📚 A-Level Biology: Tuberculosis – Causes, Transmission and Prevention | A-Level 生物：结核病的病因、传播与防治
Tuberculosis (TB) is a bacterial infection that primarily affects the lungs, though it can also involve other organs. It remains one of the top infectious killers worldwide, making it a key topic in A-Level Biology for both its microbiology and public health significance.
结核病（TB）是一种主要影响肺部的细菌感染，但也可能累及其他器官。它仍然是全球最主要的感染性致死疾病之一，因此成为 A-Level 生物学中兼具微生物学与公共卫生意义的重要考点。
1. What Is Tuberculosis? | 什么是结核病？
Tuberculosis is a chronic infectious disease caused by the bacterium Mycobacterium tuberculosis. In most cases, the infection is contained by the immune system, leading to a latent state. However, when immunity is compromised, the bacterium can multiply and cause active disease with severe tissue damage.
结核病是由结核分枝杆菌（Mycobacterium tuberculosis）引起的慢性传染病。多数情况下，感染会被免疫系统控制，从而进入潜伏状态。然而，当免疫力下降时，细菌可大量繁殖并引起活动性疾病，造成严重的组织损伤。
The disease is characterised by the formation of tubercles (granulomas) in infected tissues, caseous necrosis, and fibrosis. Pulmonary tuberculosis is the most common form, but extrapulmonary TB can affect lymph nodes, pleura, bones, kidneys, and the meninges.
该病的特征是感染组织中形成结核结节（肉芽肿）、干酪样坏死和纤维化。肺结核是最常见的形式，但肺外结核可累及淋巴结、胸膜、骨骼、肾脏和脑膜。
2. The Causative Agent | 病原体：结核分枝杆菌
Mycobacterium tuberculosis is an obligate aerobic rod-shaped bacterium. Its most notable feature is its cell wall, which is rich in mycolic acids and lipids. This unusual structure gives the bacterium an acid-fast staining property and provides resistance to many chemical disinfectants and antibiotics.
结核分枝杆菌是一种专性需氧的杆状细菌。其最显著的特征是富含分枝菌酸和脂质的细胞壁。这种特殊的结构使细菌具有抗酸染色特性，并对许多化学消毒剂和抗生素具有抵抗力。
Key features of the pathogen include:
该病原体的关键特征包括：
- Slow growth: Its generation time is 15–20 hours, far longer than most bacteria (e.g. E. coli divides every 20 minutes). This slows both disease progression and laboratory culture.
- 中文：生长缓慢：其倍增时间约为 15–20 小时，远长于大多数细菌（例如大肠杆菌每 20 分钟分裂一次）。这减慢了疾病进展和实验室培养速度。
- Obligate aerobe: It thrives in oxygen-rich tissues, especially the lung apices, which explains the characteristic upper-lobe involvement in pulmonary TB.
- 中文：专性需氧：它喜好富氧组织，尤其是肺尖部，这解释了肺结核好发于上叶的特点。
- Intracellular survival: It can survive and multiply inside macrophages by inhibiting phagolysosome fusion, allowing it to evade immune destruction.
- 中文：胞内生存：它能够通过抑制吞噬溶酶体融合在巨噬细胞内生存和繁殖，从而逃避免疫杀伤。
3. Causes and Risk Factors | 病因与危险因素
The direct cause of TB is infection with Mycobacterium tuberculosis. However, infection does not always lead to disease. The outcome depends on bacterial virulence, the host immune status, and environmental factors.
结核病的直接病因是感染结核分枝杆菌。然而，感染并不总是导致发病。最终结果取决于细菌毒力、宿主免疫状态和环境因素。
Major risk factors for progression from latent to active TB include:
从潜伏感染进展为活动性结核的主要危险因素包括：
- Immunosuppression: HIV infection, especially with CD4⁺ T-cell counts below 200 cells/µL, greatly increases reactivation risk.
- 中文：免疫抑制：HIV 感染，尤其是 CD4⁺ T 细胞计数低于 200 个/µL 时，大大增加复燃风险。
- Malnutrition: Protein-energy malnutrition impairs cell-mediated immunity, particularly T-cell function.
- 中文：营养不良：蛋白质-能量营养不良损害细胞介导免疫，尤其是 T 细胞功能。
- Diabetes mellitus: Hyperglycaemia impairs macrophage and neutrophil activity, raising TB risk about two- to three-fold.
- 中文：糖尿病：高血糖损害巨噬细胞和中性粒细胞活性，使结核病风险增加约 2–3 倍。
- Smoking: Tobacco smoke damages the mucociliary clearance and alveolar macrophages, increasing susceptibility.
- 中文：吸烟：烟草烟雾损害黏液纤毛清除功能和肺泡巨噬细胞，增加易感性。
- Overcrowding: Poor ventilation in crowded homes or prisons facilitates droplet transmission.
- 中文：拥挤环境：拥挤的家庭或监狱中通风不良，促进飞沫传播。
4. Transmission: How Does TB Spread? | 传播途径
TB is transmitted via airborne droplets. When a person with active pulmonary TB coughs, sneezes, speaks, or sings, they release droplet nuclei (1–5 µm in diameter) containing Mycobacterium tuberculosis. These tiny droplets can remain suspended in the air for hours.
结核病通过空气飞沫传播。当活动性肺结核患者咳嗽、打喷嚏、说话或唱歌时，会释放含有结核分枝杆菌的飞沫核（直径 1–5 µm）。这些微小飞沫可在空气中悬浮数小时。
Transmission occurs when a susceptible person inhales these aerosols:
当易感者吸入这些气溶胶时，就会发生传播：
- Droplet nuclei are small enough to bypass the mucociliary escalator of the upper respiratory tract.
- 中文：飞沫核足够小，可绕过上呼吸道的黏液纤毛传送带。
- They reach the alveoli, where they are phagocytosed by alveolar macrophages.
- 中文：它们到达肺泡，被肺泡巨噬细胞吞噬。
- If the macrophages fail to kill the bacteria, infection becomes established.
- 中文：如果巨噬细胞不能杀死细菌，感染便会建立。
Direct contact with fomites or contaminated surfaces is not a significant route because the bacterium is not highly resistant to desiccation and is poorly transmitted by fomites. TB is not transmitted through handshakes, shared food, or blood transfusion.
直接接触污染物或污染表面并非重要传播途径，因为该细菌对干燥环境的抵抗力不强，物体表面传播能力很低。结核病不会通过握手、共餐或输血传播。
5. Pathogenesis and the Immune Response | 发病机制与免疫应答
Once inhaled, the bacteria are phagocytosed by alveolar macrophages. The key virulence mechanism is the ability to prevent phagosome–lysosome fusion, allowing the bacteria to survive within the macrophage.
吸入后，细菌被肺泡巨噬细胞吞噬。其关键毒力机制是能够阻止吞噬体与溶酶体融合，使细菌得以在巨噬细胞内生存。
The immune response unfolds in stages:
免疫应答分阶段展开：
- Innate response: Macrophages attempt to kill bacteria using reactive oxygen species and lysosomal enzymes. Interferon-gamma (IFN-γ) secreted by natural killer cells activates the macrophages in the early phase.
- 中文：固有免疫应答：巨噬细胞试图利用活性氧和溶酶体酶杀死细菌。自然杀伤细胞分泌的干扰素-γ（IFN-γ）在早期激活巨噬细胞。
- Adaptive response: Infected macrophages present bacterial antigens via MHC class II molecules to CD4⁺ helper T cells. This triggers a Th1 response, characterised by IFN-γ production, which further activates macrophages to kill intracellular bacteria.
- 中文：适应性免疫应答：受感染的巨噬细胞通过 MHC II 类分子将细菌抗原呈递给 CD4⁺ 辅助性 T 细胞。这触发 Th1 型应答，以产生 IFN-γ 为特征，进一步激活巨噬细胞以杀灭胞内细菌。
- Granuloma formation: If bacteria survive despite this, the immune system walls them off by forming a granuloma (tubercle). This structure contains a central core of infected macrophages surrounded by foamy macrophages, epithelioid cells, and a rim of lymphocytes and fibrous tissue.
- 中文：肉芽肿形成：如果细菌在此过程中存活，免疫系统会通过形成肉芽肿（结核结节）将其包裹隔离。该结构中心是受感染的巨噬细胞核心，周围绕以泡沫状巨噬细胞、类上皮细胞以及淋巴细胞和纤维组织构成的边缘。
Granuloma = containment, not cure → bacteria can remain viable for decades
肉芽肿 = 控制而非清除 → 细菌可存活数十年
6. Latent vs Active TB | 潜伏性感染与活动性结核
In about 90% of infected individuals, the granuloma walls off the bacteria successfully, and the infection becomes latent. Latent TB is asymptomatic and not contagious. The person has a positive tuberculin skin test or interferon-gamma release assay, but no clinical signs and no detectable bacteria in sputum.
约 90% 的感染者中，肉芽肿能成功包裹细菌，感染进入潜伏状态。潜伏性结核病无症状并且不具传染性。此人结核菌素皮肤试验或 γ-干扰素释放试验呈阳性，但无临床表现，痰中也检测不到细菌。
Active TB occurs when the granuloma breaks down, often due to immunosuppression. The bacteria spill out through the airways, causing extensive tissue damage:
活动性结核发生于肉芽肿破坏时，常见于免疫抑制。细菌通过气道播散，造成广泛组织损伤：
| Feature | Latent TB | Active TB |
| 中文：特征 | 中文：潜伏性结核 | 中文：活动性结核 |
| Symptoms | None | Cough, fever, night sweats, weight loss |
| 中文：症状 | 无 | 咳嗽、发热、盗汗、体重下降 |
| Infectious | No | Yes (especially pulmonary) |
| 中文：传染性 | 无 | 有（尤其是肺结核） |
| Sputum smear (AFB) | Negative | Positive |
| 中文：痰涂片（抗酸杆菌） | 阴性 | 阳性 |
| Chest X-ray | Normal or healed lesions | Cavitation, infiltrates, upper-lobe opacities |
| 中文：胸部 X 线 | 正常或陈旧病灶 | 空洞、浸润影、上叶实变 |
Classic symptoms of active pulmonary TB include a persistent cough lasting more than three weeks, haemoptysis (coughing up blood), fever, night sweats, fatigue, and unintentional weight loss.
活动性肺结核的典型症状包括持续超过三周的咳嗽、咯血（咳血）、发热、盗汗、乏力和不明原因的体重下降。
7. Diagnosis | 诊断方法
A-Level Biology students should understand the main diagnostic approaches for TB, particularly the basis of each test:
A-Level 生物学学生应理解结核病的主要诊断方法，尤其是每种检测的原理：
- Sputum smear microscopy: Using the Ziehl–Neelsen stain, acid-fast bacilli appear red against a blue background because mycolic acid in the cell wall retains the primary stain even after acid-alcohol decolourisation.
- 中文：痰涂片镜检：使用 Ziehl–Neelsen 抗酸染色，抗酸杆菌在蓝色背景下呈红色，因为细胞壁中的分枝菌酸在酸性酒精脱色后仍保留初染液。
- Culture: The gold standard for diagnosis, but M. tuberculosis takes 2–8 weeks to grow on Lowenstein–Jensen medium due to its slow generation time.
- 中文：培养：诊断金标准，但由于结核分枝杆菌生长缓慢，在Löwenstein–Jensen 培养基上需 2–8 周才能生长。
- Nucleic acid amplification tests (NAAT): PCR-based detection of mycobacterial DNA (e.g. IS6110 insertion sequence) delivers results in hours, enabling rapid detection and drug-resistance screening.
- 中文：核酸扩增试验（NAAT）：基于 PCR 检测分枝杆菌 DNA（例如 IS6110 插入序列），数小时即可出结果，可快速检测和耐药筛查。
- Tuberculin skin test (TST) / IGRA: These detect cell-mediated immunity rather than active bacteria. TST measures delayed-type hypersensitivity after intradermal injection of purified protein derivative (PPD). IGRA measures IFN-γ release by T cells exposed to specific TB antigens (e.g. ESAT-6 and CFP-10).
- 中文：结核菌素皮肤试验（TST）/ γ-干扰素释放试验（IGRA）：这些检测的是细胞介导免疫而非活动性细菌。TST 通过皮内注射纯蛋白衍生物（PPD）后测量迟发型超敏反应。IGRA 测量 T 细胞暴露于特异性结核抗原（如 ESAT-6 和 CFP-10）后释放的 IFN-γ 水平。
8. Prevention: Vaccination and Public Health | 预防：疫苗与公共卫生措施
Prevention operates at three levels: avoiding initial exposure, preventing latent infection from becoming active, and preventing transmission from infectious cases.
预防措施分为三个层面：避免初始暴露、防止潜伏感染发展为活动性，以及防止传染性病例的传播。
BCG vaccination | BCG 疫苗
The Bacille Calmette–Guérin (BCG) vaccine is a live attenuated strain of Mycobacterium bovis. It stimulates cell-mediated immunity, particularly a Th1-type response that activates macrophages to contain the bacterium.
卡介苗（BCG）是牛分枝杆菌的减毒活疫苗。它刺激细胞介导免疫，尤其是 Th1 型应答，帮助激活巨噬细胞来遏制细菌。
- BCG is highly effective (≈80%) against severe disseminated TB (e.g. miliary TB and TB meningitis) in children.
- 中文：BCG 对儿童重症播散性结核（如粟粒性结核和结核性脑膜炎）具有高度保护效力（约 80%）。
- Its efficacy against adult pulmonary TB is variable (range 0–80%), so it does not reliably control the main infectious form.
- 中文：其对成人肺结核的保护效力不稳定（0–80%），因此不能可靠地控制主要传染形式。
- In the UK, BCG is offered to high-risk groups rather than the entire population, reflecting the changing epidemiology.
- 中文：在英国，BCG 只提供给高风险人群而非全体人口，这反映了流行病学的变化。
Public health measures | 公共卫生措施
- Early detection and treatment: Identifying infectious patients and treating them with effective antibiotics reduces the pool of transmissible cases.
- 中文：早期发现和治疗：识别传染性患者并给予有效抗生素治疗，可减少可传播病例的储备库。
- Infection control: Respiratory isolation, good ventilation, natural sunlight (UV kills mycobacteria), and the use of surgical masks in healthcare settings.
- 中文：感染控制：呼吸道隔离、良好通风、自然阳光（紫外线可杀灭分枝杆菌）以及医疗场所佩戴口罩。
- Contact tracing: Close contacts of active TB patients are screened, and those with latent infection may be offered preventive chemotherapy (isoniazid).
- 中文：接触者追踪：对活动性结核患者的密切接触者进行筛查，潜伏感染者可接受预防性化疗（异烟肼）。
- Improving social conditions: Reducing overcrowding, improving nutrition and housing, and addressing poverty lower the incidence of TB in populations.
- 中文：改善社会条件：减少拥挤、改善营养和住房、解决贫困问题，可降低人群中结核病的发病率。
9. Treatment and Drug Resistance | 治疗与耐药性
Active TB requires a combination of antibiotics over a prolonged period. The standard regimen follows the DOTS strategy (Directly Observed Therapy, Short-course). The first-line drugs are usually denoted by the acronym RIPE:
活动性结核需要联合使用多种抗生素并长期治疗。标准方案遵循 DOTS 策略（直接督导短程化疗）。一线药物通常用缩写 RIPE 表示：
| Drug | Mechanism of action | 中文：药物 | 中文：作用机制 |
| Rifampicin | Inhibits bacterial DNA-dependent RNA polymerase, blocking transcription. | 利福平 | 抑制细菌 DNA 依赖的 RNA 聚合酶，阻断转录。 |
| Isoniazid | Inhibits synthesis of mycolic acids in the cell wall. | 异烟肼 | 抑制细胞壁分枝菌酸的合成。 |
| Pyrazinamide | Disrupts membrane transport and acidifies the phagosome; active only at acidic pH (inside macrophages). | 吡嗪酰胺 | 破坏膜转运并使吞噬体酸化；仅在酸性 pH（巨噬细胞内）时有效。 |
| Ethambutol | Inhibits arabinosyltransferases, blocking cell wall arabinogalactan synthesis. | 乙胺丁醇 | 抑制阿拉伯糖基转移酶，阻断细胞壁阿拉伯半乳聚糖合成。 |
The typical regimen has two phases: an intensive phase of 2 months with all four drugs, followed by a continuation phase of 4 months with isoniazid and rifampicin. The long duration is necessary because the slow-growing and intracellular persister bacteria are difficult to eradicate.
典型方案分为两个阶段：2 个月强化期使用全部四种药物，随后是 4 个月巩固期仅用异烟肼和利福平。疗程长是必要的，因为慢生长和胞内持留菌难以根除。
Drug resistance arises from spontaneous chromosomal mutations during replication. Multi-drug-resistant TB (MDR-TB) is defined as resistance to at least isoniazid and rifampicin, the two most powerful first-line drugs. Extensively drug-resistant TB (XDR-TB) adds resistance to any fluoroquinolone and at least one second-line injectable agent.
耐药性来源于复制过程中发生的自发染色体突变。耐多药结核病（MDR-TB）定义为至少对异烟肼和利福平这两种最强一线药物耐药。广泛耐药结核病（XDR-TB）则在此基础上还对所有氟喹诺酮类药物和至少一种二线注射剂耐药。
Non-adherence to treatment is the primary driver of acquired resistance, because patients may stop taking drugs once symptoms improve. DOTS ensures that a healthcare worker directly observes the patient swallowing each dose, guaranteeing complete therapy.
治疗依从性差是获得性耐药的主要驱动力，因为患者可能在症状改善后便停药。DOTS 确保医务人员直接监督患者吞服每一剂药物，从而保证完成全程治疗。
10. TB and HIV Co-infection | 结核病与 HIV 合并感染
HIV infection is the strongest risk factor for developing active TB. At A-Level, the biological link is important:
HIV 感染是发展为活动性结核的最强危险因素。在 A-Level 中，生物学联系很重要：
- HIV destroys CD4⁺ helper T cells, which are essential for activating macrophages and maintaining granuloma integrity.
- 中文：HIV 破坏 CD4⁺ 辅助性 T 细胞，而后者对于激活巨噬细胞和维持肉芽肿完整性至关重要。
- The resulting loss of IFN-γ production allows dormant bacteria to escape the granuloma and multiply, leading to reactivation.
- 中文：由此导致 IFN-γ 产生减少，使休眠细菌得以逃出肉芽肿并繁殖，导致复燃。
- TB is one of the most common opportunistic infections and a leading cause of death among people living with HIV worldwide.
- 中文：结核病是最常见的机会性感染之一，也是全球 HIV 感染者死亡的主要原因之一。
This is why TB screening is recommended for all HIV-positive individuals, and why co-infected patients require coordinated antiretroviral therapy (ART) and anti-TB treatment.
因此建议对所有 HIV 阳性者进行结核病筛查，合并感染者需要协调开展抗逆转录病毒治疗（ART）和抗结核治疗。
11. Exam Focus: Common Questions and Pitfalls | 考点聚焦：常见问题与易错点
CIE A-Level Biology often tests this topic through short-answer questions, data interpretation, and essay-style questions. Below are the most frequently examined concepts:
CIE A-Level 生物学常通过简答题、数据解读和论述题来考查这一主题。以下是最常考的概念：
- Why is TB slow to develop and slow to treat? Because the bacterium's generation time is 15–20 hours, and it can persist intracellularly in a dormant state.
- 中文：为什么结核病发展慢、治疗也慢？因为细菌倍增时间为 15–20 小时，并且能以休眠状态在细胞内持留。
- Why is the BCG vaccine not universally effective? Because it is a live attenuated vaccine that provides variable protection against adult pulmonary TB, possibly due to exposure to environmental mycobacteria and genetic variation in immune responses.
- 中文：为什么 BCG 疫苗并非普遍有效？因为它是减毒活疫苗，对成人肺结核的保护效力不稳定，可能是因为环境中分枝杆菌暴露和免疫应答的遗传差异。
- How does TB affect the gas exchange surface? Granulomas, fibrosis, and cavitation reduce the surface area for gas exchange, increase the diffusion distance, and can cause haemoptysis.
- 中文：结核病如何影响气体交换表面？肉芽肿、纤维化和空洞形成减少气体交换面积，增加扩散距离，并可导致咯血。
- Antibiotic resistance: Explain the genetic basis of MDR-TB and why combination therapy reduces the risk of resistance emerging.
- 中文：抗生素耐药性：解释 MDR-TB 的遗传学基础，以及为什么联合治疗能降低耐药出现的风险。
A common pitfall is confusing "infection" with "disease." Latent TB infection is not the same as active TB disease, and only active pulmonary TB is infectious.
常见错误是混淆"感染"与"疾病"。潜伏结核感染不同于活动性结核病，只有活动性肺结核才具有传染性。
12. Summary | 总结
To revise effectively, remember the "chain of infection" for TB:
为高效复习，记住结核病的"感染链"：
Causative agent (Mycobacterium tuberculosis) → Reservoir (humans) → Exit (respiratory droplets) → Transmission (airborne) → Entry (inhalation) → Susceptible host
病原体（结核分枝杆菌）→ 储存宿主（人类）→ 排出途径（呼吸道飞沫）→ 传播方式（空气传播）→ 侵入途径（吸入）→ 易感宿主
Key take-home points for the exam:
考试要点总结：
- M. tuberculosis is an acid-fast, slow-growing, intracellular pathogen with an unusual lipid-rich cell wall.
- 中文：结核分枝杆菌是抗酸、生长缓慢、胞内寄生的病原体，具有富含脂质的特殊细胞壁。
- Transmission is airborne via droplet nuclei; close contact and poor ventilation increase spread.
- 中文：传播通过飞沫核经空气完成；密切接触和通风不良增加传播。
- The immune response involves macrophages, CD4⁺ Th1 cells, and granuloma formation; HIV co-infection is the greatest risk factor for reactivation.
- 中文：免疫应答涉及巨噬细胞、CD4⁺ Th1 细胞和肉芽肿形成；HIV 合并感染是复燃的最大危险因素。
- Prevention relies on BCG vaccination, early diagnosis, infection control, and social measures.
- 中文：预防依赖 BCG 疫苗接种、早期诊断、感染控制和社会措施。
- Treatment uses long-term combination therapy (RIPE); MDR-TB results from inadequate adherence and genetic mutations.
- 中文：治疗采用长期联合用药（RIPE）；耐多药结核病源于依从性差和基因突变。
Mastering these concepts will allow you to answer both short-answer and essay questions confidently.
掌握这些概念，你将能够自信地应对简答题和论述题。
Published by TutorHao | Biology Revision Series | aleveler.com
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