Antibiotics and Antibiotic Resistance
BiologyΒ· Unit 10: Infectious DiseaseΒ· 45 min read
1. Mode of Action of Antibioticsβ β ββββ± 15 min
Antibiotics are antimicrobial compounds that kill or inhibit the growth of bacteria, without causing significant harm to the host organism. This effect is called selective toxicity, and it works because bacterial cells have key structural and functional differences from human eukaryotic cells.
Selective Toxicity
The ability of an antibiotic to target and kill/inhibit bacteria without damaging the host's human cells
Example:
Penicillin targets peptidoglycan cell wall synthesis, which human cells do not carry out.
Most antibiotics target one of four key bacterial-specific processes: 1. Cell wall synthesis, 2. Cell membrane structure and function, 3. Protein synthesis (translation), 4. Nucleic acid replication and transcription
Explain why penicillin is toxic to bacteria but does not harm human cells
- 1
Penicillin targets the synthesis of peptidoglycan, the main structural component of bacterial cell walls.
- 2
Human cells do not have a cell wall, and never produce peptidoglycan, so penicillin has no target in human cells.
- 3
When growing bacteria are exposed to penicillin, they cannot form new cell walls during binary fission, so they become osmotically unstable.
- 4
Water enters the bacterial cell by osmosis, causing cell lysis (breakdown), while human cells remain completely unaffected.
Exam tip:
CIE regularly asks for explanations of selective toxicity: always mention the specific target in bacteria and why it is absent in human cells to get full marks.
2. Evolution of Antibiotic Resistance by Natural Selectionβ β β βββ± 20 min
Antibiotic resistance can be innate (naturally present in a bacterial species) or acquired. Acquired resistance develops when a previously susceptible bacterial population gains resistance through genetic change, selected for by antibiotic exposure.
Acquired resistance evolves via standard natural selection, following these key steps:
Random spontaneous mutation creates a resistance allele in one bacterial cell
Antibiotic exposure kills all non-resistant bacterial cells
The resistant cell survives exposure and reproduces asexually by binary fission
All offspring inherit the resistance allele, so over time the entire population becomes resistant
Describe how a population of Staphylococcus aureus becomes methicillin-resistant (MRSA)
- 1
Random mutations occur spontaneously during DNA replication in S. aureus cells. One mutation creates an allele for methicillin resistance.
- 2
When methicillin is used to treat the infection, all non-resistant susceptible S. aureus cells are killed.
- 3
The mutant resistant cell survives methicillin exposure, because the drug cannot bind to its target enzyme.
- 4
The resistant cell reproduces asexually by binary fission, passing the resistance allele to all offspring. Over time, the entire population becomes resistant, forming MRSA.
3. Mechanisms of Antibiotic Resistanceβ β β βββ± 18 min
Bacteria use four main mechanisms to avoid the effect of antibiotics:
Enzymatic breakdown: Bacteria produce an enzyme that breaks down the antibiotic. Example: penicillinase (beta-lactamase) breaks down penicillin.
Reduced permeability: Bacteria modify their cell membrane structure to prevent the antibiotic from entering the cell.
Altered target site: The protein target that the antibiotic binds to is modified, so the antibiotic can no longer attach. Example: MRSA has an altered transpeptidase enzyme that penicillin cannot bind.
Efflux pumping: Bacteria use membrane proteins to pump the antibiotic out of the cell before it can act.
Resistance genes are often carried on plasmids (small circular DNA separate from the bacterial chromosome). This allows resistance genes to spread between different bacterial species via horizontal gene transfer through conjugation, which contributes to rapid spread of multi-drug resistance.
Explain how penicillinase causes penicillin resistance
- 1
Some bacteria carry the gene that codes for the enzyme penicillinase (beta-lactamase), often on a plasmid.
- 2
When penicillin enters the bacterial cell, penicillinase breaks the beta-lactam ring of the penicillin molecule.
- 3
The broken beta-lactam ring means penicillin can no longer inhibit cell wall synthesis, so the drug becomes inactive.
- 4
The inactive penicillin cannot harm the bacteria, so the cell survives, and the resistance gene can be passed to other bacteria via conjugation.
4. Causes, Consequences and Preventionβ β ββββ± 15 min
Human overuse and misuse of antibiotics is the primary driver of the rapid increase in antibiotic resistance globally. Common misuses include:
Prescribing antibiotics for viral infections (which antibiotics cannot treat)
Overuse of antibiotics in agriculture for growth promotion and preventative disease control in livestock
Patients stopping their antibiotic course early before all bacteria are killed
Overuse of broad-spectrum antibiotics when narrow-spectrum would be sufficient
Consequences of widespread antibiotic resistance include increased mortality from common treatable infections, higher healthcare costs, and increased risk of infection during surgeries and chemotherapy, which rely on prophylactic antibiotics. Prevention strategies focus on reducing misuse of existing antibiotics and developing new antimicrobial drugs.
Explain why stopping an antibiotic course early promotes antibiotic resistance
- 1
At the start of an antibiotic course, the most susceptible bacteria are killed first.
- 2
Partially resistant bacteria survive longer, because they are less affected by the drug.
- 3
If the course is stopped early, the antibiotic concentration drops, and these partially resistant bacteria survive, reproduce, and pass on their resistance alleles.
- 4
Over repeated exposure, the bacterial population becomes fully resistant to the antibiotic.
Exam tip:
You may be asked to outline prevention strategies for antibiotic resistance: remember the key points of reducing agricultural use, avoiding prescriptions for viral infections, and completing full courses.
5. Common Pitfalls
Wrong move:
Claiming antibiotics work against viruses because they are infectious agents
Why:
Antibiotics only target bacterial-specific structures/processes that viruses do not have
Correct move:
Antibiotics only kill/inhibit bacteria, they have no effect on viruses
Wrong move:
Stating antibiotics cause mutations that create resistance in bacteria
Why:
Mutations are random and spontaneous, occurring before antibiotic exposure. Antibiotics do not induce resistance mutations
Correct move:
Antibiotics select for existing random resistance mutations that already exist in the bacterial population
Wrong move:
Confusing innate and acquired antibiotic resistance
Why:
The two have different origins, and CIE examiners test the distinction
Correct move:
Innate resistance is naturally present in a bacterial species, while acquired resistance develops in a previously susceptible population
Wrong move:
Forgetting that resistance can spread between different bacterial species
Why:
Many students only think of resistance passing through vertical asexual reproduction, not horizontal transfer
Correct move:
Resistance genes on plasmids can spread between different species via horizontal gene transfer
Wrong move:
Claiming antibiotic resistance is only a problem for people who take antibiotics regularly
Why:
Resistant bacteria can spread between people, just like any other pathogen
Correct move:
Antibiotic resistant bacteria can cause outbreaks in hospitals and communities, affecting anyone
6. Quick Reference Cheatsheet
Concept | Key CIE Exam Fact |
|---|---|
Selective Toxicity | Antibiotics target bacterial structures absent in human cells |
4 Resistance Mechanisms | Breakdown, reduced permeability, altered target, efflux pumping |
Evolution of Resistance | Random mutation β antibiotic selection β resistant population |
Top 3 Causes of Resistance | Overuse, incomplete courses, agricultural overuse |
Common Exam Example | MRSA = methicillin-resistant Staphylococcus aureus |
7. Frequently Asked
Why don't antibiotics work against viruses?
Antibiotics target bacterial-specific structures (e.g. peptidoglycan cell walls, bacterial ribosomes) that viruses do not have. Viruses use host cell machinery for replication, so there is no bacterial-specific target for antibiotics to attack.
Do antibiotics cause resistance mutations in bacteria?
No. Mutations that cause resistance are random and occur spontaneously before exposure to antibiotics. Antibiotics only select for existing resistant bacteria, increasing the frequency of resistance in the population.
When this came up on past exams
AI-estimated based on syllabus patterns β cross-check with official past papers for accuracy. Use only as revision-focus signals.
- 2022 Β· 2
Discuss spread of MRSA resistance
- 2021 Β· 1
Mode of antibiotic action MCQ
- 2023 Β· 4
Evolution of antibiotic resistance
Going deeper
What's Next
Antibiotics and antibiotic resistance is a core applied topic that connects directly to natural selection (the foundation of evolution) and the study of infectious disease. This topic is regularly assessed in both multiple choice and extended response questions, so mastering the role of natural selection and the different mechanisms of resistance is critical for exam success. Understanding antibiotic resistance also gives context to modern issues in public health and biotechnology, where researchers are working to develop new antimicrobial treatments to combat drug-resistant superbugs.