📚 Microbiology | 微生物考点精讲
Microbiology is a core topic in IB and AQA Biology, covering the diversity, structure, and significance of microorganisms — including bacteria, viruses, fungi, and protists. A thorough understanding of microbial growth, pathogenicity, and industrial applications is essential for success in both coursework and examinations. This revision guide synthesises key concepts across examination specifications, providing a clear and structured approach to the subject.
微生物学是 IB 和 AQA 生物学中的核心主题，涵盖细菌、病毒、真菌和原生生物等微生物的多样性、结构和重要意义。透彻理解微生物的生长、致病机理及工业应用，对于课程学习和考试成功至关重要。本考点精讲综合各考试大纲的关键概念，以清晰的结构化方式呈现这一学科内容。
1. Introduction to Microbiology | 微生物学导论
Microbiology is the study of organisms that are too small to be seen with the naked eye. These include bacteria, archaea, viruses, fungi, and protists. Microorganisms are ubiquitous — they inhabit soil, water, air, and the bodies of plants and animals. While many microbes are harmless or even beneficial, some are pathogens capable of causing disease. In IB and AQA specifications, students are expected to recognise the diversity of microbial life and understand the fundamental roles microbes play in ecosystems, human health, and biotechnology.
微生物学是研究肉眼无法看到的生物体的学科，包括细菌、古菌、病毒、真菌和原生生物。微生物无处不在 —— 栖息于土壤、水、空气以及动植物体内。许多微生物无害甚至有益，但有些是能引发疾病的病原体。在 IB 和 AQA 考纲中，学生需要认识微生物生命的多样性，并理解微生物在生态系统、人类健康和生物技术中的基本角色。
2. Classification of Microorganisms | 微生物分类
Microorganisms are classified into several major groups based on cellular structure and genetic makeup. Bacteria and archaea are prokaryotes, lacking a membrane-bound nucleus. Eukaryotic microbes include fungi, protoctists (protists), and algae. Viruses are acellular, consisting only of nucleic acid and protein, and are not classified within the three domains of life. AQA and IB courses both emphasise the three-domain system (Archaea, Bacteria, Eukarya) and the use of ribosomal RNA analysis in modern classification. The key distinguishing features include cell wall composition, membrane lipid structure, and sensitivity to antibiotics.
微生物根据细胞结构和遗传组成分为几大类群。细菌和古菌是原核生物，缺乏膜包裹的细胞核。真核微生物包括真菌、原生生物和藻类。病毒是无细胞结构的，仅由核酸和蛋白质组成，不被归入三域生命系统。AQA 和 IB 课程都强调三域分类系统（古菌域、细菌域、真核域）以及利用核糖体 RNA 分析进行的现代分类。关键的区分特征包括细胞壁成分、膜脂结构以及对抗生素的敏感性。
3. Bacterial Structure and Function | 细菌结构与功能
Bacteria are unicellular prokaryotes with distinctive structural features. They possess a cell wall, typically composed of peptidoglycan, which maintains cell shape and prevents osmotic lysis. The Gram stain technique classifies bacteria into Gram-positive (thick peptidoglycan layer, purple stain) and Gram-negative (thin peptidoglycan layer, additional outer lipopolysaccharide membrane, pink stain). Many bacteria have flagella for motility, pili for adhesion and conjugation, and a capsule that provides protection against phagocytosis. The bacterial chromosome is a single circular DNA molecule located in the nucleoid region, and small plasmids often carry antibiotic resistance genes. Ribosomes are 70S, smaller than those of eukaryotes, making them targets for antibiotics.
细菌是单细胞原核生物，具有独特的结构特征。它们拥有主要由肽聚糖构成的细胞壁，维持细胞形状并防止渗透裂解。革兰氏染色法将细菌分为革兰氏阳性菌（厚肽聚糖层，染成紫色）和革兰氏阴性菌（薄肽聚糖层，另有一层外脂多糖膜，染成粉红色）。许多细菌具有用于运动的鞭毛、用于附着和接合的菌毛，以及能抵抗吞噬作用的荚膜。细菌染色体是位于拟核区的单一环状 DNA 分子，小的质粒常携带抗生素抗性基因。核糖体为 70S，比真核生物的小，这使它们成为抗生素的靶点。
4. Viruses: Structure and Replication | 病毒结构与复制
Viruses are obligate intracellular parasites that possess no cellular machinery of their own. A typical virus consists of a nucleic acid core (either DNA or RNA, single- or double-stranded) surrounded by a protective protein coat called a capsid. Some viruses, like HIV and influenza, also have a lipid envelope derived from the host cell membrane. The lytic cycle involves viral attachment, injection of genetic material, replication of viral components using host ribosomes and enzymes, assembly of new virions, and lysis of the host cell. The lysogenic cycle, seen in bacteriophages such as lambda, integrates the viral DNA into the host genome as a prophage, where it can remain dormant before entering the lytic cycle. Understanding these cycles is essential for explaining how viruses spread and how antiviral drugs work.
病毒是专性胞内寄生物，自身没有任何细胞器。典型的病毒由核酸核心（DNA 或 RNA，单链或双链）和称为衣壳的保护性蛋白质外壳组成。某些病毒，如 HIV 和流感病毒，还具有来自宿主细胞膜的脂质包膜。裂解循环包括病毒附着、注射遗传物质、利用宿主核糖体和酶复制病毒组分、组装新病毒粒子以及裂解宿主细胞。溶原循环见于λ噬菌体等，病毒 DNA 以前噬菌体形式整合到宿主基因组中，可保持休眠后进入裂解循环。理解这些循环对解释病毒传播方式及抗病毒药物作用至关重要。
5. Fungi and Protists | 真菌和原生生物
Fungi are eukaryotic organisms that can be unicellular (yeasts) or multicellular (moulds, mushrooms). Their cell walls contain chitin, not peptidoglycan. Fungi are heterotrophic, obtaining nutrients through extracellular digestion by secreting enzymes. They play vital roles as decomposers in nutrient cycling and are used in food production (bread, cheese). Protists are a diverse group of eukaryotic microorganisms. They include animal-like protozoa (e.g., Plasmodium, the malaria parasite), plant-like algae (e.g., Chlorella), and fungus-like slime moulds. Many protists are motile, using flagella, cilia, or pseudopodia. In IB Biology, students often study the life cycle of Plasmodium as an example of a pathogenic protist.
真菌是真核生物，可以是单细胞（酵母）或多细胞（霉菌、蘑菇）。它们的细胞壁含有几丁质，而非肽聚糖。真菌为异养生物，通过分泌酶进行胞外消化获取营养。它们在营养循环中作为分解者发挥重要作用，并用于食品生产（面包、奶酪）。原生生物是一组多样的真核微生物，包括类似动物的原生动物（如疟原虫）、类似植物的藻类（如小球藻）和类似真菌的黏菌。许多原生生物能利用鞭毛、纤毛或伪足运动。在 IB 生物学中，学生通常以疟原虫的生活史作为致病原生生物的例子进行学习。
6. Culturing Microorganisms | 微生物培养
Microorganisms can be grown in the laboratory using nutrient agar plates or broth. Aseptic technique is critical to prevent contamination. Key steps include sterilising equipment (autoclaving at 121°C for 15 minutes), working near a Bunsen burner flame to create an updraft, and flaming the neck of culture bottles. Selective media allow only specific microbes to grow; differential media enable identification based on metabolic reactions, such as MacConkey agar differentiating lactose fermenters. Inoculated plates are incubated at a suitable temperature — in schools, usually below 25°C to discourage pathogen growth. Serial dilution and viable cell counts are common quantitative methods, and the ‘most probable number’ technique is used when organisms cannot be cultured on solid media. IB and AQA specifications both expect students to design and interpret culturing experiments.
微生物可在实验室中使用营养琼脂平板或肉汤进行培养。无菌操作技术对于防止污染至关重要。关键步骤包括设备灭菌（高压灭菌 121°C、15 分钟）、在接近本生灯火焰处操作以形成上升气流、以及灼烧培养瓶瓶颈。选择性培养基仅允许特定微生物生长；鉴别培养基则能基于代谢反应进行鉴定，如麦康凯琼脂区分乳糖发酵菌。接种后的平板在适宜温度下培养 —— 学校通常低于 25°C 以抑制病原体生长。连续稀释和活菌计数是常用的定量方法；当微生物无法在固体培养基上培养时，使用“最大可能数”技术。IB 和 AQA 考纲都期望学生能设计并解读培养实验。
7. Bacterial Growth Curve | 细菌生长曲线
When bacteria are inoculated into fresh liquid medium, a characteristic growth curve is observed when cell numbers are plotted against time. The four phases are: lag phase (adaptation, no significant increase), exponential (log) phase (rapid binary fission, doubling at a constant rate), stationary phase (nutrient depletion and waste accumulation balance cell division and death), and death phase (decline in viable cells). The mean generation time varies between species; E. coli can double every 20 minutes under optimal conditions. Students must be able to calculate growth rates using the formula N = N0 × 2n, where n = number of generations. In AQA required practicals and IB internal assessments, spectrophotometry or plating can be used to measure growth.
当细菌被接种到新鲜液体培养基中，以细胞数量对时间作图会获得一条特征性生长曲线。四个阶段为：延滞期（适应，无显著增加）、指数（对数）期（快速二分裂，恒定速率倍增）、稳定期（营养耗尽和废物积累使细胞分裂与死亡平衡）和死亡期（活菌数下降）。不同物种的平均代时各异；在最优条件下大肠杆菌每 20 分钟倍增一次。学生须能使用公式 N = N0 × 2n 计算生长速率，其中 n=世代数。在 AQA 必修实验和 IB 内部评估中，可使用分光光度法或平板计数法测量生长。
8. Factors Affecting Microbial Growth | 影响微生物生长的因素
Microbial growth is influenced by a range of environmental factors. Temperature is paramount: psychrophiles grow best below 20°C, mesophiles (including most human pathogens) between 20°C and 45°C, and thermophiles above 45°C. pH also affects enzyme activity; most bacteria prefer neutral pH, whereas fungi favour slightly acidic conditions. Oxygen requirements classify microorganisms into obligate aerobes, obligate anaerobes, facultative anaerobes, and microaerophiles. Water availability (measured as water activity, aw) determines the lower limit for growth — halophiles thrive in high salt concentrations that lower aw. Nutrients such as carbon, nitrogen, and essential trace elements must be present. In food preservation, manipulating these factors (refrigeration, acidification, salting) is used to inhibit microbial growth.
微生物的生长受多种环境因素影响。温度至关重要：嗜冷菌在 20°C 以下最佳生长，嗜温菌（包括多数人类病原体）为 20°C 至 45°C，嗜热菌在 45°C 以上。pH 也影响酶活性；大多数细菌偏好中性 pH，而真菌喜微酸条件。依需氧情况可将微生物分为专性需氧菌、专性厌氧菌、兼性厌氧菌和微需氧菌。水分活度（以 aw 衡量）决定生长的低限 —— 嗜盐菌在高盐浓度下生长旺盛，因为盐降低了 aw。碳、氮及必需微量元素等营养物质也必须存在。在食品保藏中，通过调控这些因素（冷藏、酸化、盐腌）可抑制微生物生长。
9. Microbial Ecology and Nutrient Cycles | 微生物生态与营养循环
Microorganisms are essential drivers of biogeochemical cycles. In the nitrogen cycle, nitrogen-fixing bacteria (e.g., Rhizobium) convert atmospheric N2 into ammonia; nitrifying bacteria (Nitrosomonas, Nitrobacter) oxidise ammonia to nitrite and nitrate; and denitrifying bacteria return N2 to the atmosphere. In the carbon cycle, saprotrophic bacteria and fungi decompose organic matter, releasing CO2 through respiration. Methanogenic archaea produce methane in anaerobic environments. Mycorrhizal fungi form mutualistic associations with plant roots, enhancing phosphate and water uptake. In IB Biology, students are required to understand these microbial roles and their implications for soil fertility and climate regulation. AQA specifications similarly emphasise the importance of microorganisms in recycling chemical elements.
微生物是生物地球化学循环的关键驱动者。在氮循环中，固氮细菌（如根瘤菌）将大气中的 N2 转化为氨；硝化细菌（亚硝化单胞菌、硝化杆菌）将氨氧化为亚硝酸盐和硝酸盐；反硝化细菌则将 N2 归还大气。在碳循环中，腐生细菌和真菌分解有机物，通过呼吸释放 CO2。产甲烷古菌在厌氧环境中产生甲烷。菌根真菌与植物根系形成互惠共生，增强磷酸盐和水分的吸收。在 IB 生物学中，学生须理解这些微生物作用及其对土壤肥力与气候调节的影响。AQA 考纲同样强调微生物在化学元素循环中的重要性。
10. Pathogens and Mechanisms of Disease | 病原体与致病机制
Pathogens are microorganisms that cause disease. Bacteria can produce toxins — exotoxins are secreted proteins (e.g., tetanospasmin from Clostridium tetani), while endotoxins are lipopolysaccharides released when Gram-negative bacteria lyse. Viruses damage host cells by hijacking cellular machinery and inducing lysis or apoptosis. Fungi can produce mycotoxins and cause superficial or systemic infections, especially in immunocompromised individuals. Protists such as Plasmodium invade and destroy red blood cells. The transmission of pathogens can be direct (contact, droplet) or indirect (vectors, contaminated food and water). Koch’s postulates provide a historical framework for linking a specific microbe to a specific disease, though they have limitations. Both IB and AQA courses require students to discuss specific diseases, including their causative agents, symptoms, and modes of transmission.
病原体是能引起疾病的微生物。细菌可产生毒素 —— 外毒素为分泌性蛋白质（如破伤风梭菌的破伤风痉挛毒素），内毒素则是革兰氏阴性菌裂解时释放的脂多糖。病毒通过劫持细胞机构和诱导裂解或凋亡来损伤宿主细胞。真菌可产生真菌毒素，在免疫功能低下个体中引起浅表或系统性感染。像疟原虫这样的原生生物侵入并破坏红细胞。病原体的传播可以是直接途径（接触、飞沫）或间接途径（媒介、污染的食物和水）。科赫法则为特定微生物与特定疾病的关联提供了历史性框架，尽管其存在局限性。IB 和 AQA 课程都要求学生讨论具体疾病，包括其病原体、症状及传播方式。
11. Antibiotics and Resistance | 抗生素与耐药性
Antibiotics are chemical agents that selectively kill or inhibit bacteria without harming host cells. They target prokaryotic-specific structures: penicillin inhibits cell wall synthesis by blocking transpeptidase enzymes; tetracycline binds to the 30S ribosomal subunit, preventing protein synthesis. Broad-spectrum antibiotics affect a wide range of bacteria, while narrow-spectrum drugs target specific groups. Antimicrobial resistance arises through spontaneous mutations or horizontal gene transfer (conjugation, transformation, transduction). Resistance mechanisms include enzymatic degradation (beta-lactamases), target site modification, and efflux pumps. The widespread use and misuse of antibiotics in medicine and agriculture have accelerated the spread of resistant strains, leading to concerns about ‘superbugs’ like MRSA. Students must be able to relate resistance to natural selection and evaluate strategies for controlling resistance.
抗生素是能选择性杀灭或抑制细菌而不伤害宿主细胞的化学制剂。它们靶向原核生物特有的结构：青霉素通过阻断转肽酶来抑制细胞壁合成；四环素与 30S 核糖体亚基结合，阻止蛋白质合成。广谱抗生素作用于范围广泛的细菌，窄谱药物则靶向特定类群。抗菌素耐药性通过自发突变或水平基因转移（接合、转化、转导）产生。耐药机制包括酶降解（β-内酰胺酶）、靶位点修饰和主动外排泵。抗生素在医疗和农业中的广泛使用及滥用已加速耐药菌株的传播，引发对 MRSA 等“超级细菌”的担忧。学生须能将耐药性与自然选择联系起来，并评估控制耐药性的策略。
12. Biotechnology Applications | 生物技术应用
Microorganisms are exploited in numerous biotechnological processes. Bacteria and fungi are used in industrial fermentation to produce antibiotics, insulin, enzymes, and biofuels. E. coli and yeast are genetically engineered to express human proteins such as growth hormone and factor VIII. The CRISPR-Cas9 system, derived from a bacterial immune mechanism, has revolutionised gene editing. In food production, lactic acid bacteria are crucial for yoghurt and cheese manufacture; Saccharomyces cerevisiae is used in baking and brewing. In environmental biotechnology, microbial bioremediation uses bacteria or fungi to degrade pollutants, such as oil spills. Students in IB Biology may carry out practical work on factors affecting fermentation, while AQA practicals often include investigations of yeast respiration under different conditions.
微生物在众多生物技术过程中得到开发利用。细菌和真菌用于工业发酵来生产抗生素、胰岛素、酶和生物燃料。大肠杆菌和酵母经基因工程改造后可表达人类蛋白质，如生长激素和凝血因子 VIII。源于细菌免疫机制的 CRISPR-Cas9 系统彻底改变了基因编辑。在食品生产中，乳酸菌对于酸奶和奶酪制造至关重要；酿酒酵母用于烘焙和酿造。在环境生物技术中，微生物生物修复利用细菌或真菌降解污染物，如石油泄漏。IB 生物学学生可进行影响发酵因素的实践研究，而 AQA 实践常包括不同条件下酵母呼吸作用的探究。
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