The structure of bacteria and viruses, how pathogens cause disease, the culture and growth of microorganisms, and the action and resistance of antibiotics.

What this dot point is asking

Edexcel wants you to describe the structure of bacteria and viruses, explain how pathogens cause disease, describe how microorganisms are cultured and how their growth is measured, and explain how antibiotics work and how resistance arises. Exponential-growth calculations and the natural-selection explanation of resistance are common exam tasks.

Bacteria and viruses

How pathogens cause disease

Culturing and growth

Microorganisms are grown on nutrient agar or broth using aseptic technique to avoid contamination (flaming the inoculating loop, working near a Bunsen flame, sealing plates). Plates are incubated at $25 \text{ °C}$ in school labs rather than body temperature to reduce the risk of culturing human pathogens. A population in fresh medium shows four phases of growth:

• Lag phase: little growth as cells adjust and make the enzymes needed.
• Exponential (log) phase: rapid doubling while resources are plentiful, following $N = N_0 \times 2^n$ where $n$ is the number of divisions.
• Stationary phase: the reproduction rate equals the death rate as nutrients run low and waste accumulates.
• Death (decline) phase: death rate exceeds reproduction as conditions deteriorate.

Growth can be measured by counting colonies (viable count) or by turbidity (how cloudy a broth is).

Antibiotics and resistance

Antibiotics kill bacteria (bactericidal) or stop them growing (bacteriostatic), for example by disrupting peptidoglycan cell-wall synthesis or inhibiting the bacterial 70S ribosome. Because human cells have no cell wall and use 80S ribosomes, this gives selective toxicity. Antibiotics do not affect viruses, which lack these structures and hide inside host cells. Antibiotic resistance arises by natural selection: a random mutation (or a resistance gene on a plasmid) makes some bacteria resistant; when antibiotics are used, the non-resistant bacteria die but the resistant ones survive and reproduce, so the resistance allele becomes more common and a resistant strain develops. Resistance genes can also be passed directly between bacteria on plasmids (horizontal gene transfer).

Examples in context

Example 1. MRSA in hospitals. Methicillin-resistant Staphylococcus aureus is a strain that survives many common antibiotics because resistance alleles were selected for by heavy antibiotic use. It spreads in hospitals where many patients are treated with antibiotics, creating strong selection pressure. Control relies on strict hygiene, isolating patients and using antibiotics only when needed, a direct application of natural-selection reasoning.

Example 2. Why colds are not treated with antibiotics. The common cold is caused by viruses, which antibiotics cannot affect because they have no bacterial cell wall or 70S ribosome to target and replicate inside host cells. Prescribing antibiotics for viral infections gives no benefit and adds to the selection pressure driving resistance, which is why doctors avoid it.

Try this

Q1. Explain why antibiotics do not work against viral infections. [2 marks]

• Cue. Antibiotics target bacterial structures such as the cell wall; viruses lack these and reproduce inside host cells.

Q2. Explain how the overuse of antibiotics leads to resistant bacteria. [3 marks]

• Cue. Random mutation makes some bacteria resistant; antibiotics kill the non-resistant ones; resistant bacteria survive, reproduce and pass on the resistance allele.