Antibiotics

Revise Antibiotics for Biology 4 (IAL) WBI14 (A2 Level) — revision notes and instant AI marking.

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Summary — What This Chapter Covers

What antibiotics are, and the difference between bactericidal (kill) and bacteriostatic (inhibit growth) antibiotics
The four main ways antibiotics attack bacteria: cell wall enzymes, ribosomes, cell membranes, and DNA coiling
Why antibiotics harm bacteria but not human cells (and don't touch viruses at all)
Core Practical 14 — testing antibiotic effectiveness using aseptic technique and the disc diffusion method
How to interpret "clear zones" on an agar plate
Hospital-acquired infections (HAIs) and how poor hygiene spreads them
How antibiotic resistance evolves by natural selection, using MRSA as the case study
Hospital strategies to slow resistance: narrow-spectrum antibiotics, no unnecessary prescriptions, rotating antibiotics

1. What Are Antibiotics?

When you get a bacterial infection, your doctor might prescribe an antibiotic — a chemical substance that damages bacterial cells while leaving your own (human) cells largely unharmed. The most famous one, penicillin, was discovered by Sir Alexander Fleming in 1928, almost by accident, when he noticed mould killing off bacteria in a petri dish he'd forgotten about.

Think of an antibiotic like a key that only fits bacterial locks. Bacterial cells (prokaryotic) are built differently from your cells (eukaryotic) — they have cell walls, different ribosomes, and different enzymes. Antibiotics are designed to exploit exactly those differences, which is why they can be so selective.

Bactericidal vs Bacteriostatic

Antibiotics fall into two broad categories based on what they actually do to bacteria:

Type	What it does	Key detail
Bactericidal	Actively kills bacterial cells	Bacteria die directly from the antibiotic's action
Bacteriostatic	Inhibits bacterial growth processes (doesn't necessarily kill)	At a high enough dose, bacteriostatic antibiotics will also cause bacterial death

Remember this distinction

"-cidal" = kills (like "suicide", "homicide"). "-static" = stays still/stops growing (think "static" = not moving). Bacteriostatic antibiotics essentially press pause on bacterial reproduction, giving your immune system time to mop up the infection — but push the dose high enough, and even these become lethal to the bacteria.

Practice Question 1

A student says "bacteriostatic antibiotics never kill bacteria." Explain why this statement is not entirely accurate.

2. How Antibiotics Attack Bacteria

Antibiotics work by interfering with bacterial growth or metabolism. There isn't just one mechanism — different antibiotics target different structures. Edexcel wants you to know four main routes of attack:

① Inhibiting cell wall enzymes (e.g. Penicillin)

Bacterial cell walls are built using specific enzymes that form bonds between wall components. Penicillin blocks these enzymes, so the cell can't properly build or repair its wall. As the bacterium tries to grow, water keeps entering the cell by osmosis (because the cytoplasm is more concentrated than the surroundings), and without a strong wall to resist that pressure, the cell literally bursts (lyses).