Microbiology — Eduqas A-Level Biology

Microbiology is the study of microorganisms, including bacteria, viruses, fungi, and protozoa. In WJEC A-Level Biology, this topic focuses on the structure, classification, growth, and control of microbes, with an emphasis on their roles in disease, biotechnology, and the environment. You'll explore how bacteria divide by binary fission, how viruses replicate inside host cells, and how we can control microbial growth using antibiotics, antiseptics, and sterilisation techniques. Understanding microbiology is crucial for grasping concepts in immunology, genetics, and ecology, and it has direct applications in medicine, food production, and industry.

This topic is particularly important because it links to real-world issues like antibiotic resistance, vaccine development, and the use of microorganisms in genetic engineering. You'll learn about the aseptic techniques used in labs to prevent contamination, the different phases of bacterial growth (lag, log, stationary, death), and how to calculate generation time. You'll also study the structure of viruses, including the lytic and lysogenic cycles of bacteriophages, and the differences between prokaryotic and eukaryotic cells. By the end, you should appreciate how microbes can be both harmful pathogens and beneficial tools in biotechnology.

Microbiology builds on your knowledge of cell structure and biochemistry. It also connects to topics like respiration (e.g., using yeast in anaerobic respiration), photosynthesis (e.g., cyanobacteria), and genetics (e.g., bacterial transformation). Mastering this topic will help you answer synoptic questions in the exam and prepare you for further study in biomedical sciences, pharmacy, or environmental biology.

• →Bacterial cell structure: prokaryotic cells lack a nucleus and membrane-bound organelles; they have a cell wall (peptidoglycan), plasma membrane, cytoplasm, ribosomes (70S), and sometimes a capsule, flagella, or plasmids.
• →Binary fission: bacteria reproduce asexually by replicating their DNA, elongating, and dividing into two identical daughter cells. Generation time can be as short as 20 minutes under optimal conditions.
• →Viral structure and replication: viruses are acellular, consisting of genetic material (DNA or RNA) inside a protein capsid. They replicate inside host cells via the lytic cycle (immediate lysis) or lysogenic cycle (viral DNA integrates into host genome).
• →Control of microbial growth: methods include physical (heat, radiation, filtration) and chemical (disinfectants, antiseptics, antibiotics). Antibiotics like penicillin inhibit cell wall synthesis; resistance arises via mutation or horizontal gene transfer.
• →Aseptic techniques: procedures to prevent contamination, such as sterilising equipment, using Bunsen burners to create convection currents, and working in a laminar flow hood.

• Classification of bacteria by shape and Gram stain reaction
• Understanding the relationship between Gram stain reaction and cell wall structure
• Principles and application of aseptic techniques
• Methods for culturing microorganisms in the laboratory
• Techniques for monitoring population growth including viable counts, serial dilutions, plating, and colony counting

• 💡Ensure you can describe the step-by-step process of the Gram stain technique
• 💡Practice calculations involving serial dilutions and colony forming units (CFU)
• 💡Be prepared to explain why specific aseptic techniques are necessary to prevent contamination
• 💡Understand how to interpret growth curves and the logarithmic nature of bacterial population growth
• 💡When describing bacterial growth curves, always label the four phases (lag, log, stationary, death) and explain what is happening at each stage. Mention that the log phase shows exponential growth because each bacterium divides by binary fission, and that the stationary phase occurs when nutrients are depleted or waste products accumulate.
• 💡For questions on antibiotic resistance, use specific examples like MRSA (methicillin-resistant Staphylococcus aureus) and explain how natural selection favours resistant bacteria. Mention that resistance genes can be transferred via plasmids (conjugation) or transduction (by bacteriophages).
• 💡When comparing prokaryotic and eukaryotic cells, be precise: prokaryotes have 70S ribosomes, no mitochondria, and a single circular chromosome; eukaryotes have 80S ribosomes, mitochondria, and linear chromosomes. This is a common exam question.

• Failure to maintain aseptic conditions leading to contamination
• Incorrect calculation of dilution factors in serial dilutions
• Confusing viable counts with total cell counts
• Misinterpreting the relationship between Gram stain results and cell wall composition
• Misconception: All bacteria are harmful. Correction: Many bacteria are beneficial, e.g., gut flora aids digestion, and bacteria are used in fermentation to produce yoghurt, cheese, and antibiotics.
• Misconception: Viruses are living organisms. Correction: Viruses are not considered living because they cannot reproduce or carry out metabolic processes outside a host cell; they are obligate intracellular parasites.
• Misconception: Antibiotics kill viruses. Correction: Antibiotics target bacterial structures like cell walls or ribosomes; they are ineffective against viruses. Antiviral drugs work differently, e.g., by inhibiting viral enzymes.