📚 A-Level CIE Biology: Microorganisms Key Points | A-Level CIE 生物：微生物 考点精讲
Microorganisms are microscopic organisms that exist as single cells, cell clusters, or acellular entities. They include bacteria, viruses, fungi, and protoctists. In the CIE A-Level Biology syllabus, understanding their structure, growth, and applications is essential for topics ranging from cell biology to biotechnology and infectious diseases.
微生物是微小的生物，以单细胞、细胞群体或无细胞实体的形式存在，包括细菌、病毒、真菌和原生生物。在 CIE A-Level 生物课程中，理解微生物的结构、生长和应用是细胞生物学、生物技术及传染病等核心主题的基础。
1. Classification and Diversity of Microorganisms | 微生物的分类与多样性
Microorganisms are not a single taxonomic group; they span several kingdoms and even acellular forms. Bacteria belong to the kingdom Prokaryotae, characterised by the absence of a membrane-bound nucleus. Fungi, such as yeasts and moulds, are eukaryotic and have cell walls made of chitin. Protoctists are a diverse group of unicellular eukaryotes, including protozoa like Amoeba and Plasmodium. Viruses are acellular and are not classified into any kingdom, as they lack cellular structure and can only replicate inside host cells.
微生物并非一个单一的生物分类群，它们跨越多个界，甚至包括无细胞形式。细菌属于原核生物界，特点是没有膜包被的细胞核。真菌（如酵母和霉菌）是真核生物，细胞壁由几丁质构成。原生生物是一大类单细胞真核生物，包括变形虫和疟原虫等。病毒是无细胞结构，不归入任何生物界，因为它们没有细胞结构，只能在宿主细胞内复制。
The table below summarises the main groups of microorganisms relevant to the CIE specification:
下表总结了与 CIE 考试大纲相关的主要微生物类群：
| Group | Key Features | Examples |
|---|---|---|
| Bacteria (Prokaryotae) | No nucleus, 70S ribosomes, peptidoglycan cell wall, circular DNA, plasmids | Escherichia coli, Vibrio cholerae |
| Viruses | Acellular, nucleic acid (DNA or RNA) + protein capsid, some have lipid envelope | HIV, Influenza virus, λ phage |
| Fungi | Eukaryotic, cell wall of chitin, saprotrophic or parasitic, hyphae or unicellular yeast | Saccharomyces cerevisiae, Penicillium |
| Protoctists | Eukaryotic, mostly unicellular, autotrophic or heterotrophic | Plasmodium (malaria), Amoeba |
2. Structure of Bacteria | 细菌的结构
A typical bacterial cell possesses a cell wall made of peptidoglycan, which provides shape and protection against osmotic lysis. External to the cell wall, some bacteria have a slime capsule that aids in adhesion and evasion of the immune system. The cell surface membrane controls the movement of substances in and out of the cell. Unlike eukaryotic cells, bacteria lack membrane-bound organelles; their ribosomes are 70S, smaller than those in eukaryotic cytoplasm.
典型的细菌细胞具有由肽聚糖构成的细胞壁，提供形状并防止渗透裂解。在细胞壁外侧，一些细菌具有荚膜，有助于附着和逃避宿主免疫系统。细胞膜控制物质的进出。与真核细胞不同，细菌没有膜包被的细胞器；其核糖体为 70S，小于真核细胞质中的核糖体。
The genetic material of bacteria consists of a single circular chromosome located in the nucleoid region, and small extrachromosomal DNA molecules called plasmids. Plasmids often carry genes for antibiotic resistance and are used as vectors in genetic engineering. Some bacteria possess flagella for locomotion and pili for attachment or conjugation.
细菌的遗传物质包括位于拟核区的单个环状染色体，以及称为质粒的小型染色体外 DNA 分子。质粒常携带抗生素抗性基因，并在基因工程中用作载体。一些细菌具有用于运动的鞭毛和用于附着或接合的菌毛。
Key structural features you must link to function:
必须将结构与功能联系起来的要点：
- Peptidoglycan cell wall: maintains shape, protects from lysis; target of penicillin.
- 肽聚糖细胞壁：维持形状，防止裂解；是青霉素的作用靶点。
- 70S ribosomes: protein synthesis; differ from 80S eukaryotic ribosomes, allowing selective antibiotic action.
- 70S 核糖体：合成蛋白质；与 80S 真核核糖体不同，因此可被抗生素选择性抑制。
- Plasmids: carry accessory genes, can replicate independently; used in recombinant DNA technology.
- 质粒：携带额外基因，能独立复制；用于重组 DNA 技术。
3. Basic Features of Viruses | 病毒的基本特征
Viruses are acellular, obligate intracellular parasites. They consist of a nucleic acid core (either DNA or RNA) surrounded by a protective protein coat called a capsid. Some viruses, such as HIV and influenza virus, also have a lipid envelope derived from the host cell membrane, studded with glycoproteins for host recognition.
病毒是无细胞的专性细胞内寄生物。它们由核酸核心（DNA 或 RNA）和称为衣壳的保护性蛋白质外壳组成。一些病毒，如 HIV 和流感病毒，还具有来自宿主细胞膜的脂质包膜，上面镶嵌着用于识别宿主的糖蛋白。
Viral replication follows a general sequence: attachment to host cell receptors, entry or injection of nucleic acid, replication of viral genome and synthesis of capsid proteins using host machinery, self-assembly of new virus particles, and release by lysis or budding. Bacteriophages like the lambda phage exhibit two life cycles: the lytic cycle, where the host cell is lysed, and the lysogenic cycle, where viral DNA integrates into the host genome as a prophage.
病毒的复制遵循一般顺序：吸附到宿主细胞受体，注入或进入核酸，利用宿主机器复制病毒基因组并合成衣壳蛋白，新病毒颗粒自组装，然后通过裂解或出芽释放。像 λ 噬菌体这样的噬菌体表现出两种生活周期：裂解周期（宿主细胞被裂解）和溶原周期（病毒 DNA 作为原噬菌体整合到宿主基因组中）。
HIV, as a retrovirus, uses reverse transcriptase to convert its RNA genome into DNA, which is then integrated into the host chromosome. This makes eradication difficult and is a key concept in AIDS pathology.
HIV 作为逆转录病毒，利用逆转录酶将其 RNA 基因组转化为 DNA，然后整合到宿主染色体中。这使得根除病毒十分困难，这也是 AIDS 病理学中的一个关键概念。
4. Fungi and Protoctists | 真菌与原生生物
Fungi are eukaryotic organisms with cell walls composed of chitin. They exist as unicellular yeasts or multicellular moulds forming a mycelium of hyphae. Fungi are heterotrophic and obtain nutrients by extracellular digestion (saprotrophic nutrition). They play vital roles in decomposition and in industrial processes, such as the production of antibiotics by Penicillium and ethanol by yeast.
真菌是真核生物，细胞壁由几丁质组成。它们以单细胞酵母或形成菌丝体（由菌丝构成）的多细胞霉菌形式存在。真菌为异养生物，通过胞外消化获取营养（腐生营养）。它们在分解作用以及工业生产中至关重要，例如青霉产生抗生素、酵母生产乙醇。
Protoctists are a diverse kingdom of mainly unicellular eukaryotes. They display a wide range of nutritional modes: some are autotrophic (algae), others are heterotrophic (protozoa). Medically important examples include Plasmodium, the causative agent of malaria, which has a complex life cycle involving both human and mosquito hosts. Another example is Amoeba, which moves using pseudopodia and feeds by phagocytosis.
原生生物是一个多样化的界，主要为单细胞真核生物。它们展现出多种营养方式：有些是自养的（藻类），有些是异养的（原生动物）。医学上重要的例子包括引起疟疾的疟原虫，它具有涉及人类和蚊子宿主的复杂生活史。另一个例子是变形虫，利用伪足运动并通过吞噬作用摄食。
5. Aseptic Technique and Culturing Microorganisms | 无菌技术与微生物培养
Aseptic technique is essential to prevent contamination by unwanted microorganisms and to ensure safety when handling pathogenic species. Key procedures include flaming the inoculating loop until red-hot, passing the necks of culture bottles through a flame, lifting the lid of a Petri dish at an angle, and working near a Bunsen burner to create an updraft. All equipment and media must be sterilised, usually by autoclaving at 121 °C for 15 minutes under high pressure.
无菌技术对于防止不受欢迎的微生物污染以及确保处理病原菌种时的安全至关重要。关键步骤包括将接种环灼烧至红热、将培养瓶口在火焰上快速通过、以一定角度打开培养皿盖，并在本生灯附近操作以形成上升气流。所有器械和培养基都必须灭菌，通常使用高压灭菌器在 121 °C 下高压处理 15 分钟。
Microorganisms are cultured on nutrient agar plates or in nutrient broth. For isolation of single colonies, the streak plate method is used to dilute the inoculum across the agar surface. After incubation, plates are often inverted to prevent condensation from dripping onto the colonies. Selective media can be used to favour the growth of specific microbes, a principle applied in genetic engineering when selecting for transformed bacteria.
微生物在营养琼脂平板或营养肉汤中培养。为了分离单菌落，采用划线平板法将接种物在琼脂表面稀释。培养结束后，平板通常倒置，以防止冷凝水滴落到菌落上。选择性培养基可用于促进特定微生物的生长，这一原理在基因工程中用于筛选转化后的细菌。
6. Microbial Growth Curve | 微生物的生长曲线
In a closed batch culture, the population growth of microorganisms follows a characteristic sigmoid curve with four distinct phases: lag phase, exponential (log) phase, stationary phase, and death phase. During the lag phase, cells adapt to the new environment and synthesise necessary enzymes; there is little or no increase in cell number. In the exponential phase, the population doubles at a constant rate under optimal conditions, with the growth rate being proportional to the current population size.
在封闭的分批培养中，微生物种群生长遵循一条特征性的 S 形曲线，包括四个明显的阶段：延滞期、指数（对数）期、稳定期和衰亡期。在延滞期，细胞适应新环境并合成必需的酶，细胞数目很少或没有增加。在指数期，种群在最优条件下以恒定的速率倍增，生长速率与当前种群大小成正比。
The stationary phase begins when nutrient levels decline and waste products accumulate; the rate of cell division equals the rate of cell death, causing the population size to plateau. Finally, the death phase occurs when nutrient depletion and toxic buildup lead to a decline in viable cell numbers. This death can follow an exponential pattern. These dynamics are crucial when designing industrial fermentation processes to harvest desired products during specific growth phases.
稳定期开始于营养水平下降和代谢废物积累时，细胞分裂速率等于细胞死亡速率，导致种群规模趋于平稳。最后，衰亡期在营养耗尽和有毒物质积累导致活细胞数下降时发生，该死亡也可能呈指数形式。这些动态对于设计工业发酵过程，在特定生长阶段收获目标产物至关重要。
Methods to measure microbial growth include direct cell counting with a haemocytometer, turbidity measurements using a spectrophotometer (optical density), dry mass determination, and viable plate counts. Each method has limitations, such as the inability of turbidity to distinguish live from dead cells.
测量微生物生长的方法包括用血球计数板直接计数、用分光光度计测量浊度（光密度）、测定干重，以及活菌平板计数。每种方法都有局限性，例如浊度法无法区分活细胞和死细胞。
7. Factors Affecting Microbial Growth | 影响微生物生长的因素
Environmental factors profoundly influence the growth and survival of microorganisms. Temperature is one of the most critical factors; each microbe has a minimum, optimum, and maximum growth temperature. Psychrophiles thrive at low temperatures, mesophiles at moderate temperatures (including human pathogens), and thermophiles at high temperatures. pH also affects enzyme activity and membrane integrity, with most bacteria preferring near-neutral pH, while fungi often grow best in slightly acidic conditions.
环境因素深刻影响着微生物的生长和生存。温度是最关键的因素之一；每种微生物都有最低、最适和最高生长温度。嗜冷菌在低温下生长，嗜温菌（包括人类病原菌）在中等温度下生长，嗜热菌在高温下生长。pH 也会影响酶活性和膜的完整性，大多数细菌偏爱接近中性的 pH，而真菌通常在微酸性条件下生长最好。
Oxygen availability classifies microorganisms as obligate aerobes (require O₂), obligate anaerobes (killed by O₂), facultative anaerobes (can grow with or without O₂, but better with O₂), and microaerophiles (require low O₂ tension). Water activity (aw) is another key factor; low water availability limits growth, which is why drying and salting preserve food. Nutrient concentration and the presence of essential growth factors also dictate the rate and extent of growth.
根据对氧气的需求，微生物可分为专性需氧菌（需要 O₂）、专性厌氧菌（O₂ 可致死）、兼性厌氧菌（有氧或无氧均可生长，有氧更好）和微需氧菌（需要低氧浓度）。水分活度是另一个关键因素；低水分利用率会抑制生长，这就是干燥和盐渍保藏食物的原理。营养浓度和必需生长因子的存在也决定了生长速率和程度。
8. Antibiotics: Mode of Action and Resistance | 抗生素：作用方式与抗性
Antibiotics are chemical substances produced by microorganisms that kill or inhibit the growth of other microorganisms. Penicillin, derived from the fungus Penicillium, acts by inhibiting transpeptidase enzymes that cross-link peptidoglycan chains in the bacterial cell wall. This weakens the wall, causing osmotic lysis in actively growing cells. Because human cells lack peptidoglycan, penicillin exhibits selective toxicity.
抗生素是由微生物产生的化学物质，能杀死或抑制其他微生物的生长。青霉素来源于真菌青霉，通过抑制转肽酶起作用，该酶催化细菌细胞壁肽聚糖链的交联。这会削弱细胞壁，导致活跃生长的细胞发生渗透裂解。由于人类细胞缺乏肽聚糖，青霉素表现出选择毒性。
Antibiotic resistance arises through genetic mutations or the acquisition of resistance genes via horizontal gene transfer (conjugation, transformation, or transduction). Resistance plasmids (R‑plasmids) often carry multiple resistance genes. Mechanisms of resistance include enzymatic degradation of the antibiotic (e.g. β‑lactamase breaking the β‑lactam ring of penicillin), alteration of the antibiotic target, and efflux pumps that expel the drug. The misuse and overuse of antibiotics accelerate the spread of resistant strains, making infections harder to treat.
抗生素抗性通过基因突变或通过水平基因转移（接合、转化或转导）获得抗性基因而产生。抗性质粒常携带多个抗性基因。抗性机制包括酶促降解抗生素（例如 β‑内酰胺酶破坏青霉素的 β‑内酰胺环）、改变抗生素作用靶点以及利用外排泵排出药物。抗生素的滥用和过度使用加速了耐药菌株的传播，使感染更加难以治疗。
To combat resistance, it is essential to complete prescribed antibiotic courses and use antibiotics only when necessary. In genetic engineering, antibiotic resistance genes serve as selectable markers to identify successfully transformed bacterial cells.
为了应对耐药性，必须完成处方的抗生素疗程，并且只在必要时使用抗生素。在基因工程中，抗生素抗性基因可作为选择标记，用于鉴定成功转化的细菌细胞。
9. Microorganisms in Biotechnology | 微生物在生物技术中的应用
Microorganisms are exploited in numerous biotechnological processes. The yeast Saccharomyces cerevisiae is used in baking (CO₂ causes dough to rise) and in alcoholic fermentation, where it converts glucose to ethanol and CO₂ under anaerobic conditions. Lactic acid bacteria ferment lactose to lactic acid, producing yoghurt and cheese; the lowered pH coagulates milk proteins and inhibits spoilage organisms.
微生物被应用于众多生物技术过程中。酵母 Saccharomyces cerevisiae 用于烘焙（CO₂ 使面团发酵膨胀）和酒精发酵，在厌氧条件下将葡萄糖转化为乙醇和 CO₂。乳酸菌将乳糖发酵成乳酸，生产酸奶和奶酪；降低的 pH 使乳蛋白凝固，并抑制腐败微生物。
One of the most powerful applications is the use of bacteria as hosts for recombinant DNA technology. The human insulin gene is isolated, inserted into a plasmid vector next to a promoter, and introduced into E. coli by transformation. Transformed bacteria are selected using antibiotic resistance markers, then grown in large fermenters. The expressed insulin is extracted, purified, and used to treat diabetes. This method is more effective and causes fewer immune reactions than previously used animal insulin.
最有力的应用之一是利用细菌作为重组 DNA 技术的宿主。将人胰岛素基因分离出来，插入到携带启动子的质粒载体中，通过转化导入大肠杆菌。利用抗生素抗性标记筛选转化细菌，然后在大型发酵罐中培养。表达的胰岛素经过提取和纯化，用于治疗糖尿病。这种方法比以往使用的动物胰岛素更有效，且引起的免疫反应更少。
Enzyme immobilisation using microorganisms or isolated enzymes improves industrial process efficiency. For example, immobilised lactase is used to produce lactose‑free milk. The immobilised enzyme is held in beads and can be reused, increasing the yield and reducing costs.
使用微生物或分离酶进行酶固定化提高了工业过程的效率。例如，固定化乳糖酶用于生产无乳糖牛奶。固定化的酶被包裹在珠子中，可重复使用，从而提高了产量并降低了成本。
10. Microorganisms and Infectious Diseases | 微生物与传染病
Many microorganisms are pathogens causing infectious diseases. CIE candidates must know the causative agents, modes of transmission, and control measures for key diseases. For cholera, the bacterium Vibrio cholerae is transmitted via contaminated water and food, producing a toxin that causes severe diarrhoea and dehydration. Prevention relies on clean water supplies and proper sanitation.
许多微生物是引起传染病的病原体。CIE 考生必须了解主要疾病的病原体、传播途径和防控措施。对于霍乱，霍乱弧菌通过受污染的水和食物传播，产生毒素导致严重腹泻和脱水。预防依赖于清洁的供水和良好的卫生设施。
Tuberculosis (TB) is caused by Mycobacterium tuberculosis, spread by airborne droplets. The bacteria survive inside macrophages and can remain latent. Antibiotic treatment requires long courses, and the emergence of multidrug‑resistant TB strains is a serious global health issue. Malaria is caused by the protoctist Plasmodium, transmitted by female Anopheles mosquitoes. The parasite invades red blood cells and liver cells, and control strategies include insecticide‑treated nets, antimalarial drugs, and mosquito vector control.
结核病由结核分枝杆菌引起，通过空气飞沫传播。该菌能在巨噬细胞内存活并可保持潜伏状态。抗生素治疗需要长期疗程，而耐多药结核菌株的出现是一个严重的全球健康问题。疟疾由原生生物疟原虫引起，通过雌性按蚊传播。寄生虫侵入红细胞和肝细胞，控制策略包括使用经杀虫剂处理的蚊帐、抗疟药物以及媒介蚊虫控制。
HIV/AIDS is caused by the Human Immunodeficiency Virus, a retrovirus that destroys helper T‑cells, weakening the immune system. Transmission occurs via bodily fluids such as blood, semen, and breast milk. Prevention emphasizes safe sex, sterile needles, and antiretroviral therapy to keep viral load low. Understanding the biology of these pathogens underpins the development of effective prevention and treatment strategies.
HIV/AIDS 由人类免疫缺陷病毒引起，它是一种逆转录病毒，破坏辅助 T 细胞，削弱免疫系统。传播途径包括血液、精液和母乳等体液。预防强调安全性行为、使用无菌针头以及通过抗逆转录病毒治疗将病毒载量维持在低水平。理解这些病原体的生物学是制定有效预防和治疗策略的基础。
Published by TutorHao | Biology Revision Series | aleveler.com
更多咨询请联系16621398022（同微信）
或截图后用「扫一扫 → 相册」选择本图