Antibiotic Classes

Classification by chemical structure

Chemical structure is the main criterion for dividing antibiotics into classes. Historically, the first drugs were natural compounds synthesized by actinomycetes or bacteria. Later, scientists learned to modify their structure, creating semisynthetic and fully synthetic analogues.

Main antibiotic families:

• Beta-lactams: the largest group, containing the beta-lactam ring.
• Macrolides and azalides: based on a macrocyclic lactone ring linked to carbohydrate residues (erythromycin, clarithromycin, azithromycin).
• Aminoglycosides: aminosugars linked by a glycosidic bond. Divided into generations: Generation I (streptomycin, kanamycin), Generation II (gentamicin), and Generation III (tobramycin, sisomicin).
• Tetracyclines: the molecule consists of four fused six-membered rings. They can be natural (tetracycline) or semisynthetic (doxycycline).
• Glycopeptides: substituted peptide compounds (vancomycin, teicoplanin).
• Lincosamides: include two cyclic compounds (lincomycin, clindamycin).
• Amphenicols: derivatives of dioxyaminophenylpropane, the main representative is chloramphenicol (levomycetin).
• Ansamycins (rifamycins): rifamycin, rifampicin.
• Polypeptides: contain residues of polypeptide compounds (polymyxins).
• Polyenes: a specific group of antifungal antibiotics (amphotericin B, nystatin, levorin).
• Other antibiotics: drugs that do not belong to the main classes (fusidic acid).

Type and spectrum of antimicrobial action

Antimicrobial drugs are intended for the etiotropic treatment of infections. In addition to antibiotics, these include synthetic chemotherapeutic agents (sulfonamides, quinolones, nitroimidazole and nitrofuran derivatives).

Based on their effect on the microbial cell, drugs are divided into two categories:

1. Microbicidal (bactericidal, fungicidal) — cause the death of the pathogen.
2. Microbiostatic (bacteriostatic, fungistatic) — inhibit the growth and reproduction of microbes.

The spectrum of action determines the number of microorganism species sensitive to the drug:

• Narrow spectrum: affect a limited number of bacteria. For example, penicillins, macrolides, and lincosamides are active primarily against gram-positive flora. Monobactams and polypeptides are active against gram-negative flora.
• Broad spectrum: active against a large number of different microbial species.

Features of beta-lactam antibiotics

This is the most important class of antibacterial agents. Representatives of various subgroups differ in the structural features of the beta-lactam core:

This large group also includes cephamycins (containing a 7-α-methoxy group), oxacephems, penems, nocardicins, and clavulanates (e.g., clavulanic acid, which has an oxazolidine ring system).

Drug requirements and side effects

An ideal antibiotic must meet strict requirements: have microbicidal or microbiostatic action at a minimal concentration, retain its activity in tissues, and be completely harmless to the host organism without disrupting its physiological state.

However, in practice, antibiotic therapy is always associated with risks. The main side effects include dysbiosis and allergic reactions of varying severity — from urticaria to anaphylactic shock. If a patient has a history of severe allergy to beta-lactam antibiotics, they are strictly contraindicated. In such cases, physicians use alternative regimens, prescribing macrolides or tetracyclines.

A specific complication in the treatment of certain infections is the Jarisch-Herxheimer reaction. For example, when initiating therapy for syphilis with penicillins, a massive death of *Treponema pallidum* occurs. A huge amount of their endotoxins is released into the bloodstream all at once, producing an acute systemic response: the patient develops a fever, chills, tachycardia, and exacerbation of rashes.

For successful treatment of severe lesions (e.g., neurosyphilis), drugs must cross the blood-brain barrier (BBB). High doses are used for this: benzylpenicillin is administered at 20 million IU per day, and ceftriaxone at 2 g per day intravenously for a course of 2 weeks.

Class and exam questions

What else to study on this topic

• Synthetic antimicrobial agents (sulfonamides, fluoroquinolones, nitrofuran derivatives)
• Aminoglycoside generations: from streptomycin to sisomicin
• Antifungal antibiotics (polyenes: amphotericin B, nystatin)
• Features of neurosyphilis therapy and crossing the blood-brain barrier
• Structure of the beta-lactam ring in various subgroups (penams, cephems, carbapenems)

Other topics in the "Additional Topics" section

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