Calculate survival in a model antibiotic exposure
Connect resistant survivors with incomplete elimination using counts.
Section B14.1 · Drugs
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
What remains after the stated model exposure?
What you will learn
- Calculate survivors and the killed proportion
- Keep model outcomes separate from clinical thresholds
Calculate survival in a model antibiotic exposure
Invented model, no reproduction during exposure: 900 susceptible plus 100 resistant initially; 0 susceptible plus 100 resistant afterwards. Survival 10%; survivors all resistant. Original teaching model.
| Stage | Susceptible | Resistant |
|---|---|---|
| Before exposure | 900 | 100 |
| After exposure | 0 | 100 |
| Feature | Meaning |
|---|---|
| Starting counts | 900 susceptible + 100 resistant = 1000 |
| After exposure | 0 susceptible + 100 resistant = 100 |
| Starting denominator | 10% survive; 90% killed |
| Survivor denominator | 100% of survivors resistant; not complete elimination |
Calculate survivors and the killed proportion
The invented exposure table begins with 900 bacteria susceptible to antibiotic A and 100 resistant to A, a total of 1000. After the stated exposure, all 900 susceptible bacteria are killed and all 100 resistant bacteria survive in this deliberately simplified model. No reproduction occurs during the interval, so before-and-after counts can be compared directly.
The total remaining count is 100, giving a survival proportion of 100/1000 × 100 = 10%. The count killed is 900, or 90% of the starting total. A large percentage decrease therefore does not mean that every bacterium has been eliminated. Identify the remaining resistant bacteria rather than calling a 90% decrease complete clearance.
Keep model outcomes separate from clinical thresholds
The remaining population in the supplied model consists entirely of resistant bacteria: 100 out of 100 survivors, or 100%. That survivor composition is a different denominator from the 10% of the starting population that survived. Label the denominator carefully. The 100% value does not mean that the antibiotic caused every original susceptible bacterium to become resistant.
These are invented counts with unusually clean survival outcomes for reasoning practice, not actual clinical results or a safe-effectiveness threshold. Real outcomes depend on bacterial strain, drug, exposure conditions and other factors. The model shows how resistant survivors can limit elimination; it does not establish a personal medication choice, treatment duration or dose.
Sources for this lesson
Pause and explain
What denominator gives the survival percentage?
Worked example
Calculate killed and surviving percentages for 1000 starting and 100 surviving bacteria.
Show the worked solution
- Survival is 100/1000 = 10%.
- Killed proportion is 900/1000 = 90%.
Answer Survived 10%; killed 90%. Resistant bacteria remain, so elimination is incomplete.
Try a topic test
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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 starting total, survivors and killed count.
Give me a hint
Add the starting categories, then compare the final total.
Compare my reasoning
- 900+100 = 1000 initially; 100 survive.
- 1000−100 = 900 are killed.
Starting total 1000; survivors 100; killed count 900.
Look for these in your work
- Use both starting categories
- Calculate surviving and killed totals
Calculate survival percentage and the resistant percentage among survivors.
Give me a hint
Use different denominators for the two questions.
Compare my reasoning
- Survival: 100/1000 × 100 = 10%.
- Resistant among survivors: 100/100 × 100 = 100%.
Survival is 10% of the starting population; resistance is 100% among the survivors.
Look for these in your work
- Label the two denominators
- Calculate both percentages
Evaluate: killing 90% proves complete elimination and means every original bacterium became resistant.
Give me a hint
Use the remaining count and identity.
Compare my reasoning
- 100 resistant bacteria remain, so elimination is incomplete.
- The susceptible bacteria were killed, not converted into resistant bacteria.
Elimination is incomplete because 100 resistant bacteria remain. The supplied model kills susceptible bacteria rather than converting every original bacterium into a resistant one.
Look for these in your work
- Use the survivor count
- Correct the conversion claim
Common mistakes
- Calling a large reduction complete elimination.
- Using survivor composition as the survival percentage of the starting population.
What can you explain now?
What denominator gives the survival percentage?
Compare with the explanation
The starting total of 1000
Survivors are compared with the starting population.
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Make it stick
Calculate both denominators and explain why resistant survivors mean incomplete elimination.
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.