What you need to know
- 6.13bacteriostaticDescribes an antibiotic that stops bacteria growing and dividing, without killing them. versus bactericidalDescribes an antibiotic that kills bacteria. antibioticA substance that kills bacteria or stops them growing, without harming human cells. Antibiotics do not work on viruses..
Resistance and hospital codes are 6B 5. Zones on a plate are CP14. This page is the two definitions and why they matter. Not a prescribing list.
By the end, you should be able to…
- Define bacteriostatic and bactericidal.
- Link wall-weakening to lysis (cidal) and blocked protein synthesis to stalled growth (static).
- State why antibiotics do not treat viral infections.
Build your understanding
Stop growth, or kill
A bacteriostaticDescribes an antibiotic that stops bacteria growing and dividing, without killing them. antibioticA substance that kills bacteria or stops them growing, without harming human cells. Antibiotics do not work on viruses. inhibits bacterial growth and division but does not directly kill the cells. Numbers stay roughly constant; the host’s phagocyteA white blood cell, such as a neutrophil or macrophage, that engulfs and digests pathogens. and other defences must clear the living bacteria. A bactericidalDescribes an antibiotic that kills bacteria. antibiotic kills bacteria, for example by weakening the cell wallA rigid layer outside the cell membrane. In plants it is made mainly of cellulose; it supports the cell and resists bursting. so water enters by osmosisThe net movement of water molecules from a higher water potential to a lower water potential across a partially permeable membrane. and the cells lyse.
The names are about outcome for the bacterium, not about how expensive the tablet is. Trade names and doses are not 6.13.
A mechanism that matches the word
Blocking 70S protein synthesis can be bacteriostatic: without new proteins the cell cannot grow, but it may remain viable until immunity removes it. Preventing peptidoglycan cross-links can be bactericidal: the wall cannot hold the cell as it takes up water. Those are teaching examples of the distinction, not a full pharmacy catalogue.
In a closed culture with no immune system, a static drug holds viable countA count of living cells only, for example from colonies grown on agar.; a cidal drug makes viable count fall. In a person with very few phagocytes, a static drug may fail because living bacteria remain when the course stops.
Follow the process
Bacteriostatic: the drug blocks a growth process, for example 70S protein synthesis → the cell cannot divide → it stays alive until phagocytes clear it. Bactericidal: the drug weakens the wall, for example peptidoglycan cross-links → water enters by osmosis → the cell lyses. Viruses have neither a peptidoglycan wall nor their own 70S ribosomeA small organelle made of rRNA and protein, where translation takes place., so neither word applies to them.
Not for viruses
Antibiotics target bacterial structures such as cell walls or 70S ribosomes. Viruses have a capsidThe protein coat of a virus, made of capsomeres., not peptidoglycan, and they use the host’s ribosomes (6A 1). That is why antibiotics do not treat HIVHuman immunodeficiency virus: an RNA retrovirus that infects T helper cells and can lead to AIDS., EbolaAn enveloped RNA virus that causes Ebola virus disease, a severe haemorrhagic fever. or a viral cold — a 6.15 prescription code as well as a 6.13 fact.
Plant-extract zones in CP9 are not named antibiotics. CP14 uses antibiotic discs. Both need aseptic techniqueMethods that stop unwanted microorganisms contaminating a culture and stop cultures escaping, such as flaming loops and using sterile equipment.; only 6.13 asks static versus cidal.
NOT: neither action works on viruses. They have no peptidoglycan and no 70S ribosomes of their own.
- Static and cidal describe the outcome, not the price of the drug
- In a person with very few phagocytes, a static drug may fail
- In a closed culture, a static drug holds the viable count and a cidal drug lowers it
- Doses and trade names are not required
- Plant-extract zones are not these named antibiotic actions
Two traps this diagram blocks
- Do not treat bactericidal as a synonym for a stronger dose.
- Do not prescribe the mechanism to a virus.
From the diagram alone
Question
Explain the difference between a bacteriostatic and a bactericidal antibiotic, using one bacterial target for each. [4]
Show a model answer
A bacteriostatic antibiotic stops growth without directly killing the cell, for example by blocking protein synthesis on 70S ribosomes. Phagocytes then have to clear the living cells. A bactericidal antibiotic kills the cell, for example by preventing peptidoglycan cross-links so the wall fails and the cell lyses as water enters.
| Type | Effect on bacteria | What the host must still do |
|---|---|---|
| Bacteriostatic | Stops growth and division | Clear living cells (phagocytosis, etc.) |
| Bactericidal | Kills cells (e.g. lysis) | Remove debris; less reliant on killing them itself |
Make your reasoning visible
Build the explanation
X happens because Y, leading to Z. X: viable countA count of living cells only, for example from colonies grown on agar. falls with a bactericidalDescribes an antibiotic that kills bacteria. drug and only levels off with a bacteriostaticDescribes an antibiotic that stops bacteria growing and dividing, without killing them. one. Y: cidal treatment kills cells, while static treatment stops division and leaves them alive. Z: a host with very few phagocyteA white blood cell, such as a neutrophil or macrophage, that engulfs and digests pathogens. may not clear the survivors of a static drug. Incomplete: “antibioticA substance that kills bacteria or stops them growing, without harming human cells. Antibiotics do not work on viruses. kill all germs, including viruses.” Improved: “A bactericidal antibiotic can lyse bacteria by damaging the wall. A bacteriostatic one stops growth. Viruses have no peptidoglycan and no 70S ribosome, so neither works on them.”
Define both words
“Antibiotics kill germs” collapses the distinction.
Viruses lack the target
No wall, no 70S ribosome of their own.
Key explanations
- The host still matters. A static drug depends on phagocytes to remove the living bacteria.
- Viruses use host ribosomeA small organelle made of rRNA and protein, where translation takes place. and have a capsidThe protein coat of a virus, made of capsomeres. rather than peptidoglycan, so these antibiotic targets are absent.
Avoid this misconception
The claim: Stronger antibiotics are always bactericidal, and they work on viruses if you take enough.
The correction: Cidal versus static is a mode of action, not a strength slogan. Viral infections are not treated with antibiotics because the targets are bacterial.
Check your understanding
Try each question before opening the suggested answer.
Practice question
Retrieval and explanation · suggested answer
Define bacteriostatic and bactericidal antibiotics.
Show suggested answer
Bacteriostatic: inhibits growth and division without directly killing. Bactericidal: kills bacteria, for example by destroying the cell wall so they lyse.
Practice question
Retrieval and explanation · suggested answer
Why might a bacteriostatic drug fail if the immune system is very weak?
Show suggested answer
Living bacteria remain. Without phagocytes and other defences they are not cleared and can resume growth when the drug stops.
Practice question
Retrieval and explanation · suggested answer
Why do antibiotics not treat HIV?
Show suggested answer
HIV is a virus. Antibiotics target bacterial structures such as cell walls or 70S ribosomes, which viruses lack.
Take the idea further
Apply the same specification knowledge to an unfamiliar problem. Any extra assumptions are supplied in the question.
Stretch question
Fictional viable counts in broth: antibiotic X holds CFU constant; antibiotic Y makes CFU fall. Classify X and Y, then say why a large CP14 zone for X still does not prove it would cure a T-helper-depleted patient.
Show a hint
Static needs host killing.
Explore the reasoning
X is bacteriostatic; Y is bactericidal. A zone only shows the lawn did not grow near the disc. In a patient with almost no T helpers and poor phagocytosis, static X leaves living bacteria. Zone size also depends on diffusion, not just killing.
Recall this lesson
Lesson recall questions
Question 1 of 5: Define a bacteriostatic antibiotic.
Bacteriostatic vs bactericidal
Practice question
Retrieval and explanation · suggested answer
Define a bacteriostatic antibiotic.
Lesson: 6B 4: Antibiotics: Treating Bacterial Disease
Try this from memory first.
Put your biology to work
Explore where these ideas appear in the world. Enrichment, not extra required recall.
NHS antibiotics are not a virus medicine
6B · Antibiotics: Treating Bacterial DiseaseNHS information on antibiotics states they treat bacterial infections and do not work for viral colds. That public page matches 6.13’s viral-target point. It is not a list…Specification 6.13Read & use the scienceClose article
Why this matters
NHS information on antibiotics states they treat bacterial infections and do not work for viral colds. That public page matches 6.13’s viral-target point. It is not a list of drugs to memorise and not advice for anyone’s own prescription. The laboratory label describes how growth is affected under test conditions. It does not by itself predict which treatment is best for a person; infection site, organism and host factors matter too.
Back to the specification
Pearson Edexcel IAL Biology · Issue 2, February 2021 · Unit 4, pp. 29–32. The application extends beyond required recall.
- 6.13 understand the difference between bacteriostatic and bactericidal antibiotics
Use the science
Why is “take antibiotics for flu, just in case” a 6.13 and 6.15 error?
Explore a possible response
Influenza is viral, so the drug has no bacterial target, and unnecessary use selects resistance (6.15).
Explore the source
Killing bacteria is not the only useful outcome
6B · Antibiotics: Treating Bacterial DiseaseBactericidal drugs kill bacteria in a laboratory test; bacteriostatic drugs mainly stop growth under the tested conditions. A clinical review notes that the distinction can vary with organism,…Specification 6.13Read & use the scienceClose article
Why this matters
Bactericidal drugs kill bacteria in a laboratory test; bacteriostatic drugs mainly stop growth under the tested conditions. A clinical review notes that the distinction can vary with organism, concentration and test conditions. It is therefore poor reasoning to assume a drug labelled “cidal” must always be the best treatment. The immune system, infection site, dosing and resistance all affect outcomes. Students still need the clean specification definitions, but the clinical extension shows why a binary laboratory label is not a whole treatment decision. An antibiotic has a bacterial target and does not treat a viral infection merely because symptoms look severe.
Back to the specification
Pearson Edexcel IAL Biology · Issue 2, February 2021 · Unit 4, pp. 29–32. The application extends beyond required recall.
- 6.13 understand the difference between bacteriostatic and bactericidal antibiotics
Use the science
Why should a clinician not choose an antibiotic solely because a laboratory classifies it as bactericidal?
Explore a possible response
The label depends on testing conditions and does not include pathogen susceptibility in the patient, infection site, dose, resistance or host factors.
Explore the source
- Bactericidal versus bacteriostatic antibacterialsJournal of Antimicrobial Chemotherapy · Abstract