📚 A-Level Biology: Microorganisms Key Points Revision | A-Level 生物：微生物 考点精讲
Microorganisms are a core topic in A-Level Biology, linking cell structure, biochemistry, disease, and ecology. This article distils essential concepts from classification to immune defence, providing bilingual explanations to reinforce understanding and exam preparedness.
微生物是A-Level生物学的核心主题，连接了细胞结构、生物化学、疾病与生态学。本文提炼了从分类到免疫防御的关键概念，提供双语讲解以加强理解，助力备考。
1. Classification and Types of Microorganisms | 微生物的分类与类型
Microorganisms, or microbes, are organisms too small to be seen with the naked eye. They include bacteria, viruses, fungi, and protists. Viruses are acellular and considered non-living by most definitions; they require host cells to replicate. Bacteria are prokaryotes; fungi and protists are eukaryotes.
微生物是肉眼看不见的生物体，包括细菌、病毒、真菌和原生生物。病毒是非细胞结构，在多数定义下被视为非生命体，需要宿主细胞才能复制。细菌是原核生物；真菌和原生生物是真核生物。
Bacteria are classified by cell shape: coccus (spherical), bacillus (rod-shaped), spirillum (spiral), and vibrio (comma-shaped). They can also be grouped by cell wall composition (Gram-positive vs. Gram-negative) and metabolic characteristics (aerobic, anaerobic, facultative).
细菌可按细胞形态分类：球菌（球形）、杆菌（杆状）、螺菌（螺旋形）和弧菌（逗号形）。也可根据细胞壁组成（革兰氏阳性与阴性）以及代谢特征（需氧、厌氧、兼性）分组。
Fungi range from unicellular yeasts to multicellular moulds and mushrooms; they possess chitinous cell walls. Protists are a diverse group, including protozoa (animal-like, e.g. Plasmodium) and algae (plant-like, e.g. Chlorella).
真菌从单细胞的酵母到多细胞的霉菌和蘑菇；其细胞壁含有几丁质。原生生物类群多样，包括原生动物（类动物，如疟原虫）和藻类（类植物，如小球藻）。
2. Bacterial Cell Structure and Function | 细菌细胞结构与功能
Prokaryotic cells lack a membrane-bound nucleus and organelles. Their genetic material is a single, circular DNA molecule found in the nucleoid region. Many bacteria also possess small circular DNA plasmids that can carry antibiotic resistance genes.
原核细胞缺乏有膜包被的细胞核和细胞器。其遗传物质为存在于拟核区的单个环状DNA分子。许多细菌还含有小型的环状DNA质粒，可携带抗生素抗性基因。
The bacterial cell wall contains peptidoglycan (murein), a polymer of sugars and amino acids that provides structural support and prevents osmotic lysis. Gram-positive bacteria have a thick peptidoglycan layer; Gram-negative bacteria have a thin layer sandwiched between an inner and an outer membrane, the outer containing lipopolysaccharides (LPS).
细菌细胞壁含有肽聚糖（胞壁质），是一种由糖和氨基酸组成的多聚物，提供结构支撑并防止渗透裂解。革兰氏阳性菌有厚肽聚糖层；革兰氏阴性菌的肽聚糖较薄，位于内外膜之间，外膜含有脂多糖(LPS)。
Bacteria may possess a polysaccharide capsule that aids in evading phagocytosis, flagella for motility, and pili (fimbriae) for attachment or conjugation. The 70S ribosome (composed of 50S and 30S subunits) is the site of protein synthesis and a target for many antibiotics.
细菌可能具有多糖荚膜，帮助逃避吞噬作用；鞭毛用于运动；菌毛（纤毛）用于附着或接合。70S核糖体（由50S和30S亚基组成）是蛋白质合成的场所，也是许多抗生素的作用靶点。
3. Viruses: Structure and Replication | 病毒的结构与复制
Viruses are obligate intracellular parasites composed of a nucleic acid core (DNA or RNA) surrounded by a protein capsid. Some viruses have a lipid envelope derived from the host cell membrane, studded with glycoprotein spikes that mediate host recognition.
病毒是专性细胞内寄生物，由核酸核心（DNA或RNA）和蛋白质衣壳组成。某些病毒具有来自宿主细胞膜的脂质包膜，包膜上嵌有糖蛋白刺突，介导宿主识别。
The viral replication cycle includes attachment, penetration, uncoating, synthesis of viral components, assembly, and release. Bacteriophages may follow a lytic cycle (host cell lysis) or a lysogenic cycle (viral genome integrates into host DNA as a prophage).
病毒的复制周期包括附着、侵入、脱壳、病毒组分的合成、装配和释放。噬菌体可遵循裂解周期（宿主细胞裂解）或溶原周期（病毒基因组以前噬菌体形式整合进宿主DNA）。
RNA retroviruses, such as HIV, carry the enzyme reverse transcriptase, which synthesises DNA from the RNA template. This DNA integrates into the host genome, establishing a persistent infection that can later produce new virions.
RNA逆转录病毒，如HIV，携带逆转录酶，能以RNA为模板合成DNA。该DNA整合到宿主基因组，建立持续感染，之后可产生新的病毒颗粒。
Viral specificity is determined by attachment proteins that recognise specific receptor molecules on host cells. This explains tissue tropism and host range.
病毒的特异性由其附着蛋白决定，这些蛋白识别宿主细胞表面的特定受体分子，从而解释组织嗜性和宿主范围。
4. Fungi and Protists | 真菌与原生生物
Fungi are eukaryotic, heterotrophic organisms with chitin cell walls. Their basic structural unit is the hypha (plural hyphae); a network of hyphae forms a mycelium. In multicellular fungi, hyphae may be septate (divided by cross-walls) or aseptate (coenocytic).
真菌是真核、异养生物，细胞壁含几丁质。其基本结构单位为菌丝；菌丝网络构成菌丝体。在多细胞真菌中，菌丝可有隔（被横壁分隔）或无隔（多核体）。
Fungi reproduce both sexually and asexually via spores, which are often dispersed by wind or water. Yeasts reproduce asexually by budding. Fungi are saprotrophs that secrete extracellular enzymes, then absorb the digested nutrients — this is known as extracellular digestion.
真菌通过孢子进行有性和无性繁殖，孢子常由风或水传播。酵母通过出芽进行无性繁殖。真菌为腐生生物，分泌胞外酶，然后吸收消化后的营养物质——这称为胞外消化。
Protists include protoctists such as Plasmodium, the causative agent of malaria, which has a complex life cycle involving a mosquito vector and human host. Algal protists are photosynthetic and important primary producers in aquatic ecosystems.
原生生物包括原生动物，例如疟原虫是疟疾的病原体，具有复杂的生命周期，涉及蚊媒介和人类宿主。藻类原生生物可进行光合作用，是水生生态系统中的重要初级生产者。
5. Microbial Growth and Reproduction | 微生物的生长与繁殖
Bacteria primarily reproduce by binary fission, an asexual process in which a single cell divides into two identical daughter cells. Under optimal conditions, this can result in exponential population growth, with a generation time as short as 20 minutes for species like Escherichia coli.
细菌主要通过二分裂进行无性繁殖，一个母细胞分裂成两个相同的子细胞。在最优条件下，种群可呈指数增长，像大肠杆菌这样的物种，代时短至20分钟。
The bacterial growth curve in a batch culture consists of four phases: lag phase (cells adapt, no increase in number), exponential (log) phase (rapid doubling), stationary phase (nutrients deplete, wastes accumulate, death rate equals birth rate), and death phase (cells die faster than they are replaced).
分批培养中细菌的生长曲线分为四个阶段：延滞期（细胞适应，数量不增）、指数期（快速倍增）、稳定期（营养耗尽，废物积累，死亡率等于出生率）和死亡期（死亡速率超过新生速率）。
Factors affecting microbial growth include temperature, pH, oxygen availability, water activity, and nutrient supply. Obligate aerobes require oxygen; obligate anaerobes are killed by oxygen; facultative anaerobes can switch metabolism.
影响微生物生长的因素包括温度、pH、氧气可用性、水分活度和营养供应。专性需氧菌需要氧气；专性厌氧菌会被氧气杀死；兼性厌氧菌可切换代谢方式。
6. Nutritional Patterns in Microbes | 微生物的营养模式
Microorganisms exhibit diverse nutritional modes. Autotrophs synthesise their own organic molecules: photoautotrophs (e.g. cyanobacteria, algae) use light energy; chemoautotrophs (e.g. nitrifying bacteria) obtain energy from oxidising inorganic compounds like NH₃ or Fe²⁺.
微生物展现出多样的营养模式。自养生物合成自身有机物：光能自养型（如蓝细菌、藻类）利用光能；化能自养型（如硝化细菌）通过氧化无机物如NH₃或Fe²⁺获得能量。
Heterotrophs obtain carbon from organic compounds. Saprotrophs decompose dead organic matter; parasites derive nutrients from a living host, causing harm; mutualists engage in mutually beneficial relationships, such as Rhizobium bacteria in legume root nodules fixing nitrogen for the plant in exchange for carbohydrates.
异养生物从有机物中获取碳。腐生菌分解死亡有机质；寄生物从活宿主获取营养并造成伤害；互利共生者之间互相受益，如豆科植物根瘤中的根瘤菌为植物固氮，以换取碳水化合物。
Obligate intracellular parasites, like viruses and some bacteria (e.g. Chlamydia), rely entirely on the host’s metabolic machinery for energy and biomolecule synthesis, unable to grow outside the host cell.
专性胞内寄生物，如病毒和某些细菌（如衣原体），完全依赖宿主的代谢机器来获取能量和合成生物分子，无法在宿主细胞外生长。
7. Pathogenicity and Mechanisms of Disease | 致病机理
Pathogenicity refers to a microorganism’s ability to cause disease. Virulence factors include adhesion molecules (pili, capsules), enzymes that damage host tissues (coagulase, collagenase), and toxins. Endotoxins are lipopolysaccharide components of Gram-negative outer membranes that trigger inflammation; exotoxins are soluble proteins secreted by bacteria that target specific cells.
致病性指微生物引起疾病的能力。毒力因子包括黏附分子（菌毛、荚膜）、破坏宿主组织的酶（凝固酶、胶原酶）以及毒素。内毒素是革兰氏阴性菌外膜的脂多糖成分，能触发炎症；外毒素是细菌分泌的可溶性蛋白，靶向特定细胞。
Pathogens can damage host directly by destroying cells and tissues, or indirectly by triggering excessive immune responses. For example, the bacterium Mycobacterium tuberculosis survives inside macrophages, forming granulomas, while Vibrio cholerae causes severe watery diarrhoea via the cholera toxin that disrupts ion transport in intestinal epithelium.
病原体可直接破坏细胞和组织，或间接引发过度的免疫反应。例如，结核分枝杆菌在巨噬细胞内存活并形成肉芽肿；霍乱弧菌通过霍乱毒素破坏肠上皮离子转运，引起严重水样腹泻。
The ability to form biofilms — communities of microorganisms encased in a self-produced extracellular matrix — enhances resistance to antibiotics and host defences. Biofilms are implicated in chronic infections and medical device contamination.
形成生物被膜的能力——由自身产生的胞外基质包裹的微生物群落——增强了对抗生素和宿主防御的抵抗。生物被膜与慢性感染和医疗器械污染有关。
8. Antibiotics and Antimicrobial Resistance | 抗生素与耐药性
Antibiotics are chemical substances that kill or inhibit the growth of bacteria. Bactericidal antibiotics kill bacteria directly (e.g. penicillin); bacteriostatic antibiotics inhibit growth, allowing the immune system to eliminate the pathogen (e.g. tetracycline). Antibiotics target structures or processes unique to prokaryotes, such as cell wall synthesis, 70S ribosomes, and DNA gyrase.
抗生素是杀死或抑制细菌生长的化学物质。杀菌性抗生素直接杀死细菌（如青霉素）；抑菌性抗生素抑制生长，由免疫系统清除病原体（如四环素）。抗生素靶向原核生物独有的结构或过程，如细胞壁合成、70S核糖体和DNA旋转酶。
Antimicrobial resistance arises through mutations or horizontal gene transfer (transformation, transduction, conjugation). Resistance plasmids (R-factors) often carry multiple resistance genes. Key resistance mechanisms include enzymatic degradation of the drug (e.g. β-lactamase), alteration of the drug target, reduced permeability, and efflux pumps.
抗菌素耐药性通过突变或水平基因转移（转化、转导、接合）产生。耐药性质粒（R因子）常携带多重耐药基因。主要耐药机制包括：酶降解药物（如β-内酰胺酶）、改变药物靶点、降低通透性和外排泵。
Overuse and misuse of antibiotics in medicine and agriculture accelerate the spread of resistant strains, leading to the global challenge of multi-drug-resistant organisms like MRSA (methicillin-resistant Staphylococcus aureus). Prudent antibiotic stewardship is essential.
医疗和农业中抗生素的过度使用和滥用加速了耐药菌株的传播，导致多重耐药菌如MRSA（耐甲氧西林金黄色葡萄球菌）的全球性挑战。谨慎使用抗生素至关重要。
9. Beneficial Roles of Microorganisms | 微生物的有益作用
Microorganisms are indispensable to ecosystem nutrient cycling. Decomposers break down organic matter, releasing carbon, nitrogen, and phosphorus. Nitrogen-fixing bacteria (e.g. Rhizobium, Azotobacter) convert atmospheric N₂ into ammonia; nitrifying bacteria oxidise ammonia to nitrate, which plants can absorb. Denitrifying bacteria return nitrogen to the atmosphere.
微生物对生态系统养分循环不可或缺。分解者分解有机质，释放碳、氮、磷。固氮菌（如根瘤菌、固氮菌）将大气N₂转化为氨；硝化细菌将氨氧化为硝酸盐，供植物吸收；反硝化细菌将氮送回大气。
In biotechnology, microorganisms are used to produce bread, yogurt, cheese, alcoholic beverages, and fermented foods via anaerobic respiration or fermentation. Recombinant DNA technology employs bacteria to produce human insulin, growth hormone, and enzymes.
在生物技术中，微生物用于通过厌氧呼吸或发酵生产面包、酸奶、奶酪、酒精饮料和发酵食品。重组DNA技术利用细菌生产人胰岛素、生长激素和酶。
Wastewater treatment relies on aerobic and anaerobic microbes to degrade organic pollutants. Bioremediation uses microorganisms to clean up oil spills and toxic chemicals. Genetically engineered microbes can synthesise biofuels and biodegradable plastics.
污水处理依赖好氧和厌氧微生物降解有机污染物。生物修复利用微生物清理石油泄漏和有毒化学品。基因工程微生物可以合成生物燃料和可生物降解塑料。
10. Laboratory Cultivation and Enumeration | 实验室培养与计数
To study microorganisms, aseptic technique must be used to prevent contamination and ensure safety. Equipment is sterilised using an autoclave (high-pressure steam at 121 °C) or by flaming. Cultures on solid media (agar plates) or in liquid broth are incubated at controlled temperatures.
研究微生物时必须采用无菌技术，以防止污染并确保安全。设备用高压灭菌器（121 °C高压蒸汽）或灼烧灭菌。固体培养基（琼脂平板）或液体培养基于受控温度下培养。
Selective media encourage the growth of certain microbes while inhibiting others; differential media allow visual distinction of species based on metabolic traits. Total viable count can be determined by serial dilution and spread plating, counting colony-forming units (CFU). Each visible colony typically arises from a single cell or clump.
选择性培养基促进特定微生物生长同时抑制其他菌；鉴别培养基基于代谢特性使不同菌种可见区分。总活菌数可通过连续稀释和涂布平板计数菌落形成单位(CFU)获得，每个可见菌落通常源于一个单细胞或细胞团。
A hemocytometer or turbidimetry (measuring absorbance with a spectrophotometer) can estimate total cell count, though these methods do not distinguish live from dead cells. Growth can be monitored using log-transformed plate counts to construct growth curves.
血细胞计数板或比浊法（用分光光度计测吸光度）可估算总细胞数，但这些方法不能区分活细胞和死细胞。可通过对数转化后的平板计数监测生长，绘制生长曲线。
11. Transmission and Control of Infectious Diseases | 传染病的传播与控制
Infectious diseases can be transmitted directly (contact, droplet inhalation, sexual contact) or indirectly (via fomites, food, water, vectors). Vectors may be mechanical (passive carriage, e.g. fly on faeces) or biological (the pathogen reproduces inside the vector, e.g. mosquito for malaria).
传染病可直接传播（接触、飞沫吸入、性接触）或间接传播（通过污染物、食物、水、媒介生物）。媒介可是机械性的（被动携带，如苍蝇沾染粪便）或生物性的（病原体在媒介体内繁殖，如按蚊传播疟疾）。
Control strategies include hygiene (handwashing, sanitation), quarantine and isolation, safe food and water practices, vector control (insecticides, bed nets), vaccination, and public health education. The World Health Organisation emphasises a One Health approach linking human, animal, and environmental health.
控制策略包括卫生（洗手、环境卫生）、检疫隔离、安全食品与用水、媒介控制（杀虫剂、蚊帐）、疫苗接种和公共卫生教育。世界卫生组织强调“一体化健康”方针，关联人类、动物和环境健康。
Epidemiology studies the patterns, causes, and effects of disease in populations. Key parameters include basic reproduction number (R₀), incidence, prevalence, and mortality rate. Contact tracing and screening help break chains of transmission.
流行病学研究疾病在人群中的模式、原因和影响。关键参数包括基本再生数(R₀)、发病率、患病率和死亡率。接触者追踪和筛查有助于阻断传播链。
12. Immune Defences against Microbes | 机体对微生物的免疫防御
The human body has innate (non-specific) defences: physical barriers (skin, mucous membranes), chemical barriers (lysozyme in tears, stomach acid), phagocytes (neutrophils, macrophages) that engulf pathogens, and the inflammatory response that recruits immune cells and causes vasodilation.
人体具有先天（非特异性）防御：物理屏障（皮肤、黏膜）、化学屏障（眼泪中的溶菌酶、胃酸）、吞噬病原体的吞噬细胞（中性粒细胞、巨噬细胞）以及炎症反应，后者招募免疫细胞并导致血管舒张。
Adaptive (specific) immunity involves lymphocytes. B lymphocytes produce antibodies that neutralise toxins, agglutinate pathogens, and mark them for phagocytosis (opsonisation). T helper cells activate other immune cells; cytotoxic T cells kill infected host cells.
适应性（特异性）免疫涉及淋巴细胞。B淋巴细胞产生抗体，可中和毒素、凝集病原体并标记它们供吞噬（调理作用）。辅助性T细胞激活其他免疫细胞；细胞毒性T细胞杀死受感染的宿主细胞。
After an infection or vaccination, memory B cells and memory T cells persist for years, conferring long-term immunity. Herd immunity arises when a high proportion of a population is immune, protecting susceptible individuals by reducing pathogen circulation.
感染或疫苗接种后，记忆B细胞和记忆T细胞可存留多年，赋予长期免疫力。当一个群体中高比例个体具有免疫力时，通过减少病原体循环保护易感者，即产生群体免疫。
Vaccines typically contain inactivated pathogens, attenuated strains, or purified antigens (including mRNA or viral vectors) that stimulate an immune response without causing disease, establishing immunological memory.
疫苗通常含有灭活病原体、减毒株或纯化抗原（包括mRNA或病毒载体），可在不致病的情况下激发免疫应答，建立免疫记忆。
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