Antibiotics: Mechanisms, Selectivity and Resistance | 抗生素：作用机制、选择性与耐药性

📚 Antibiotics: Mechanisms, Selectivity and Resistance | 抗生素：作用机制、选择性与耐药性

Antibiotics are chemical agents that kill or inhibit the growth of bacteria, and they have transformed the treatment of bacterial infections. In A-Level Biology, you need to understand how antibiotics achieve selective toxicity, how they disrupt bacterial cell structures and processes, and how bacteria evolve resistance through mutation and horizontal gene transfer.

抗生素是杀死细菌或抑制细菌生长的化学药物，它们已经彻底改变了细菌感染的治疗方式。在 A-Level 生物课程中，你需要理解抗生素如何实现选择毒性，如何破坏细菌的细胞结构和生命活动，以及细菌如何通过突变和水平基因转移产生耐药性。

1. What Is an Antibiotic? | 什么是抗生素？

An antibiotic is a substance produced by microorganisms, or made synthetically, that at low concentrations selectively inhibits or kills bacteria. The term ‘antibiotic’ originally referred to natural products such as penicillin from the fungus Penicillium, but many modern drugs are semi-synthetic or fully synthetic derivatives.

抗生素是由微生物产生或人工合成的一类物质，在低浓度下能够选择性地抑制或杀死细菌。”抗生素”一词最初指天然产物，例如来自青霉真菌的青霉素，但许多现代药物是半合成或全合成的衍生物。

Antibiotics target bacteria, not viruses, fungi or human cells in general, because they interfere with structures or metabolic pathways that are unique to bacteria.

抗生素通常作用于细菌，而不是病毒、真菌或人体细胞，因为它们干扰的是细菌特有的结构或代谢途径。

Common examples include penicillins, tetracyclines, aminoglycosides and macrolides. Each class has a different mechanism and spectrum of activity.

常见例子包括青霉素类、四环素类、氨基糖苷类和大环内酯类。每一类都有不同的作用机制和抗菌谱。

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2. Selective Toxicity | 选择毒性

Selective toxicity means the drug damages the pathogen while causing minimal harm to the host. It is the central principle of antimicrobial chemotherapy.

选择毒性是指药物能损伤病原体，而对宿主细胞伤害最小。这是抗微生物化疗的核心原则。

Bacteria possess features that human cells lack, such as a peptidoglycan cell wall, 70S ribosomes, and unique enzymes for folic acid synthesis. Antibiotics exploit these differences.

细菌具有人体细胞缺乏的结构，例如肽聚糖细胞壁、70S 核糖体以及独特的叶酸合成酶。抗生素正是利用这些差异发挥作用。

For example, penicillins block peptidoglycan cross-linking, while human cells have no cell wall and therefore are not affected by this mode of action.

例如，青霉素类阻断肽聚糖的交联，而人体细胞没有细胞壁，因此不受这种作用方式的影响。

The therapeutic index compares the dose that is toxic to the host with the dose that is effective against the pathogen. A high therapeutic index means the antibiotic is relatively safe.

治疗指数将药物对宿主产生毒性的剂量与对病原体有效的剂量进行比较。治疗指数高意味着抗生素相对安全。

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3. Bactericidal vs Bacteriostatic Antibiotics | 杀菌性抗生素与抑菌性抗生素

Antibiotics can be classified as bactericidal if they kill bacteria, or bacteriostatic if they mainly inhibit bacterial growth and rely on the host immune system to eliminate the pathogens.

抗生素可分为杀菌性抗生素和抑菌性抗生素：杀菌性抗生素能杀死细菌，而抑菌性抗生素主要抑制细菌生长，依靠宿主免疫系统清除病原体。

Penicillins and cephalosporins are usually bactericidal because they weaken the cell wall, causing osmotic lysis. Tetracyclines are typically bacteriostatic because they stop protein synthesis without immediately killing the cell.

青霉素类和头孢菌素类通常是杀菌性的，因为它们削弱细胞壁，引起渗透性裂解。四环素类通常是抑菌性的，因为它们阻止蛋白质合成，但不会立即杀死细胞。

The distinction is not absolute; some drugs may be bactericidal at high concentrations and bacteriostatic at low concentrations.

这种区分并不是绝对的；某些药物在高浓度下可能具有杀菌作用，在低浓度下则表现为抑菌作用。

Bacteriostatic drugs can still cure infections if the immune system is functional, but they may be less suitable for immunocompromised patients.

如果免疫系统功能正常，抑菌性药物仍可治愈感染，但对免疫功能低下的患者可能不太适合。

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4. Broad-Spectrum and Narrow-Spectrum Antibiotics | 广谱与窄谱抗生素

Broad-spectrum antibiotics act against a wide range of Gram-positive and Gram-negative bacteria. Narrow-spectrum antibiotics are effective against a limited group of bacteria.

广谱抗生素对多种革兰氏阳性菌和革兰氏阴性菌都有作用。窄谱抗生素只对有限的细菌群有效。

Broad-spectrum agents, such as tetracyclines, are useful in serious infections when the pathogen is unknown, but they also kill normal microbiota and can cause secondary infections or antibiotic-associated diarrhoea.

四环素等广谱药物在病原体未知的严重感染中很有用，但它们也会杀死正常菌群，可能引起继发感染或抗生素相关性腹泻。

Narrow-spectrum agents, such as penicillin G, are preferred when the pathogen has been identified, because they reduce damage to the patient’s normal flora and lower the selection pressure for resistance.

当病原体已经确定时，优先选择窄谱药物，如青霉素 G，因为它们能减少对患者正常菌群的破坏，并降低耐药性的选择压力。

Using narrow-spectrum antibiotics where possible is a key strategy for antibiotic stewardship.

尽可能使用窄谱抗生素是抗生素管理的关键策略之一。

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5. Cell Wall Synthesis Inhibitors: Penicillins | 细胞壁合成抑制剂：青霉素类

Penicillins belong to the β-lactam class of antibiotics. They contain a β-lactam ring that is essential for their antibacterial activity.

青霉素属于 β-内酰胺类抗生素。它们含有一个 β-内酰胺环，这是其抗菌活性所必需的结构。

Penicillins inhibit transpeptidase enzymes, also called penicillin-binding proteins (PBPs). These enzymes normally form cross-links between peptide side chains in peptidoglycan, giving the bacterial cell wall its strength.

青霉素抑制转肽酶，也称为青霉素结合蛋白（PBPs）。这些酶通常在肽聚糖的肽侧链之间形成交联，使细菌细胞壁具有强度。

When cross-linking is blocked, the cell wall becomes weak, especially in growing bacteria. Water enters by osmosis, and the bacterium bursts, a process called osmotic lysis.

当交联被阻断时，细胞壁变弱，尤其是在正在生长的细菌中。水分通过渗透进入细胞，细菌破裂，这一过程称为渗透性裂解。

Penicillins are therefore most effective against actively dividing bacteria that are synthesising new peptidoglycan.

因此，青霉素对正在活跃分裂、合成新肽聚糖的细菌最有效。

Because human cells do not have peptidoglycan cell walls, penicillins have excellent selective toxicity.

由于人体细胞没有肽聚糖细胞壁，青霉素具有很好的选择毒性。

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6. Other Targets: Protein Synthesis and Nucleic Acid Synthesis | 其他作用靶点：蛋白质合成与核酸合成

Many antibiotics target bacterial ribosomes. Bacterial ribosomes are 70S, made of a 50S and a 30S subunit, while eukaryotic cytoplasmic ribosomes are 80S. This difference allows selective inhibition.

许多抗生素以细菌核糖体为靶点。细菌核糖体为 70S，由 50S 和 30S 亚基组成，而真核细胞质核糖体为 80S。这一差异使药物能够选择性抑制细菌。

Aminoglycosides such as streptomycin bind to the 30S subunit and cause misreading of mRNA. Tetracyclines also bind to the 30S subunit and block attachment of tRNA, preventing protein synthesis.

链霉素等氨基糖苷类与 30S 亚基结合，导致 mRNA 被错误读取。四环素类也与 30S 亚基结合，阻断 tRNA 的结合，从而阻止蛋白质合成。

Macrolides such as erythromycin bind to the 50S subunit and inhibit translocation of the growing polypeptide chain.

红霉素等大环内酯类与 50S 亚基结合，抑制生长中多肽链的移位。

Some antibiotics, such as quinolones, inhibit bacterial DNA gyrase or topoisomerase, enzymes involved in DNA supercoiling and replication. Others block RNA polymerase or folic acid synthesis.

一些抗生素，如喹诺酮类，抑制细菌 DNA 旋转酶或拓扑异构酶，这些酶参与 DNA 超螺旋化和复制。其他药物则阻断 RNA 聚合酶或叶酸合成。

Sulfonamides inhibit folic acid synthesis, which is essential for nucleotide production.

Published by TutorHao | A-Level Biology Revision Series | aleveler.com

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