Definition and Classification
of Antibiotics
Antibiotics are a class of drugs that can inhibit or kill bacteria and are used to prevent and treat bacterial infections. It can be divided into many types according to its chemical structure and mechanism of action. (1) β-lactams are the most widely used antibacterial drugs in clinic. Include penicillins, cephalosporins, carbapenems, etc. Penicillins, such as amoxicillin, play an antibacterial role by inhibiting the synthesis of bacterial cell walls, and are mainly used to treat Gram-positive bacterial infections, such as respiratory tract infections caused by Streptococcus pneumoniae. From the first generation to the fifth generation, the antibacterial spectrum of cephalosporins has been gradually expanded, and the effect on Gram-negative bacteria has been enhanced. For example, ceftriaxone has a good effect in the treatment of Neisseria gonorrhoeae infection. Carbapenems, such as meropenem, have a very broad antibacterial spectrum, and have strong antibacterial activity against a variety of drug-resistant bacteria, including extended-spectrum beta-lactamase (ESBLs) -producing bacteria, which are often used in severe nosocomial infection. (2) Aminoglycosides such as gentamicin and amikacin are commonly used. They mainly act on bacterial ribosomes and inhibit protein synthesis. These drugs have strong antibacterial activity against Gram-negative bacilli, such as Escherichia coli, Pseudomonas aeruginosa and so on. However, aminoglycosides have some ototoxicity and nephrotoxicity, and need to be carefully monitored when used. (3) Macrolides include erythromycin and azithromycin. They block bacterial protein synthesis by binding to the 50s subunit of the bacterial ribosome. Macrolides have antibacterial activity against Gram-positive bacteria, some Gram-negative bacteria and atypical pathogens such as mycoplasma and chlamydia. Azithromycin is widely used in the treatment of respiratory tract infections, especially Mycoplasma pneumoniae pneumonia, due to its unique pharmacokinetic characteristics, high tissue concentration and long half-life. (4) Quinolones such as levofloxacin and moxifloxacin. Its mechanism of action is to inhibit bacterial DNA gyrase (topoisomerase II) and (or) topoisomerase IV, and hinder bacterial DNA replication. Quinolones have a broad antibacterial spectrum and are effective against both Gram-negative and Gram-positive bacteria, as well as anaerobic bacteria and intracellular pathogens such as Legionella and Mycobacterium tuberculosis. (5) Tetracyclines include tetracycline and doxycycline. It plays an antibacterial role mainly by inhibiting bacterial protein synthesis. Tetracyclines are effective against a variety of pathogens, such as rickettsia, chlamydia, mycoplasma, spirochetes, etc., and have application value in the treatment of some special infections such as tsutsugamushi disease. However, long-term use may lead to adverse reactions such as tooth discoloration and abnormal bone development.
II. Mechanism
of Action of Antibiotics
(1) Inhibition of cell wall synthesis, such as beta-lactam antibiotics, the cell wall of bacteria is essential for maintaining the morphology of bacteria and protecting bacteria from the influence of external osmotic pressure. These drugs can combine with the key enzymes in the process of bacterial cell wall synthesis, prevent the synthesis of cell wall peptidoglycan, resulting in cell wall defect, and bacterial swelling, rupture and death due to water infiltration in hypotonic environment. (2) Antibacterial drugs such as polymyxins that affect cell membrane permeability can act on bacterial cell membranes. They can combine with phospholipids in the cell membrane, increase the permeability of the cell membrane, and cause the leakage of important substances such as nucleotides, amino acids and potassium ions, thus inhibiting the growth of bacteria or causing the death of bacteria. (3) Inhibition of protein synthesis Aminoglycosides, macrolides and tetracyclines all act on bacterial ribosomes in different ways, interfering with the initiation, prolongation or termination of protein synthesis. Because the structure of bacterial ribosomes is different from that of eukaryotic ribosomes, these drugs have relatively little effect on protein synthesis in human cells while playing an antibacterial role. (4) Quinolone drugs that inhibit nucleic acid synthesis inhibit bacterial DNA gyrase and topoisomerase IV and interfere with DNA replication, transcription and repair processes. Rifampicin specifically inhibits DNA-dependent RNA polymerase of bacteria and hinders the synthesis of mRNA, thus achieving the purpose of antibacterial.
