
| Comparison | Final total 20% below original; no hourly rate without time data |
Calculate resistance frequency before and after exposure
The invented population table begins with 900 susceptible and 100 resistant bacteria, so the resistant proportion is 100/1000 × 100 = 10%. After the specified antibiotic exposure, 0 susceptible and 100 resistant bacteria remain. The resistant count is unchanged at 100, but the resistant proportion among the 100 survivors is now 100%. This stage is differential survival, not reproduction.
In the next stated stage, the surviving resistant bacteria reproduce three times by doubling, with all descendants retaining the resistance trait in this simplified model. The sequence is 100→200→400→800 resistant bacteria. There are no susceptible descendants in this supplied branch. The final resistant proportion stays 100%, while its absolute count has increased eightfold from the survivor count.
Describe reproduction without inventing individual conversion
From the original total of 1000 to the final model count of 800, the total is 200 lower, a 20% decrease. That total decrease can coexist with a resistant-count increase from the initial 100 to 800. Different denominators answer different questions: total population change, resistant abundance and resistant frequency must be labelled separately.
The clean counts illustrate the selection-and-reproduction mechanism; they are not measured clinical growth rates or universal generation times. No time per doubling is supplied, so no bacteria-per-hour rate can be calculated. Resistance traits existed before exposure in the model, and it does not claim that antibiotics purposefully create those traits in every cell. Avoiding unnecessary exposure reduces avoidable opportunities for the selective first stage.
Sources for this lesson
Pause and explain
What count follows three later doublings from 100 survivors?
Worked example
Compare the resistant proportion and count before and immediately after exposure.
Show the worked solution
- Before: 100/1000 = 10%, count 100.
- After: 100/100 = 100%, count still 100.
Answer Resistance rises from 10% to 100% among survivors while the resistant count remains 100 during exposure.
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.
Calculate resistant percentages before exposure and among immediate survivors, with the resistant counts.
Give me a hint
Use each stage’s total.
Compare my reasoning
- Before: 100/(900+100) × 100 = 10%, count 100.
- After: 100/(0+100) × 100 = 100%, count 100.
Before 10% resistant, count 100; immediately after 100% resistant, count still 100.
Look for these in your work
- Use stage-specific denominators
- Distinguish frequency and count
Calculate resistant count after three doublings and its factor increase from survivors.
Give me a hint
Multiply by two at each doubling.
Compare my reasoning
- 100→200→400→800.
- 800/100 = 8.
800 resistant bacteria; eightfold increase from the survivor count.
Look for these in your work
- Apply three doublings
- Calculate the factor increase
Compare original and final totals and evaluate a claim of an exact hourly growth rate.
Give me a hint
The total starts at 1000 and ends at 800; no time per doubling is supplied.
Compare my reasoning
- Total decrease is 200, or 20% of 1000.
- The doubling count gives no hourly rate without elapsed time.
The final total is 200 lower, a 20% decrease from the original total. No exact hourly rate can be calculated because no doubling time or elapsed time is supplied.
Look for these in your work
- Calculate total change
- Reject the unsupported rate
Common mistakes
- Calling a proportion increase proof of immediate reproduction.
- Inventing a per-hour growth rate without time data.
What can you explain now?
What count follows three later doublings from 100 survivors?
Compare with the explanation
800 resistant bacteria
100 × 2 × 2 × 2 = 800.
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Make it stick
Calculate frequency, counts and total change while separating selection from reproduction and respecting missing time data.
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.