IB & CCEA Biology: Microorganisms Key Revision Points | IB CCEA 生物：微生物考点精讲

📚 IB & CCEA Biology: Microorganisms Key Revision Points | IB CCEA 生物：微生物考点精讲

This comprehensive revision guide covers the essential microbial concepts examined in IB Biology and CCEA A-Level Biology. From classification and structure to growth kinetics, aseptic techniques, ecological roles, pathogenesis and biotechnology, all key syllabus points are explained with precision. Each section presents core ideas in paired English and Chinese paragraphs, supported by tables, lists and equations where helpful.

这份综合复习指南涵盖了 IB 生物和 CCEA A-Level 生物中考查的核心微生物概念。从分类、结构到生长动力学、无菌技术、生态角色、致病机制和生物技术，所有关键大纲要点都得到了精确解释。每个部分以中英对照的段落呈现，并适当辅以表格、列表和方程。

1. Classification of Microorganisms | 微生物的分类

Microorganisms are divided into several major groups based on cellular structure and molecular phylogeny: bacteria (prokaryotes), archaea (prokaryotes distinct from bacteria), fungi (eukaryotes), protoctista (mostly unicellular eukaryotes) and viruses (acellular infectious particles). In the three‑domain system, Bacteria and Archaea are separate domains, while eukaryotes belong to Eukarya.

微生物根据细胞结构和分子系统发育可分为几大类群：细菌（原核生物）、古菌（与细菌不同的原核生物）、真菌（真核生物）、原生生物（多为单细胞真核生物）和病毒（非细胞感染颗粒）。在三域系统中，细菌域和古菌域彼此独立，真核生物则属于真核域。

A particularly useful distinction for laboratory identification is the Gram stain, which separates bacteria into Gram‑positive (thick peptidoglycan wall, purple) and Gram‑negative (thin peptidoglycan layer plus outer lipopolysaccharide membrane, pink). Mycobacteria, though Gram‑positive by phylogeny, require acid‑fast staining due to their waxy mycolic acid cell wall.

实验室鉴定中一个特别实用的区分方法是革兰氏染色，它将细菌分为革兰氏阳性菌（厚肽聚糖壁，紫色）和革兰氏阴性菌（薄肽聚糖层外加外膜脂多糖，粉红色）。分枝杆菌虽然系统发育上属于革兰氏阳性，但因为其蜡质的霉菌酸细胞壁而需要抗酸染色。

2. Structure of Bacteria | 细菌的结构

A typical bacterial cell possesses a cell wall containing peptidoglycan, a plasma membrane, circular DNA free in the cytoplasm (nucleoid), 70S ribosomes, and often plasmids – small extrachromosomal DNA circles. Many bacteria also have a capsule (polysaccharide layer) for protection, flagella for motility, and pili for attachment or conjugation.

典型的细菌细胞具有含肽聚糖的细胞壁、质膜、游离于细胞质中的环状 DNA（拟核）、70S 核糖体，并常含有质粒——小的额外染色体 DNA 环。许多细菌还具有荚膜（多糖层）用于保护、鞭毛用于运动，以及菌毛用于附着或接合。

In unfavourable conditions, some Gram‑positive genera (e.g. Bacillus and Clostridium) form highly resistant endospores. The endospore contains a dehydrated core of DNA and ribosomes encased in a tough spore coat, allowing survival through extreme heat, radiation and disinfectants.

在不利条件下，某些革兰氏阳性菌属（如芽孢杆菌属和梭菌属）会形成高度抗性的内生孢子。内生孢子含有一个脱水的 DNA 和核糖体核心，被坚韧的孢子外壳包裹，从而能忍受极端高温、辐射和消毒剂。

3. Structure of Viruses | 病毒的结构

Viruses are non‑living infectious agents consisting of a nucleic acid core (DNA or RNA, single‑ or double‑stranded) enclosed within a protein coat called a capsid. Some viruses (e.g. influenza, HIV) also possess a lipid envelope derived from the host cell membrane, embedded with glycoprotein spikes for host recognition.

病毒是非生命的感染因子，由核酸核心（DNA 或 RNA，单链或双链）和包裹其外的蛋白质外壳（称为衣壳）组成。一些病毒（如流感病毒、HIV）还具有来源于宿主细胞膜的脂质包膜，包膜上嵌有用于识别宿主的糖蛋白刺突。

Bacteriophages, like the T4 phage that infects E. coli, exhibit a complex structure with a head containing DNA, a tail sheath and tail fibres. The virus attaches to specific receptors on the host cell surface, injecting its genetic material while the capsid remains outside.

噬菌体，如感染大肠杆菌的 T4 噬菌体，展示出复杂的结构：含 DNA 的头部、尾鞘和尾丝。病毒附着在宿主细胞表面的特定受体上，注入其遗传物质，而衣壳留在胞外。

4. Culturing Microorganisms | 微生物的培养

Bacteria are grown on nutrient agar plates or in broth. Agar is a polysaccharide from seaweed that solidifies at ~40 °C and remains solid at incubation temperatures, making it an ideal gelling agent. Selective media (e.g. MacConkey agar) favour the growth of specific bacteria while inhibiting others; differential media produce visible colour changes to distinguish between metabolic types.

细菌在营养琼脂平板或肉汤中培养。琼脂是来自海藻的多糖，约 40 °C 凝固并在培养温度下保持固态，因此是理想的凝胶剂。选择性培养基（如麦康凯琼脂）有利于特定细菌生长而抑制其他菌；鉴别培养基则产生可见的颜色变化以区分代谢类型。

Obligate aerobes require oxygen; obligate anaerobes are killed by it; facultative anaerobes can grow with or without oxygen. In a thioglycollate broth tube, oxygen diffuses only into the top layers, so obligate aerobes grow at the surface, obligate anaerobes at the bottom, and facultative organisms throughout.

专性需氧菌需要氧气；专性厌氧菌会被氧气杀死；兼性厌氧菌在有氧和无氧条件下均能生长。在巯基乙酸盐肉汤管中，氧气仅扩散到上层，因此专性需氧菌在表面生长，专性厌氧菌在底部生长，兼性菌则全管分布。

5. Bacterial Growth Curve | 细菌生长曲线

A closed batch culture shows four distinct phases: lag phase (cells adapt, synthesise enzymes, no increase in number), exponential (log) phase (cells divide at a constant maximum rate, population doubles in regular intervals), stationary phase (nutrient depletion and waste accumulation cause growth rate to equal death rate), and death phase (cells die exponentially).

封闭的分批培养表现出四个明显阶段：延滞期（细胞适应，合成酶，数量不增加）、指数（对数）期（细胞以恒定最大速率分裂，群体每隔固定时间翻倍）、稳定期（营养耗尽和废物积累使生长速率等于死亡速率）和衰亡期（细胞指数式死亡）。

During exponential growth, the population N after time t can be calculated as Nₜ = N₀ × 2ⁿ, where n is the number of generations and equals t / g (g = generation time). The specific growth rate μ = (ln N₂ − ln N₁) / (t₂ − t₁) h⁻¹. A smaller generation time means a steeper log‑phase slope.

在指数生长期，时间 t 后的群体数量 N 可通过 Nₜ = N₀ × 2ⁿ 计算，其中 n 为代数，等于 t / g（g 为代时）。比生长速率 μ = (ln N₂ − ln N₁) / (t₂ − t₁) h⁻¹。代时越小，对数期斜率越陡。

6. Aseptic Technique | 无菌操作技术

Aseptic technique prevents contamination of cultures and the environment. Key practices include flaming the inoculating loop to redness, flaming the necks of bottles and tubes before and after transferring cultures, working near a Bunsen burner to create an updraft, and minimising the time that agar plates or cultures are open.

无菌技术可防止培养物和环境污染。关键操作包括：将接种环灼烧至红热，转移培养物前后灼烧瓶口和试管口，在本生灯附近工作以产生上升气流，并缩短琼脂平板或培养物敞开的时间。

Autoclaving at 121 °C, 103 kPa for 15 minutes kills all microorganisms including endospores, achieving sterilisation. Air‑borne contaminants can be monitored by exposing a nutrient agar plate to the laboratory air for a fixed time and then incubating it to observe colony growth.

在 121 °C、103 kPa 下高压蒸汽灭菌 15 分钟可杀死包括内生孢子在内的所有微生物，实现灭菌。空气中污染物可通过将营养琼脂平板暴露于实验室空气中一定时间然后培养、观察菌落生长来进行监测。

7. Roles of Bacteria in Ecosystems | 细菌在生态系统中的角色

Saprotrophic bacteria and fungi decompose dead organic matter, releasing inorganic ions such as NH₄⁺ and PO₄³⁻. In the nitrogen cycle, nitrifying bacteria (Nitrosomonas oxidises NH₄⁺ → NO₂⁻; Nitrobacter oxidises NO₂⁻ → NO₃⁻) and nitrogen‑fixing bacteria (Rhizobium in legume root nodules, free‑living Azotobacter) are essential for converting atmospheric N₂ into usable forms.

腐生细菌和真菌分解死亡有机质，释放无机离子如 NH₄⁺ 和 PO₄³⁻。在氮循环中，硝化细菌（Nitrosomonas 氧化 NH₄⁺ → NO₂⁻；Nitrobacter 氧化 NO₂⁻ → NO₃⁻）和固氮细菌（豆科根瘤中的根瘤菌，自生固氮菌如固氮菌属）对于将大气 N₂ 转化为可用形式至关重要。

Denitrifying bacteria (e.g. Pseudomonas) convert nitrate back to N₂ gas under anaerobic conditions, returning nitrogen to the atmosphere. Chemoautotrophic bacteria can synthesise organic molecules using energy from the oxidation of inorganic substances such as H₂S or NH₃, supporting food webs in deep‑sea vents.

反硝化细菌（如假单胞菌）在厌氧条件下将硝酸盐还原为 N₂ 气体，使氮返回大气。化能自养细菌能利用氧化无机物（如 H₂S 或 NH₃）所释放的能量合成有机分子，支撑深海热液喷口处的食物网。

8. Pathogens and Infectious Disease | 病原体与传染病

Pathogens cause disease by damaging host tissues directly, releasing toxins, or triggering excessive immune responses. Vibrio cholerae secretes cholera toxin that opens ion channels in intestinal cells, leading to massive water loss through diarrhoea. Mycobacterium tuberculosis survives inside lung macrophages, forming tubercles that destroy lung tissue.

病原体通过直接破坏宿主组织、释放毒素或引发过度免疫反应导致疾病。霍乱弧菌分泌霍乱毒素，打开肠道细胞离子通道，导致大量腹泻失水。结核分枝杆菌在肺巨噬细胞内存活，形成结核结节，破坏肺组织。

Human immunodeficiency virus (HIV) targets CD4⁺ T‑helper lymphocytes, progressively destroying the immune system and leaving the host susceptible to opportunistic infections. HIV is a retrovirus; its enzyme reverse transcriptase synthesises DNA from the viral RNA genome, which then integrates into the host chromosome as a provirus.

人类免疫缺陷病毒 (HIV) 攻击 CD4⁺ 辅助性 T 淋巴细胞，逐步摧毁免疫系统，使宿主容易发生机会性感染。HIV 是一种逆转录病毒；其逆转录酶能从病毒 RNA 基因组合成 DNA，该 DNA 随后以原病毒形式整合到宿主染色体中。

9. Antibiotics and Resistance | 抗生素与耐药性

Antibiotics are chemicals that kill or inhibit bacteria without harming host cells. Bactericidal antibiotics (e.g. penicillin) cause cell death by disrupting cell wall synthesis; bacteriostatic antibiotics (e.g. tetracycline) inhibit protein synthesis by binding to 70S ribosomes, preventing growth. Viruses are unaffected by antibiotics because they lack their own metabolic machinery.

抗生素是能杀死或抑制细菌而不损害宿主细胞的化学物质。杀菌性抗生素（如青霉素）通过破坏细胞壁合成导致细胞死亡；抑菌性抗生素（如四环素）通过与 70S 核糖体结合抑制蛋白质合成，从而阻止生长。病毒不受抗生素影响，因为它们缺乏自身的代谢机制。

Antibiotic resistance arises through mutation and horizontal gene transfer. Resistance genes can be carried on plasmids and transferred between bacteria by conjugation (via sex pili), transduction (via bacteriophages), or transformation (uptake of naked DNA from the environment). The misuse of antibiotics selects for resistant strains, creating serious clinical challenges like MRSA.

抗生素耐药性源于突变和水平基因转移。耐药基因可由质粒携带，并通过接合（经性菌毛）、转导（经噬菌体）或转化（从环境中摄取裸 DNA）在细菌间转移。抗生素的滥用会筛选出耐药菌株，导致诸如耐甲氧西林金黄色葡萄球菌 (MRSA) 等严重临床问题。

Mechanism	Explanation
Enzymatic degradation	β‑lactamases hydrolyse the β‑lactam ring of penicillin.
Target site alteration	Mutation in ribosomal protein prevents tetracycline binding.
Efflux pumps	Membrane proteins actively export antibiotic molecules.
Reduced permeability	Porin channels in Gram‑negative outer membrane are altered.

10. Microorganisms in Biotechnology | 微生物在生物技术中的应用

Yeast (Saccharomyces cerevisiae) is used in baking and brewing. It ferments sugars anaerobically: C₆H₁₂O₆ → 2C₂H₅OH + 2CO₂. The CO₂ causes dough to rise; ethanol is the desired product in alcoholic beverages. In aerobic conditions, yeast respires completely, producing biomass rather than ethanol.

酵母（酿酒酵母）用于烘焙和酿造。它在厌氧条件下发酵糖类：C₆H₁₂O₆ → 2C₂H₅OH + 2CO₂。CO₂ 使面团膨胀；乙醇是酒精饮料中所需的产物。在有氧条件下，酵母进行完全呼吸，产生生物量而非乙醇。

Lactic acid bacteria (e.g. Lactobacillus) ferment lactose to lactic acid in yoghurt and cheese production, lowering the pH and coagulating milk proteins. The low pH also inhibits spoilage organisms. In biotechnology, genetically modified bacteria produce human insulin, growth hormone, and enzymes for industrial processes.

乳酸菌（如乳杆菌）在酸奶和奶酪生产中把乳糖发酵为乳酸，降低 pH 并凝固乳蛋白。低 pH 也能抑制腐败微生物。在生物技术中，转基因细菌可生产人胰岛素、生长激素以及用于工业过程的酶。

Bioremediation: Pseudomonas species can degrade oil pollutants.
Bioremediation：假单胞菌属物种能降解石油污染物。
Sewage treatment: aerobic bacteria oxidise organic matter in activated sludge.
污水处理：好氧细菌在活性污泥中氧化有机质。
Biofuels: methanogens produce methane gas from organic waste.
生物燃料：产甲烷菌从有机废物中产生甲烷气体。

11. Practical Skills: Measuring Growth | 实践技能：测量微生物生长

To construct a growth curve, viable cell counts (colony‑forming units, CFU mL⁻¹) are often obtained by serial dilution and spread plating. A dilution of 10⁻⁶ yielding 150 colonies on a plate from 0.1 mL inoculum gives a count of 150 ÷ 0.1 × 10⁶ = 1.5 × 10⁹ CFU mL⁻¹.

为绘制生长曲线，通常通过连续稀释和涂布平板获得活菌计数（菌落形成单位，CFU mL⁻¹）。若 10⁻⁶ 稀释液涂布 0.1 mL 得到 150 个菌落，则计数为 150 ÷ 0.1 × 10⁶ = 1.5 × 10⁹ CFU mL⁻¹。

Turbidity measured by a spectrophotometer at 600 nm provides a quicker but indirect estimate of total cell mass. A calibration curve relating absorbance to CFU or dry mass is needed. The generation time g can be calculated from the slope of the logarithmic plot of cell number vs time.

用分光光度计在 600 nm 处测量浊度能更快但间接地估计总细胞质量。需要一条将吸光度与 CFU 或干重相关联的标准曲线。代时 g 可由细胞数对数–时间图的斜率计算得出。

Safety must be observed: all cultures should be treated as potentially pathogenic; plates are sealed and incubated at safe temperatures (usually below 37 °C in schools to discourage human pathogen growth); and all materials are autoclaved before disposal.

必须遵守安全规范：所有培养物均应视为潜在致病源处理；平板密封后在安全温度下培养（学校通常低于 37 °C 以抑制人类病原体生长）；所有材料在废弃前均需高压灭菌。

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IB & CCEA Biology: Microorganisms Key Revision Points | IB CCEA 生物：微生物考点精讲