10.2.110-11 · Infectious Disease and Immunity

Penicillin — Cambridge AS & A Level Biology 9700 Revision Notes & Topical Questions

5 min read 9 parts Lesson 6 of 21 1 quiz

📚 Cambridge AS & A Level Biology 9700 Hub / Topical Revision

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Lesson summary

10.2.1 Penicillin — CAMBRIDGE AS & A LEVEL BIOLOGY 9700 — REVISION NOTES notes: Explain how penicillin kills growing bacteria but not viruses or human cells, and describe how antibiotic sensitivity is tested.Definition — an antibiotic kills or inhibits the growth of bacteria with…

Bactericidal
Bacteriostatic
Selective toxicity
Fleming (1928)
Florey and Chain
Mass production

Part 1 of 7What antibiotics are

Definition

Definition an antibiotic kills or inhibits the growth of bacteria without harming the host's cells.

Origin

Origin many were first found as chemicals made by fungi or other bacteria competing for resources.
Penicillin is made by fungi of the genus Penicillium.
Other antibiotics are made synthetically.

Bactericidal

Bactericidal it kills bacteria.

Bacteriostatic

Bacteriostatic it stops bacteria multiplying and leaves the immune system to clear the infection.

Selective toxicity

Selective toxicity antibiotics target features found in bacteria but not in human cells.

Targets

Targets synthesis of cell walls, proteins in the cell surface membrane, bacterial enzymes, DNA synthesis and protein synthesis.

Part 2 of 7Discovery and development

1Fleming (1928)

Penicillium mould on a plate of Staphylococcus had a clear zone around it where bacteria could not grow.

2Florey and Chain

Florey and Chain they purified penicillin and developed it for patients in the 1940s, testing it first on mice and then on patients.

3Mass production

Mass production it transformed the outlook for pneumonia, wound infections and syphilis in the Second World War.

4Semi-synthetic penicillins

Semi-synthetic penicillins modified side chains resist breakdown by penicillinase or can be taken by mouth.

5Dose

Dose it must be high enough to inhibit bacteria in blood and tissues but low enough to avoid harming the patient.

6Allergy

Allergy some people are allergic to penicillin and need alternative drugs.

Part 3 of 7How penicillin works

Cell wall

Cell wall bacteria are surrounded by a wall of peptidoglycan, sugar polymers cross-linked by short peptide chains.
The wall is strong and prevents the cell bursting.

Osmosis

Osmosis the cytoplasm has a lower water potential than the surroundings, so water tends to enter and creates pressure the wall must withstand.

Growth

Growth autolysins make small holes in the wall so it can stretch, and transpeptidases form new cross-links.

Penicillin

Penicillin it binds to the active site of transpeptidases and stops cross-link formation.

Result

Result the autolysins keep making holes, so the wall becomes weaker, takes in water and bursts (osmotic lysis).

Growing cells only

Growing cells only once growth is complete there are no new holes or cross-links, so penicillin acts mainly on growing bacteria.

Part 4 of 7Effect of penicillin on different organisms

Organism	Effect of penicillin	Reason
Growing bacteria	Killed	Peptidoglycan wall disrupted
Human cells	Not harmed	No cell wall and no peptidoglycan
Viruses	No effect	Not cells; no wall or metabolism of their own, and they use host machinery for transcription and translation
Fungi	No effect	Walls made of chitin, not peptidoglycan

Gram-negative bacteria

Gram-negative bacteria an outer membrane keeps penicillin from the wall, so they are less sensitive.

Spectrum

Spectrum broad-spectrum antibiotics act on many types of bacteria, and narrow-spectrum ones act on few.

Other modes

Other modes some inhibit protein synthesis on bacterial 70S ribosomes or DNA replication.
- (human ribosomes are 80S)

Viral infections

Viral infections cholera and TB can be treated with antibiotics, but HIV cannot.

Part 5 of 7Testing antibiotic sensitivity

Spread a bacterial lawn on nutrient agar using aseptic technique.

Place paper discs soaked in different antibiotics on the surface.

Include a control disc soaked in sterile water.

Secure the lid with tape without fully sealing it, to avoid anaerobic conditions.

Incubate at about 25 to 30 °C; school plates stay at or below about 30 °C because 37 °C favours human pathogens.

Measure the clear zone of inhibition around each disc.

Calculate the area using πr², a better indicator than diameter.

Sterilise the plates after the experiment.

9Interpretation

Interpretation a larger zone means a more effective antibiotic, assuming equal concentration and diffusion.

10Precautions

Precautions sterilise equipment and work close to a flame.

Part 6 of 7More from the specification

Part 7 of 7More on this topic

Viruses
- they are not cells, so they cannot be targeted as bacteria are.
- A virus uses host mechanisms for transcription and translation, so these cannot be targeted without harming the host.

Key terms

Term	Meaning
Selective toxicity	Harming bacteria without harming host cells.
Bactericidal	Killing bacteria.
Bacteriostatic	Stopping bacteria multiplying.
Autolysin	A bacterial enzyme that makes holes in the wall to allow growth.
Transpeptidase	An enzyme that forms peptide cross-links in peptidoglycan.
Broad-spectrum antibiotic	An antibiotic that acts on many types of bacteria.

Avoid these mistakes

Saying penicillin kills all bacteria equally — it acts on growing bacteria, and some are less sensitive.

Saying penicillin works on viruses — viruses are not cells and cannot be targeted.

Saying penicillin stops bacteria making any wall — it stops the cross-links so the wall weakens.

Sealing agar plates completely — this risks anaerobic conditions and growth of dangerous pathogens.

Last updated: 18 October 2026