📚 Microorganisms: Essential WJEC A-Level Revision | 微生物：WJEC A-Level 考点精讲
Microorganisms are a central topic in WJEC A-Level Biology, spanning fundamental principles of structure, infectious disease, industrial applications and experimental techniques. A thorough grasp of microbial classification, growth kinetics and interactions with the human body is essential for success in both written papers and practical assessments. This revision guide breaks down the key concepts, model answers and common pitfalls to help you master the topic.
微生物是 WJEC A-Level 生物学的核心主题，涵盖结构原理、传染病、工业应用和实验技术等多个方面。牢牢掌握微生物的分类、生长动力学及其与人体之间的相互作用，对于在笔试和实践考核中取得好成绩至关重要。这份考点精讲将关键概念、典型答案和常见错误逐一分解，助你彻底攻克这一专题。
1. Classification and Structure of Microorganisms | 微生物的分类与结构
WJEC requires you to distinguish between the three major groups: bacteria, viruses and fungi. Bacteria are prokaryotic cells lacking membrane-bound organelles, with a circular DNA molecule, 70S ribosomes and a peptidoglycan cell wall. Viruses are acellular, consisting of a nucleic acid core (DNA or RNA) surrounded by a protein capsid, and some have a lipid envelope. Fungi are eukaryotic; yeasts are single-celled whilst moulds form hyphae and mycelium, and their cell walls contain chitin.
WJEC 要求你区分三大类微生物：细菌、病毒和真菌。细菌是原核细胞，没有膜包围的细胞器，含有环状 DNA、70S 核糖体和肽聚糖细胞壁。病毒是非细胞结构，仅由核酸核心（DNA 或 RNA）和蛋白质衣壳组成，有些病毒还具有脂质包膜。真菌属于真核生物；酵母是单细胞的，而霉菌形成菌丝和菌丝体，其细胞壁中含有几丁质。
Always link structure to function. For example, the bacterial capsule helps evade phagocytosis, while the fungal chitin wall provides rigidity. Viruses lack ribosomes and enzymes for metabolism, so they are obligate intracellular parasites.
务必把结构与功能联系起来。例如，细菌荚膜有助于逃避吞噬作用，而真菌的几丁质细胞壁提供刚性。病毒缺乏核糖体和代谢酶，因此是专性胞内寄生物。
| Feature | Bacteria | Viruses | Fungi |
|---|---|---|---|
| Cell type | Prokaryotic | Acellular | Eukaryotic |
| Genetic material | Circular DNA + plasmids | DNA or RNA | Linear DNA in nucleus |
| Cell wall | Peptidoglycan | Absent | Chitin |
| Ribosomes | 70S | None | 80S |
| Reproduction | Binary fission | Within host cell | Budding / spores |
2. Viral Replication Cycles | 病毒复制周期
WJEC expects you to describe the lytic cycle of a bacteriophage and the replication of an enveloped animal virus such as HIV. In the lytic cycle, the phage attaches to a bacterial cell, injects its DNA, and the host machinery transcribes viral genes and replicates viral DNA. New phage particles are assembled and released via lysis. HIV, a retrovirus, attaches to CD4 receptors on T-helper cells, releases its RNA and reverse transcriptase into the cell, synthesises viral DNA from RNA, and integrates this DNA into the host genome. The provirus is then transcribed to produce new viral RNA and proteins, and new viruses bud from the cell, acquiring an envelope.
WJEC 要求你描述噬菌体的裂解周期以及 HIV 等有包膜动物病毒的复制过程。在裂解周期中，噬菌体附着于细菌细胞，注入其 DNA，宿主机器转录病毒基因并复制病毒 DNA。新的噬菌体颗粒组装并通过裂解释放。HIV 是一种逆转录病毒，它附着于辅助 T 细胞上的 CD4 受体，将 RNA 和逆转录酶释放到细胞中，以 RNA 为模板合成病毒 DNA，并将此 DNA 整合到宿主基因组中。随后前病毒被转录产生新的病毒 RNA 和蛋白质，新病毒以出芽方式从细胞中释放，并获得包膜。
The key enzyme to remember is reverse transcriptase. Also note that the lysogenic cycle, where the viral DNA integrates without immediate lysis, can be referenced for comparison. Exam questions often ask how the structure of HIV relates to its replication – for example, the envelope glycoproteins gp120 and gp41 are essential for attachment and fusion.
需要记住的关键酶是逆转录酶。还要注意溶原周期，即病毒 DNA 整合但不立即裂解，可用于对比。试题经常问到 HIV 的结构如何与其复制相关——例如，包膜糖蛋白 gp120 和 gp41 对附着和融合至关重要。
3. Bacterial Growth and the Growth Curve | 细菌生长与生长曲线
In a closed batch culture, bacterial population growth follows a characteristic curve with four phases: lag phase (cells adapt, synthesise enzymes, no increase in number), exponential (log) phase (cells divide at maximum rate under constant conditions), stationary phase (carrying capacity reached, growth rate equals death rate due to nutrient depletion and waste accumulation), and death phase (cells die at an exponential rate). The growth rate constant (k) and mean generation time (g) can be calculated using:
在封闭分批培养中，细菌种群增长遵循由四个时期构成的典型曲线：迟缓期（细胞适应，合成酶，数目不增）、指数（对数）期（在恒定条件下以最大速率分裂）、稳定期（达到环境容纳量，因营养耗尽和废物积累，生长速率等于死亡速率）和死亡期（细胞呈指数死亡）。生长速率常数 (k) 和平均代时 (g) 可用以下公式计算：
log₁₀ N = log₁₀ N₀ + (k t) / 2.303
g = 0.301 / k
where N₀ is the initial cell number, N is the number after time t, and k is the growth rate constant. WJEC practical work often involves viable count (colony-forming units) and turbidity measurements to plot growth curves.
其中 N₀ 为初始细胞数，N 为经过时间 t 后的细胞数，k 为生长速率常数。WJEC 实验工作常涉及活菌计数（菌落形成单位）和浊度测量来绘制生长曲线。
It is important to explain why the exponential phase is used to calculate generation time, and to link the stationary phase to secondary metabolite production (e.g. antibiotics). Ensure you can describe aseptic sampling and serial dilution for viable cell counting.
解释为何用指数期计算代时、并将稳定期与次级代谢产物（如抗生素）的产生联系起来很重要。确保你能描述无菌取样和用于活细胞计数的系列稀释方法。
4. Aseptic Techniques and Culturing | 无菌技术与培养
Mastering aseptic technique is essential for both practical exams and theory questions. Key steps include: flaming the inoculating loop until red–hot before and after use, working near a Bunsen burner to create an updraft, flaming the neck of the culture tube, lifting the Petri dish lid only slightly to avoid airborne contamination, and disposing of all cultures by autoclaving. For enumeration, you may carry out serial dilutions and spread 0.1 cm³ of a suitable dilution onto an agar plate, incubating at an appropriate temperature (e.g. 25 °C for school labs to avoid growth of human pathogens).
掌握无菌技术对实践考试和理论题都至关重要。关键步骤包括：使用前后将接种环在火焰上烧至红热，靠近本生灯工作以产生上升气流，灼烧培养管管口，只将培养皿盖稍微抬起以避免空气污染，并通过高压灭菌处理所有培养物。进行计数时，你可以进行系列稀释，将 0.1 cm³ 的合适稀释液涂布到琼脂平板上，在适当温度下培养（如学校实验室用 25 °C 以避免人类病原体生长）。
WJEC often asks why nutrient agar must be sterilised before use (to kill any contaminating microorganisms) and why plates are incubated upside down (to prevent condensation dripping onto the colonies). The difference between selective media and differential media may also be tested.
WJEC 常问为何营养琼脂必须在使用前灭菌（杀灭所有污染的微生物）以及为何平板需倒置培养（防止冷凝水滴落到菌落上）。选择性培养基与鉴别培养基的区别也可能被考查。
5. Microbial Roles in Food Production | 微生物在食品生产中的作用
WJEC expects detailed knowledge of yogurt production and bread making, linking microbiology to the process. For yogurt, milk is pasteurised (to kill unwanted bacteria) and then cooled to around 40–45 °C before adding a starter culture of Lactobacillus bulgaricus and Streptococcus thermophilus. The bacteria ferment lactose to lactic acid, lowering the pH, which denatures milk proteins (casein) and causes coagulation to form a gel. The acidic environment also inhibits spoilage organisms. In bread making, Saccharomyces cerevisiae (baker’s yeast) respires aerobically to produce CO₂, causing the dough to rise; any ethanol produced during brief anaerobic periods evaporates during baking.
WJEC 要求详细了解酸奶生产和面包制作，将微生物学与工艺联系起来。制作酸奶时，牛奶先经巴氏消毒（杀灭有害菌），再冷却至 40–45 °C 左右，然后加入含有 保加利亚乳杆菌 和 嗜热链球菌 的发酵剂。这些细菌将乳糖发酵为乳酸，降低 pH，使乳蛋白（酪蛋白）变性并凝固形成凝胶。酸性环境还能抑制腐败菌。在面包制作中，酿酒酵母 进行有氧呼吸产生 CO₂，使面团膨胀；短暂厌氧阶段产生的乙醇在烘烤时蒸发。
You should be able to explain the importance of controlling temperature and pH, and name the substrates and products of fermentation. Alcoholic fermentation by yeast to produce ethanol in brewing is another frequently examined example, where S. cerevisiae converts glucose to ethanol and CO₂ under anaerobic conditions.
你要能解释控制温度和 pH 的重要性，并说出发酵的底物和产物。酵母通过酒精发酵生产乙醇（酿造）是另一个常考例子，其中 酿酒酵母 在厌氧条件下将葡萄糖转化为乙醇和 CO₂。
6. Antibiotics and Mechanisms of Action | 抗生素及其作用机制
Antibiotics are chemical substances produced by microorganisms (or synthesised) that inhibit or kill other microbes. WJEC focuses on two major targets: the bacterial cell wall and protein synthesis. Penicillin is a β-lactam antibiotic that irreversibly inhibits the transpeptidase enzyme responsible for cross-linking peptidoglycan chains, leading to a weakened cell wall and osmotic lysis – it is bactericidal. Tetracycline binds to the 30S ribosomal subunit, blocking tRNA attachment and thus preventing protein synthesis; it is bacteriostatic. The selective toxicity lies in the presence of peptidoglycan only in bacteria, not human cells, and differences in ribosomal structure (70S vs 80S).
抗生素是由微生物产生（或人工合成）的可抑制或杀灭其他微生物的化学物质。WJEC 重点关注两大靶点：细菌细胞壁和蛋白质合成。青霉素是一种 β-内酰胺类抗生素，能不可逆地抑制转肽酶，该酶负责交联肽聚糖链，结果导致细胞壁变弱及渗透性裂解——青霉素是杀菌性的。四环素与 30S 核糖体亚基结合，阻止 tRNA 的附着，从而抑制蛋白质合成；它是抑菌性的。选择性毒性在于仅细菌具有肽聚糖，而人体细胞没有，以及核糖体结构差异（70S 与 80S）。
You must distinguish bactericidal from bacteriostatic. Spectrum of activity is also relevant: broad-spectrum antibiotics such as tetracycline act against a wide range of Gram-positive and Gram-negative bacteria, whilst narrow-spectrum antibiotics such as penicillin G act mainly against Gram-positives. The Gram staining procedure (crystal violet, iodine, alcohol, safranin) is linked to differences in cell wall thickness and chemistry.
你必须区分杀菌性和抑菌性。抗菌谱也很重要：四环素等广谱抗生素可对抗多种革兰氏阳性及阴性菌，而青霉素 G 等窄谱抗生素主要针对革兰氏阳性菌。革兰氏染色步骤（结晶紫、碘液、酒精、番红）与细胞壁厚度和化学组成的差异有关。
7. Antibiotic Resistance | 抗生素耐药性
Antibiotic resistance is a major theme in WJEC. Bacteria become resistant through spontaneous mutations or by acquiring resistance genes via horizontal gene transfer (conjugation, transformation, transduction). Resistance mechanisms include production of enzymes that inactivate the drug (e.g. β-lactamase breaks the β-lactam ring of penicillin), alteration of the target site (e.g. mutated ribosomal protein prevents tetracycline binding), reduced drug uptake or increased efflux pumps that expel the antibiotic from the cell.
抗生素耐药性是 WJEC 的一个重要主题。细菌通过自发突变或通过水平基因转移（接合、转化、转导）获得耐药基因而产生耐药性。耐药机制包括：产生灭活药物的酶（例如 β-内酰胺酶破坏青霉素的 β-内酰胺环）、改变靶点（例如突变的核糖体蛋白阻止四环素结合）、减少药物摄入或增加外排泵将抗生素排出细胞。
Exam questions frequently ask about the role of natural selection in the spread of resistance. In an antibiotic-treated population, susceptible bacteria are killed, while resistant ones survive and reproduce, passing the resistance allele to their offspring and increasing its frequency over generations. Misuse of antibiotics (e.g. not completing a course, using in animal feed) accelerates this process. You should be able to interpret data from antibiotic disc diffusion tests and minimum inhibitory concentration (MIC) assays.
试题经常问及自然选择在耐药性扩散中的作用。在抗生素处理后的群体中，敏感的细菌被杀死，而耐药细菌存活并繁殖，将耐药等位基因传递给后代，使其频率逐代增加。滥用抗生素（如未完成疗程、在动物饲料中使用）会加速这一过程。你应该能够解读抗生素纸片扩散试验和最低抑菌浓度 (MIC) 测定的数据。
8. Immune Response to Pathogens | 对病原体的免疫应答
A solid understanding of the human immune response to bacterial and viral infections is required. The non-specific (innate) defence includes physical barriers (skin, mucus), phagocytosis by neutrophils and macrophages, and inflammation. Specific (adaptive) immunity involves clonal selection of B and T lymphocytes. For bacteria, humoral immunity is crucial: activated B cells differentiate into plasma cells that secrete antibodies; these opsonise bacteria, agglutinate them and activate the complement system leading to lysis. For viruses, cell-mediated immunity is vital: T-helper cells activate cytotoxic T cells, which destroy infected host cells presenting viral antigens on MHC class I molecules.
你需要扎实理解人体对细菌和病毒感染的免疫应答。非特异性（先天）防御包括物理屏障（皮肤、黏液）、中性粒细胞和巨噬细胞的吞噬作用以及炎症。特异性（适应性）免疫涉及 B 和 T 淋巴细胞的克隆选择。对于细菌，体液免疫至关重要：活化的 B 细胞分化为浆细胞，分泌抗体；这些抗体可调理细菌、使其凝集并激活补体系统导致裂解。对于病毒，细胞介导的免疫极为重要：辅助 T 细胞激活细胞毒性 T 细胞，后者破坏在 MHC I 类分子上呈递病毒抗原的感染宿主细胞。
Memory cells produced during the primary response enable a faster, stronger secondary response upon re-infection, which is the basis of vaccination. WJEC may ask you to compare active and passive immunity, and natural versus artificial acquisition. Be prepared to apply your knowledge to diseases such as tuberculosis (bacterial) and influenza (viral).
初次应答中产生的记忆细胞使得再次感染时能产生更快、更强的二次应答，这是疫苗接种的基础。WJEC 可能会要求你比较主动免疫与被动免疫，以及自然获得与人工获得。要准备好将知识应用于结核病（细菌性）和流感（病毒性）等疾病。
9. Transmission of Infectious Diseases | 传染病传播
Pathogens are transmitted via several routes: direct contact (e.g. HIV, herpes), droplet infection (e.g. influenza, tuberculosis), contaminated food and water (e.g. Salmonella, cholera), vector-borne (e.g. Plasmodium via female Anopheles mosquito), and fomites. WJEC expects you to relate the mode of transmission to preventive measures. For example, water purification, hand washing, condom use, and vector control (insecticide-treated nets, drainage of breeding sites) are all directly linked to breaking the transmission cycle.
病原体通过多种途径传播：直接接触（如 HIV、疱疹）、飞沫传播（如流感、结核病）、受污染的食物和水（如沙门氏菌、霍乱）、媒介传播（如疟原虫通过雌性按蚊传播）以及污染物传播。WJEC 要求你将传播方式与预防措施联系起来。例如，水净化、洗手、使用安全套和媒介控制（经杀虫剂处理过的蚊帐、清除孳生地排水）都与打破传播循环直接相关。
Knowledge of the biology of the pathogen underpins each preventive strategy. In malaria, the complex life cycle involving sporozoites, liver schizonts and erythrocytic stages means that prophylactic drugs can target different stages. You must also understand epidemiological terms such as endemic, epidemic and pandemic.
掌握病原体的生物学特征是每种预防策略的基础。在疟疾中，涉及子孢子、肝脏裂殖体和红细胞内期的复杂生活史意味着预防性药物可以靶向不同阶段。你还必须理解地方性、流行和大流行等流行病学术语。
10. Biotechnological Applications of Microorganisms | 微生物的生物技术应用
Beyond food production, WJEC examines the use of microbes in industrial processes and environmental management. In bioremediation, naturally occurring or genetically modified bacteria break down oil spills or toxic pollutants into harmless substances. In sewage treatment, aerobic and anaerobic microorganisms on trickling filters or in activated sludge digest organic matter, reducing the biological oxygen demand (BOD) of the effluent before release. Microorganisms are also used to produce enzymes for biological washing powders (e.g. lipases, proteases from bacteria) and to manufacture human insulin via recombinant DNA technology, where the human insulin gene is inserted into E. coli or yeast and the protein is harvested from the culture.
除了食品生产，WJEC 还会考查微生物在工业过程和环境管理中的应用。在生物修复中，天然或经基因改造的细菌将泄漏的石油或有毒污染物分解为无害物质。在污水处理中，滴滤池或活性污泥中的需氧和厌氧微生物消化有机物，降低排出前污水的生物需氧量 (BOD)。微生物还被用来生产生物洗衣粉中的酶（如来自细菌的脂肪酶、蛋白酶），以及通过重组 DNA 技术生产人胰岛素：将人胰岛素基因插入大肠杆菌或酵母中，再从培养物中收集该蛋白质。
In biotechnology questions, you must describe the role of vectors (plasmids), restriction enzymes, ligase and marker genes used to identify successfully transformed cells. Ethical and safety considerations, such as containment of genetically modified organisms, are often worth marks in evaluation-style questions.
在生物技术题目中，你必须描述载体（质粒）、限制酶、连接酶以及用于识别成功转化细胞的标记基因的作用。伦理和安全方面的考虑，例如转基因生物的隔离防范，在评价类题目中常常值得得分。
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