Antibiotics and infection control — Edexcel International A Level Biology
Bacteriostatic and bactericidal antibiotics, core practical 14, and how hospital-acquired infections shape antibiotic prescribing and hospital practice.
Revision notes
3 short notes, in the order of the specification. Each one in short:
Bactericidal antibiotics kill bacteria, for example penicillin, which prevents cell wall formation so the bacteria burst by osmosis. Bacteriostatic antibiotics stop bacteria growing and dividing, for example tetracyclines, which block protein synthesis at the ribosome. The immune system then removes the bacteria.
In core practical 14 a lawn of non-pathogenic bacteria is spread on agar and discs soaked in different antibiotics, with a control disc, are placed on it. After incubation the clear zones of inhibition are measured; larger zones mean the antibiotic is more effective.
A hospital-acquired infection is one that a patient gets while in hospital. Causes include vulnerable patients, close contact, invasive procedures, poor hygiene and overuse of antibiotics, which selects for resistant bacteria. Codes of practice limit antibiotic prescribing and set hygiene, isolation and cleaning rules.
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Quick check questions
What is the difference between bacteriostatic and bactericidal?
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Bacteriostatic antibiotics stop bacteria growing and dividing. Bactericidal antibiotics kill them.
Why do antibiotics not harm human cells?
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They show selective toxicity: they target structures that bacteria have and human cells lack, such as the peptidoglycan wall and 70S ribosomes.
What is a zone of inhibition?
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The clear area around an antibiotic disc where bacteria cannot grow.
Why is a control disc of sterile water used?
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To show that the disc and water do not themselves stop bacterial growth.
Give two contributory causes of hospital-acquired infections.
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Weakened patients; poor hygiene or contact between patients; use of invasive devices; overuse of antibiotics.
Frequently asked questions
What is the difference between bacteriostatic and bactericidal antibiotics?
Bactericidal antibiotics, such as penicillin, kill bacteria, often by stopping cell wall formation so the cell bursts. Bacteriostatic antibiotics, such as tetracycline, stop bacteria growing and dividing, for example by blocking ribosomes, and rely on the immune system to remove them. Neither type works on viruses.
Why do antibiotics not work against viruses?
Antibiotics target features of bacterial cells, such as the peptidoglycan cell wall and 70S ribosomes. Viruses have neither. They reproduce inside human cells using the host's machinery, so there is nothing for the antibiotic to attack, and it cannot reach the virus inside cells.
How do you measure the effect of an antibiotic on bacteria?
Spread a lawn of non-pathogenic bacteria on agar, place discs of different antibiotics and a control disc on it, and incubate at 25 °C. Measure the diameter or area of the clear zones of inhibition. A larger zone means the antibiotic is more effective against those bacteria.
Why do hospital-acquired infections happen?
Patients in hospital often have weakened immune systems, are in close contact, and have wounds, surgery or devices such as catheters that let pathogens in. Hands, equipment and surfaces spread bacteria. Overuse of antibiotics also selects for resistant strains such as MRSA.
How are hospital infections prevented?
Hospitals use hand washing with alcohol gel, isolation of infected patients, screening for MRSA, sterilised equipment, cleaning and protective clothing. Antibiotics are prescribed only when needed, using narrow-spectrum drugs and after sensitivity tests, to reduce selection for resistant bacteria such as MRSA.
Written and checked against the Edexcel International A Level Biology (YBI11) specification · Updated October 2026