📚 Antibiotics | 抗生素
Antibiotics are one of the greatest medical breakthroughs of the 20th century, revolutionising the treatment of bacterial infections and saving countless lives. For A-Level Biology, understanding how these powerful agents work, why they are selectively toxic, and how bacteria evolve resistance is central to mastering microbiology, infection and immunity.
抗生素是20世纪最伟大的医学突破之一，彻底改变了细菌感染的治疗方式，拯救了无数生命。对于A-Level生物学而言，理解这些强效药物如何发挥作用、为何具有选择性毒性，以及细菌如何演化出耐药性，是掌握微生物学、感染与免疫部分的关键。
1. What Are Antibiotics? | 什么是抗生素？
An antibiotic is a chemical substance, produced by living organisms (such as fungi or bacteria) or made synthetically, that kills or inhibits the growth of bacteria. They are not effective against viruses, because viruses lack the cellular structures that antibiotics target, a point frequently tested in exams.
抗生素是由生物体（如真菌或细菌）产生的或人工合成的化学物质，能杀死或抑制细菌生长。它们对病毒无效，因为病毒缺乏抗生素所靶向的细胞结构，这一点在考试中经常出现。
Antibiotics work by attacking features that are unique to bacteria, such as the peptidoglycan cell wall, 70S ribosomes, and specific enzymes involved in nucleic acid synthesis. This principle of selective targeting is what makes antibiotics safe for human use.
抗生素通过攻击细菌独有的特征发挥作用，例如肽聚糖细胞壁、70S核糖体以及参与核酸合成的特定酶。这种选择性靶向原理使抗生素对人体安全可用。
2. The History and Discovery of Antibiotics | 抗生素的历史与发现
The modern antibiotic era began with Alexander Fleming's serendipitous discovery of penicillin in 1928. He noticed that a mould, Penicillium notatum, inhibited the growth of Staphylococcus bacteria on an agar plate. Florey and Chain later purified penicillin for clinical use during World War II.
现代抗生素时代始于亚历山大·弗莱明在1928年偶然发现青霉素。他注意到一种霉菌——点青霉——抑制了琼脂平板上葡萄球菌的生长。弗洛里和钱恩随后在第二次世界大战期间提纯了青霉素用于临床。
Following penicillin, a golden age of antibiotic discovery gave us streptomycin, tetracycline, chloramphenicol and many others. However, resistance appeared rapidly in many cases, demonstrating how quickly bacteria can adapt.
继青霉素之后，抗生素发现的黄金时代为我们带来了链霉素、四环素、氯霉素等多种药物。然而，在许多情况下耐药性迅速出现，表明细菌的适应速度有多快。
3. Selective Toxicity | 选择性毒性
Selective toxicity is the capacity of an antibiotic to harm the bacterial cell without damaging the host's cells. This is a core concept in A-Level Biology, explaining why antibiotics can be taken safely by humans.
选择性毒性是指抗生素能够伤害细菌细胞而不损害宿主细胞的能力。这是A-Level生物学的核心概念，解释了为什么人类可以安全使用抗生素。
It relies on structural and metabolic differences between prokaryotic and eukaryotic cells. For example, human cells lack peptidoglycan cell walls and possess 80S ribosomes, while bacteria have 70S ribosomes. Antibiotics like penicillin attack the cell wall, and tetracycline binds to the bacterial 30S ribosomal subunit, leaving human ribosomes unaffected.
它依赖于原核细胞与真核细胞之间的结构和代谢差异。例如，人类细胞没有肽聚糖细胞壁，并且拥有80S核糖体，而细菌则有70S核糖体。像青霉素这样的抗生素攻击细胞壁，而四环素则与细菌的30S核糖体亚基结合，使人核糖体不受影响。
4. Bactericidal vs. Bacteriostatic Antibiotics | 杀菌性与抑菌性抗生素
Antibiotics can be classified by their effect on bacterial populations. Bactericidal antibiotics kill bacteria directly. They often work by destroying the cell wall or disrupting essential processes such as DNA replication, leading to cell lysis. Examples include penicillin and ciprofloxacin.
抗生素可根据其对细菌群体的作用方式分类。杀菌性抗生素直接杀死细菌。它们通常通过破坏细胞壁或扰乱DNA复制等必需过程而导致细胞裂解。例如青霉素和环丙沙星。
Bacteriostatic antibiotics, on the other hand, inhibit bacterial growth and reproduction, allowing the host's immune system to eliminate the pathogen. Tetracycline and chloramphenicol are classic bacteriostatic agents that stop protein synthesis. It is important to remember that very high concentrations of some bacteriostatic drugs can become bactericidal.
另一方面，抑菌性抗生素抑制细菌生长和繁殖，让宿主的免疫系统来清除病原体。四环素和氯霉素是经典的抑菌剂，能阻止蛋白质合成。重要的是要记住，某些抑菌药物在浓度很高时也可能变为杀菌药物。
| Feature | 特征 | Bactericidal | 杀菌性 | Bacteriostatic | 抑菌性 |
|---|---|---|
| Mechanism | 机制 | Kills bacteria | 杀死细菌 | Inhibits growth | 抑制生长 |
| Outcome | 结果 | Cell lysis | 细胞裂解 | Immune clearance | 免疫清除 |
| Examples | 例子 | Penicillin, Ciprofloxacin | 青霉素, 环丙沙星 | Tetracycline, Chloramphenicol | 四环素, 氯霉素 |
5. Mechanism of Action of Penicillin | 青霉素的作用机制
Penicillin belongs to the beta-lactam family of antibiotics. It works by inhibiting the enzyme transpeptidase, which is responsible for cross-linking the peptide chains in peptidoglycan, the major structural polymer of the bacterial cell wall.
青霉素属于β-内酰胺类抗生素。它通过抑制转肽酶发挥作用，转肽酶负责交联肽聚糖中的肽链，肽聚糖是细菌细胞壁的主要结构聚合物。
Without this cross-linking, the cell wall becomes weak and unable to withstand osmotic pressure. Water enters the cell by osmosis, causing it to swell and eventually burst (lyse). This makes penicillin particularly effective against actively growing bacteria that are synthesising new cell wall material.
没有这种交联，细胞壁变得脆弱，无法承受渗透压。水通过渗透进入细胞，导致细胞膨胀并最终破裂（裂解）。这使得青霉素对正在积极生长、合成新细胞壁材料的细菌特别有效。
6. Mechanism of Action of Tetracyclines | 四环素类的作用机制
Tetracyclines are broad-spectrum antibiotics that inhibit bacterial protein synthesis. They bind reversibly to the 30S ribosomal subunit, blocking the attachment of aminoacyl-tRNA to the mRNA-ribosome complex.
四环素类是广谱抗生素，能够抑制细菌蛋白质的合成。它们可逆地与30S核糖体亚基结合，阻止氨酰tRNA附着到mRNA-核糖体复合物上。
This action prevents the addition of new amino acids to the growing polypeptide chain, effectively halting protein synthesis. Since protein synthesis is essential for bacterial growth and reproduction, tetracyclines are bacteriostatic. They do not bind to the 80S ribosomes of human cells, maintaining selective toxicity.
这一作用阻止了新的氨基酸添加到延伸中的多肽链上，从而有效地停止蛋白质合成。由于蛋白质合成对细菌的生长和繁殖至关重要，四环素是抑菌性药物。它们不与人细胞的80S核糖体结合，从而保持了选择性毒性。
7. Broad-Spectrum vs. Narrow-Spectrum Antibiotics | 广谱与窄谱抗生素
Broad-spectrum antibiotics are effective against a wide range of bacterial species, including both Gram-positive and Gram-negative bacteria. Examples include tetracycline and amoxicillin. They are often prescribed when the causative pathogen is unknown, or in cases of mixed infections.
广谱抗生素对多种细菌有效，包括革兰氏阳性菌和革兰氏阴性菌。例如四环素和阿莫西林。当致病病原体不明或在混合感染的情况下，医生常会开具这类抗生素。
Narrow-spectrum antibiotics, such as penicillin G, target only specific groups of bacteria. They are preferred when the pathogen has been identified, because they cause less disruption to the normal gut microbiota, reducing the risk of secondary infections such as C. difficile colitis. The choice between broad and narrow spectrum thus balances efficacy against the risk of dysbiosis and resistance development.
窄谱抗生素，如青霉素G，仅针对特定类群的细菌。当病原体已被鉴定时，优先使用窄谱抗生素，因为它们对正常肠道菌群的干扰较小，可降低艰难梭菌结肠炎等继发性感染的风险。因此，在广谱和窄谱之间做出选择需要在疗效与菌群失调和耐药性发展风险之间取得平衡。
8. How Bacterial Resistance Develops | 细菌耐药性如何产生
Antibiotic resistance arises through genetic changes that enable bacteria to survive drug exposure. Mutations in chromosomal genes can alter the antibiotic target, and resistance genes can be acquired from other bacteria via horizontal gene transfer: transformation, transduction, and conjugation.
抗生素耐药性源于使细菌能够在药物暴露下存活的遗传改变。染色体基因的突变可以改变抗生素靶点，而耐药基因可以通过水平基因转移从其他细菌获得：包括转化、转导和接合。
Once a resistant gene is present, natural selection drives the evolution of resistant populations. In the presence of an antibiotic, susceptible bacteria are killed, while those carrying resistance genes survive and multiply. This process is a classic example of natural selection observable in real time, and it is heavily emphasised in A-Level Biology.
一旦耐药基因出现，自然选择就会驱动耐药群体的演化。在抗生素存在时，敏感细菌被杀死，而携带耐药基因的细菌存活并增殖。这一过程是一个可实时观察的经典自然选择实例，在A-Level生物学中被反复强调。
9. Mechanisms of Resistance | 耐药性机制
Resistant bacteria employ multiple mechanisms to evade antibiotics. Enzymatic degradation is one common strategy: beta-lactamase enzymes (penicillinases) break the beta-lactam ring of penicillin, rendering it inactive. Metallo-beta-lactamases extend this resistance even to carbapenems.
耐药细菌利用多种机制来逃避抗生素。酶降解是一种常见策略：β-内酰胺酶（青霉素酶）破坏青霉素的β-内酰胺环，使其失活。金属-β-内酰胺酶甚至能将这种耐药性扩展到碳青霉烯类药物。
Other mechanisms include alteration of the target site, where a mutation changes the structure of the penicillin-binding protein (as in MRSA), preventing the antibiotic from binding. Efflux pumps actively expel antibiotics from the cell, and reduced membrane permeability limits drug entry. Bacteria often combine several mechanisms, making treatment extremely difficult.
其他机制包括靶点修饰，即突变改变青霉素结合蛋白的结构（如MRSA），使抗生素无法结合；外排泵主动将抗生素排出细胞；以及降低膜通透性，限制药物进入。细菌常常结合使用多种机制，使得治疗极为困难。
10. The Evolution of Antibiotic Resistance and MRSA | 抗生素耐药性进化与MRSA
Methicillin-resistant Staphylococcus aureus (MRSA) is a prominent example of antibiotic resistance that frequently appears in A-Level exams. MRSA strains have acquired the mecA gene, which encodes an altered penicillin-binding protein (PBP2a) with low affinity for beta-lactam antibiotics.
耐甲氧西林金黄色葡萄球菌（MRSA）是抗生素耐药性的一个突出例子，常在A-Level考试中出现。MRSA菌株获得了mecA基因，该基因编码一种对β-内酰胺类抗生素亲和力低下的变构青霉素结合蛋白（PBP2a）。
The spread of MRSA in hospitals illustrates the principles of natural selection and infection control. Antibiotic use creates a selective pressure that favours the proliferation of resistant clones. Coupled with poor hygiene, this can lead to outbreaks that are difficult to manage. The same selective pressure drives the spread of vancomycin-resistant enterococci (VRE) and multidrug-resistant Gram-negative bacteria.
MRSA在医院中的传播体现了自然选择和感染控制的原则。抗生素的使用创造了有利于耐药克隆增殖的选择压力。加之卫生条件不佳，可导致难以控制的爆发。同样的选择压力也推动了对万古霉素耐药肠球菌（VRE）和多重耐药革兰氏阴性菌的传播。
11. The Impact of Antibiotic Misuse | 抗生素滥用的影响
Antibiotic resistance is accelerated by misuse. In medicine, prescribing antibiotics for viral infections (e.g., colds, flu) provides no benefit to the patient while exposing bacteria to selective pressure. Patients who do not complete the full course of antibiotics allow partially resistant bacteria to survive and later proliferate.
滥用抗生素会加速耐药性的产生。在医学上，为病毒感染（如感冒、流感）开具抗生素处方对患者没有任何益处，却使细菌暴露于选择压力之下。未完成全疗程的患者使得部分耐药的细菌存活下来，随后大量繁殖。
In agriculture, the use of antibiotics as growth promoters in livestock has been a major driver of resistance. Resistant bacteria can transfer from animals to humans through the food chain, direct contact, or environmental contamination. This has led to strict regulations in many countries, but resistance genes already widely disseminated in the environment persist.
在农业中，将抗生素作为牲畜的生长促进剂使用，是耐药性产生的一个主要驱动因素。耐药细菌可通过食物链、直接接触或环境污染从动物传播给人类。这导致许多国家制定了严格法规，但已广泛散布于环境中的耐药基因依然存在。
12. Strategies to Reduce Antibiotic Resistance | 减少抗生素耐药性的策略
Combating antibiotic resistance requires a multifaceted approach. At the clinical level, antibiotics should be prescribed only when necessary and based on culture and sensitivity testing whenever possible. Patients must be educated to complete the full prescribed course even if they feel better, to eliminate all bacteria and reduce the chance of selecting resistant mutants.
抗击抗生素耐药性需要多管齐下的策略。在临床层面，抗生素仅在必需时开具，并应尽可能基于培养和药敏试验结果。必须教育患者完成全疗程，即使感觉好转也不可停药，以彻底清除细菌，降低筛选出耐药突变株的几率。
Vaccination programmes reduce the overall burden of bacterial diseases, thereby lowering antibiotic demand. Improved hospital hygiene, including hand washing and isolation protocols, limits the spread of resistant strains. On a global scale, research into new antibiotics, bacteriophage therapy, and antimicrobial peptides is critical, as is banning the non-therapeutic use of antibiotics in agriculture. The One Health approach, integrating human, animal, and environmental health, is essential for sustainable control.
疫苗接种计划可减少细菌性疾病的总负担，从而降低抗生素需求。改善医院卫生，包括洗手和隔离措施，可限制耐药菌株的传播。在全球范围内，对新抗生素、噬菌体疗法和抗菌肽的研究至关重要，同时应禁止在农业中非治疗用途使用抗生素。整合人类、动物和环境健康的“全健康”方法对于可持续的控制至关重要。
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