Edexcel A Level Biology B Topic 6: Microbiology and Pathogens Exam Questions

About Edexcel A Level Biology B Topic 6: Microbiology and Pathogens

Topic 6 of Edexcel A Level Biology B covers how microorganisms are cultured and how they cause disease. You learn aseptic technique, how to culture bacteria and measure their growth using serial dilutions, cell counts and turbidity, and the phases of a bacterial growth curve. The topic covers bacteria as pathogens, including toxins and tuberculosis, and other pathogenic agents such as viruses, fungi and protoctists. You then study the body's response to infection, both non-specific and specific, how antibiotics work and how resistance arises. Endemic diseases such as malaria are included. Practical questions on calculating population size from dilution plating are common.

What you need to know

• Microbial techniques Aseptic technique prevents contamination. Bacteria are grown on agar or in broth. Population size is estimated by serial dilution and plate counts, haemocytometer counts or turbidity measured with a colorimeter.
• Growth curves Bacterial growth shows a lag phase, log (exponential) phase, stationary phase and death phase. Exponential growth can be plotted on a log scale.
• Bacteria as pathogens Bacteria damage cells by releasing exotoxins or endotoxins. Mycobacterium tuberculosis infects lung macrophages and can remain inactive inside granulomas.
• Other pathogens Viruses, fungi and protoctists such as Plasmodium (malaria) also cause disease. Malaria is endemic in some regions, where the disease is always present in the population.
• Response to infection Non-specific defences include skin, inflammation, lysozyme, interferon and phagocytosis. Specific responses involve T helper, T killer and B cells, plasma cells, antibodies and memory cells.
• Antibiotics and resistance Antibiotics act on bacterial targets such as cell wall synthesis or 70S ribosomes; they can be bactericidal or bacteriostatic. Resistance arises by random mutation and spreads by selection and plasmid transfer.

Common mistakes in Topic 6: Microbiology and Pathogens

• Forgetting to multiply by the dilution factor when calculating cell numbers from a plate count.
• Saying bacteria grow fastest in the stationary phase; growth is fastest in the log phase.
• Describing endemic disease as a sudden outbreak; endemic means always present in a region.
• Saying antibiotics work against viruses; viruses lack the targets that antibiotics act on.
• Writing that bacteria develop resistance in response to the antibiotic; resistant mutants already exist and are selected.

Try three Topic 6: Microbiology and Pathogens exam-style questions