Antibiotics and hospital-acquired infections

Bactericidal and bacteriostatic

Antibiotics are chemicals that kill or inhibit bacteria. They do not work against viruses, which have no cell wall, ribosomes or metabolism of their own.

• Bactericidal antibiotics kill bacteria. For example, penicillin stops cross-links forming in the peptidoglycan cell wall, so the growing wall is weak and the cell bursts by osmotic lysis.
• Bacteriostatic antibiotics stop bacteria growing and dividing, for example tetracycline, which binds to bacterial 70S ribosomes and stops protein synthesis. The immune system then removes the bacteria.

Antibiotics target structures bacteria have but human cells lack (or have in a different form), such as peptidoglycan walls and 70S ribosomes.

Resistance

A random mutation may give a bacterium resistance. When an antibiotic is used, susceptible bacteria die but resistant ones survive and reproduce (natural selection), so the proportion of resistant bacteria rises. Resistance genes on plasmids can pass between bacteria, even of different species.

Examples: MRSA (meticillin-resistant *Staphylococcus aureus*) and *Clostridioides difficile* (C. difficile), which thrives when antibiotics have killed the normal gut bacteria.

Hospital-acquired infections

Infections caught in hospital are common because:

• many patients are ill, elderly or have weakened immune systems
• antibiotics are used a lot, selecting for resistance
• wounds, catheters and drips give bacteria a way in
• staff and patients are in close contact

Codes of practice to reduce them include: hand washing and alcohol gels; staff bare below the elbows; cleaning wards and sterilising equipment; isolating infected patients; screening patients for MRSA on admission; and prescribing antibiotics only when needed, choosing narrow-spectrum drugs where possible.

Worked example

Explain why using antibiotics often in a hospital increases the proportion of resistant bacteria.

1. Some bacteria carry a resistance allele from a random mutation.
2. The antibiotic kills susceptible bacteria; resistant ones survive.
3. Survivors reproduce and pass on the allele, with less competition.

Answer: Antibiotic use is a selection pressure favouring resistant bacteria.

Check yourself

1. What does a bactericidal antibiotic do?
   • Stops bacteria dividing without killing them
   • Kills bacteria
   • Kills viruses
   • Boosts antibody production
   Show answer

   Kills bacteria. -cidal means killing.

2. How does tetracycline act on bacteria?
   • It breaks down the cell membrane of viruses
   • It stops cell wall cross-links forming
   • It digests DNA
   • It binds to 70S ribosomes and stops protein synthesis
   Show answer

   It binds to 70S ribosomes and stops protein synthesis. Tetracycline is bacteriostatic, blocking protein synthesis.

3. Why does penicillin cause bacteria to burst?
   • It dissolves the nucleus
   • It weakens the cell wall, so water entering by osmosis bursts the cell
   • It makes the cytoplasm freeze
   • It blocks ribosomes
   Show answer

   It weakens the cell wall, so water entering by osmosis bursts the cell. Without strong cross-links the wall cannot resist osmotic pressure.

4. Why do antibiotics not cure viral infections?
   • Viruses are too large
   • Viruses are always resistant through plasmids
   • Viruses lack cell walls, ribosomes and their own metabolism
   • Antibiotics are broken down by the stomach
   Show answer

   Viruses lack cell walls, ribosomes and their own metabolism. Antibiotics target bacterial structures.

5. Which is a reason hospital-acquired infections are common?
   • Many patients have weakened immune systems
   • Hospitals use no antibiotics
   • Patients never have wounds
   • Staff never touch patients
   Show answer

   Many patients have weakened immune systems. Weak immunity plus antibiotic use and invasive procedures.

6. Which measure in a code of practice helps reduce MRSA in hospitals?
   • Prescribing broad-spectrum antibiotics for every patient
   • Wearing long sleeves and jewellery
   • Screening patients on admission and isolating carriers
   • Reducing hand washing to save time
   Show answer

   Screening patients on admission and isolating carriers. Screening and isolation stop spread.

7. How does antibiotic resistance first appear in a bacterial population?
   • A random mutation gives some bacteria resistance
   • The antibiotic causes bacteria to decide to become resistant
   • Bacteria learn resistance from the patient
   • Antibiotics create new genes directly
   Show answer

   A random mutation gives some bacteria resistance. Mutations are random; the antibiotic only selects.

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Sources

• Pearson Edexcel International Advanced Level in Biology specification (XBI11/YBI11, first teaching 2018)

Last updated 7 November 2026. Spotted a mistake? Tell us.