📚 Microorganisms: Key Exam Points for IGCSE CIE Biology | 微生物：IGCSE CIE 生物考点精讲
Microorganisms, also known as microbes, are tiny living organisms that are invisible to the naked eye. They include bacteria, viruses, fungi, and protoctists. In the IGCSE CIE Biology syllabus, understanding their structure, life processes, roles in disease, and applications in industry and the environment is essential. This revision guide breaks down every key point you need to master for the exam.
微生物，又称为微生物体，是肉眼看不见的微小生物。它们包括细菌、病毒、真菌和原生生物。在 IGCSE CIE 生物教学大纲中，理解它们的结构、生命过程、在疾病中的作用以及它们在工业与环境中的应用至关重要。这份复习指南将逐一解析你需要掌握的每个考点。
1. Introduction to Microorganisms | 微生物概述
Microorganisms are organisms that are too small to be seen without a microscope. They can be unicellular (single-celled) or multicellular, although most are unicellular. Some are beneficial, such as those used in baking, while others cause diseases. In the exam, you must be able to classify microbes into the main groups: bacteria, viruses, fungi, and protoctists.
微生物是那些不用显微镜就无法看到的生物体。它们可以是单细胞的或多细胞的，尽管大多数是单细胞的。有些是有益的，比如用于烘焙的微生物；而另一些则会引起疾病。在考试中，你必须能够将微生物分为主要类别：细菌、病毒、真菌和原生生物。
2. Types of Microorganisms | 微生物类型
There are four main types studied at IGCSE level: Bacteria (prokaryotic, single-celled with no nucleus), Viruses (non-living particles made of genetic material and protein coat), Fungi (eukaryotic, can be unicellular like yeast or multicellular like moulds with hyphae), and Protoctists (eukaryotic, unicellular organisms like Amoeba and Plasmodium).
在 IGCSE 层面学习的主要有四类：细菌（原核生物，单细胞，无细胞核）、病毒（非生命颗粒，由遗传物质和蛋白质外壳组成）、真菌（真核生物，可以是单细胞如酵母菌，或多细胞如具有菌丝的霉菌）和原生生物（真核的、单细胞生物，如变形虫和疟原虫）。
3. Structure of Bacteria, Viruses, Fungi, and Protoctists | 细菌、病毒、真菌和原生生物的结构
Bacteria possess a cell wall, cell membrane, cytoplasm, circular DNA (plasmid), and sometimes flagella. They lack a true nucleus and membrane‑bound organelles. Viruses are much smaller and contain either DNA or RNA surrounded by a capsid. Fungi, such as yeast, have a nucleus, cytoplasm, cell membrane, and a cell wall made of chitin. Hyphal fungi consist of thread‑like structures called hyphae, which form a mycelium. Protoctists like Amoeba have a nucleus, cytoplasm and often contractile vacuoles or chloroplasts.
细菌具有细胞壁、细胞膜、细胞质、环状 DNA（质粒），有时还有鞭毛。它们没有真正的细胞核和膜结合的细胞器。病毒小得多，包含 DNA 或 RNA，并被衣壳包裹。真菌，如酵母菌，有细胞核、细胞质、细胞膜以及由几丁质构成的细胞壁。菌丝真菌由称为菌丝的线状结构组成，这些菌丝形成菌丝体。原生生物如变形虫具有细胞核、细胞质，通常还有伸缩泡或叶绿体。
| Feature | Bacteria | Virus | Fungi (yeast) | Protoctists |
|---|---|---|---|---|
| Nucleus | Absent | None | Present | Present |
| Cell wall | Peptidoglycan | No | Chitin | Cellulose (in some) |
| Genetic material | Circular DNA | DNA or RNA | Linear DNA in nucleus | DNA in nucleus |
Remember that only bacteria and viruses are often tested on structural differences: bacteria are cells, whereas viruses are not considered living.
请记住，考试中常考结构差异：细菌是细胞，而病毒不被视为生物。
4. Bacterial Growth and Reproduction | 细菌的生长和繁殖
Bacteria reproduce asexually by binary fission. Under ideal conditions, one bacterium splits into two, doubling the population at regular intervals. The growth curve of a bacterial population consists of four phases: lag phase (adaptation), log/exponential phase (rapid division), stationary phase (nutrient depletion, death rate equals division rate), and death phase (population declines). Exams often ask you to describe or interpret a graph showing these phases.
细菌通过二分裂进行无性繁殖。在理想条件下，一个细菌分裂成两个，每隔一段时间种群数量翻倍。细菌种群生长曲线包括四个阶段：迟缓期（适应）、对数期/指数期（快速分裂）、稳定期（营养耗尽，死亡率等于分裂率）和死亡期（种群下降）。考试经常要求描述或解读显示这些阶段的图表。
The time taken for a population to double is called the generation time. At IGCSE level, you may need to calculate the number of bacteria after a given number of divisions using the formula 2n, where n is the number of generations.
种群数量翻倍所需的时间叫做代时。在 IGCSE 水平，你可能需要使用公式 2n 来计算经过一定次数分裂后的细菌数量，其中 n 是代数。
5. Conditions Affecting Microbial Growth | 影响微生物生长的条件
Microbial growth is influenced by temperature, pH, oxygen availability, moisture, and nutrient supply. Most pathogenic bacteria prefer body temperature (37°C) and a neutral pH. Refrigeration slows down enzyme activity, thus slowing reproduction. Extreme heat denatures proteins and kills most microbes. Lack of water (e.g. drying) inhibits growth because microorganisms need water for metabolic reactions. Anaerobic respiration allows some microbes to survive without oxygen.
微生物生长受温度、pH、氧气供应、水分和营养供应的影响。大多数致病菌喜欢体温（37°C）和中性 pH。冷藏会减慢酶活性，从而减缓繁殖。极端高温会使蛋白质变性，杀死大多数微生物。缺水（例如干燥）会抑制生长，因为微生物进行代谢反应需要水。无氧呼吸使某些微生物能在没有氧气的环境下生存。
6. Food Production Using Microorganisms | 利用微生物生产食品
Yeast (a fungus) is used in bread making and alcohol production. In bread, yeast respires anaerobically, producing carbon dioxide that makes the dough rise. In brewing, yeast ferments sugars to produce ethanol. The word equation for anaerobic respiration in yeast is: Glucose → Ethanol + Carbon dioxide (C₆H₁₂O₆ → 2 C₂H₅OH + 2 CO₂). Bacteria are used to make yogurt and cheese. Lactobacillus bacteria convert lactose (milk sugar) into lactic acid, which coagulates milk proteins and gives yogurt its sour taste.
酵母菌（一种真菌）用于面包制作和酒精生产。在面包中，酵母进行无氧呼吸，产生二氧化碳使面团膨胀。在酿酒中，酵母将糖发酵产生乙醇。酵母无氧呼吸的文字方程式是：葡萄糖 → 乙醇 + 二氧化碳（C₆H₁₂O₆ → 2 C₂H₅OH + 2 CO₂）。细菌被用来制作酸奶和奶酪。乳酸杆菌将乳糖（牛奶中的糖）转化为乳酸，乳酸使乳蛋白凝固，并赋予酸奶酸味。
Anaerobic respiration in yeast: Glucose → Ethanol + Carbon dioxide
You must be able to link the products of fermentation to their commercial uses: ethanol for alcoholic drinks, CO₂ for bread texture, lactic acid for yogurt preservation.
你必须能够将发酵产物与其商业用途联系起来：乙醇用于酒精饮料，CO₂ 用于面包质地，乳酸用于酸奶保存。
7. Food Spoilage and Preservation | 食品腐败与保藏
Microorganisms cause food spoilage by breaking down organic substances, producing unpleasant odours, slime, or toxins. Factors that accelerate spoilage include warmth, moisture, and contact with air. Food preservation methods work by inhibiting microbial growth. These include: refrigeration/freezing (slow enzyme action), heating (pasteurisation, sterilisation), drying/dehydration, adding salt or sugar (osmotic effect draws water out of microbes), pickling with vinegar (low pH), and canning (sealing in sterile conditions).
微生物通过分解有机物质导致食品腐败，产生令人不快的气味、粘液或毒素。加速腐败的因素包括温暖、潮湿和与空气接触。食品保藏方法通过抑制微生物生长来起作用。这些方法包括：冷藏/冷冻（减慢酶作用）、加热（巴氏消毒法、灭菌）、干燥/脱水、加盐或糖（渗透作用将水从微生物中吸出）、用醋腌制（低 pH）以及罐装（在无菌条件下密封）。
8. Infectious Diseases Caused by Microorganisms | 微生物引起的传染病
Microorganisms that cause disease are called pathogens. Bacteria cause diseases such as cholera (Vibrio cholerae), tuberculosis (Mycobacterium tuberculosis), and food poisoning (Salmonella). Viruses cause influenza, HIV/AIDS, and COVID‑19. Fungi cause athlete’s foot and ringworm. Protoctists cause malaria (Plasmodium). You should know the mode of transmission and methods of control for at least one example: for instance, cholera is transmitted via contaminated water, controlled by clean water supply and sanitation. Malaria is transmitted by female Anopheles mosquitoes and controlled by mosquito nets, insecticides, and antimalarial drugs.
引起疾病的微生物叫病原体。细菌引起霍乱（霍乱弧菌）、结核病（结核分枝杆菌）和食物中毒（沙门氏菌）。病毒引起流感、HIV/艾滋病和 COVID‑19。真菌引起脚癣和癣菌病。原生生物引起疟疾（疟原虫）。你应该至少掌握一种疾病的传播方式和控制方法：例如霍乱通过污染的水传播，通过清洁水源和卫生设施来控制；疟疾由雌性按蚊传播，通过蚊帐、杀虫剂和抗疟药来控制。
- Cholera: Bacterium, water‑borne, controlled by clean water.
- 霍乱：细菌，通过水传播，通过清洁水源控制。
- Malaria: Protoctist, vector‑borne (mosquito), controlled by mosquito nets.
- 疟疾：原生生物，媒介传播（蚊子），通过蚊帐控制。
9. Body Defence Against Pathogens | 人体对抗病原体的防御
The body has physical barriers such as skin (waterproof, protein keratin) and mucus in the airways (traps pathogens, cilia sweep them away). Chemical barriers include stomach acid (kills bacteria) and lysozyme in tears. If pathogens enter, the immune system responds with white blood cells: phagocytes engulf and digest pathogens (phagocytosis), and lymphocytes produce specific antibodies that bind to antigens, neutralising toxins or marking pathogens for destruction. This antibody response is specific and generates memory cells for long‑term immunity.
人体具有物理屏障，如皮肤（防水、角蛋白）和呼吸道粘液（捕获病原体，纤毛将其清除）。化学屏障包括胃酸（杀死细菌）和眼泪中的溶菌酶。一旦病原体进入，免疫系统以白细胞应对：吞噬细胞吞噬并消化病原体（吞噬作用），淋巴细胞产生特异性抗体，与抗原结合，中和毒素或标记病原体供消灭。这种抗体反应是特异性的，并产生记忆细胞以提供长期免疫。
Vaccination exploits this memory response by introducing harmless forms of the pathogen, prompting antibody production and memory cells without causing disease.
疫苗接种利用这一记忆反应，通过引入无害形式的病原体，促使抗体产生和记忆细胞形成而不会引发疾病。
10. Antibiotics and Antibiotic Resistance | 抗生素与抗生素耐药性
Antibiotics are drugs that kill bacteria or prevent their growth, but they are not effective against viruses. Penicillin, discovered by Alexander Fleming, destroys bacterial cell walls. Overuse of antibiotics exerts a selection pressure: resistant mutants survive and multiply, passing resistance genes via plasmids. This leads to antibiotic‑resistant strains such as MRSA. To combat this, doctors prescribe antibiotics only when necessary and patients must complete the full course to kill all bacteria and prevent survivors from developing resistance.
抗生素是杀死细菌或抑制其生长的药物，但对病毒无效。青霉素由弗莱明发现，能破坏细菌细胞壁。过度使用抗生素会产生选择压力：抗性突变体能存活并繁殖，通过质粒传递抗性基因。这导致耐抗生素菌株的出现，如 MRSA。为了应对，医生只在必要时处方抗生素，患者必须完成整个疗程以杀死所有细菌，防止幸存者产生抗性。
11. Decomposition and Nutrient Cycling | 分解和养分循环
Many bacteria and fungi are decomposers (saprotrophs). They secrete extracellular enzymes onto dead organic matter, breaking it down into simpler soluble substances, which they then absorb. This process releases mineral ions (e.g. nitrates, phosphates) back into the soil, making them available to plants. Decomposers thus play a vital role in the carbon cycle and nitrogen cycle. In the nitrogen cycle, decomposers convert proteins and urea into ammonium compounds (ammonification). Nitrifying bacteria then convert ammonium to nitrites and nitrates (nitrification).
许多细菌和真菌是分解者（腐生生物）。它们向死有机物分泌胞外酶，将其分解为简单的可溶物质，然后吸收。这一过程将矿物质离子（如硝酸盐、磷酸盐）释放回土壤，使植物能够获取。因此，分解者在碳循环和氮循环中起着至关重要的作用。在氮循环中，分解者将蛋白质和尿素转化为铵化合物（氨化作用）。然后硝化细菌将铵转化为亚硝酸盐和硝酸盐（硝化作用）。
Exam tip: You may need to distinguish between saprotrophic nutrition (extracellular digestion) and the role of decay in recycling nutrients.
考试提示：你可能需要区分腐生营养（胞外消化）和腐烂在养分循环中的作用。
12. Practical Investigations: Culturing Microorganisms | 实践研究：微生物培养
To study bacteria, they must be cultured on nutrient agar in sterile Petri dishes. Aseptic technique is vital: sterilise work surfaces, flame the inoculating loop, secure the lid with tape (but not completely to allow oxygen for aerobes), and keep dishes upside down to prevent condensation. Incubate at a safe temperature (usually not above 25°C in schools) to avoid cultivating pathogens. Colonies grown from a single bacterium are identical clones. You should be able to calculate colony‑forming units (CFU) per cm³ from a dilution series, and interpret zones of inhibition around antibiotic discs to assess antibiotic effectiveness.
为了研究细菌，必须在无菌培养皿中的营养琼脂上培养。无菌操作至关重要：对工作台面消毒、灼烧接种环、用胶带固定盖（但不要完全密封以便需氧菌获得氧气），并倒置培养皿防止冷凝水。在安全温度（学校通常不超过 25°C）下培养，避免培养病原体。由单个细菌长出的菌落是相同的克隆。你应该能够通过稀释系列计算每 cm³ 的菌落形成单位（CFU），并解读抗生素纸片周围的抑菌圈，以评估抗生素的有效性。
Remember that in the exam, you might be asked to suggest how to test the effectiveness of different disinfectants or antibiotics using filter paper discs soaked in the solution and observing clear zones.
请记住，在考试中可能要求你提出如何用浸泡过溶液的滤纸片测试不同消毒剂或抗生素的效果，并观察透明区域。
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