Antibacterial agents

Antimicrobial agents and their types

All antimicrobial medications are intended to eliminate the cause of the disease (etiotropic therapy). According to their target of action, they are divided into antibacterial, antituberculosis, antifungal, antiprotozoal, and antiviral drugs.

Broadly speaking, all etiotropic antibacterial agents can be divided into two large groups:

• Synthetic chemotherapeutic drugs — created artificially in a laboratory.
• Antibiotics — substances of natural, semisynthetic, or synthetic origin.

Synthetic chemotherapeutic drugs

These are fully artificial compounds that have no natural analogues produced by microorganisms. This group includes:

1. Sulfonamides.
2. Quinolones and fluoroquinolones.
3. Nitroimidazole derivatives.
4. Nitrofuran derivatives.
5. Isonicotinic acid hydrazide (INH) derivatives — used primarily as antituberculosis agents.
6. PAS (para-aminosalicylic acid) derivatives.

Antibiotics: requirements and type of action

Antibiotics are substances that suppress the growth or cause the death of bacteria, fungi, or protozoa. The first of them were exclusively natural compounds produced by actinomycetes and bacteria. Later, after decoding their structure, scientists learned to modify the molecules, creating semisynthetic drugs.

Strict requirements are imposed on effective antibiotics: they must work at minimal concentrations, not lose their activity in body tissues, and be harmless to humans (without disrupting the physiological state of the macroorganism).

According to the type of action on the cell, the following are distinguished:

• Microbocidal (bactericidal, fungicidal) — completely kill the pathogen.
• Microbostatic (bacteriostatic, fungistatic) — inhibit the growth and reproduction of microbes, giving the immune system time to destroy the infection.

Classification of antibiotics by chemical structure

Molecular structure determines a drug's belonging to a specific class and its pharmacological properties:

• Beta-lactam antibiotics: contain a beta-lactam ring. This includes penicillins, cephalosporins, carbapenems, and monobactams.
• Tetracyclines: consist of four condensed six-membered rings (natural — tetracycline, semisynthetic — doxycycline).
• Macrolides and azalides: have a macrocyclic lactone ring linked to carbohydrates (erythromycin, azithromycin).
• Glycopeptides: substituted peptide compounds (vancomycin, teicoplanin).
• Lincosamides: two cyclic compounds (lincomycin, clindamycin).
• Dioxyaminophenylpropane derivatives: amphenicols (chloramphenicol).
• Ansamycins: rifampicin.
• Polypeptides: polymyxins.
• Polyenes: amphotericin B, nystatin, levorin.

Characteristics of beta-lactams and aminoglycosides

The beta-lactam group is extremely diverse. Their core structure varies depending on the subgroup:

Aminoglycosides (aminosugars linked by a glycosidic bond) deserve special attention. Within the group, they are similar in properties, but differ in resistance profile and spectrum. They are divided into three generations:

• 1st generation (streptomycin, neomycin, kanamycin): the first drugs with a narrow spectrum, to which bacterial resistance develops rapidly.
• 2nd generation (gentamicin, tobramycin, netilmicin, sisomicin): a broader spectrum, with gentamicin acting as the main representative.
• 3rd generation (amikacin): possesses the widest spectrum, resistant to degrading bacterial enzymes. Used as a reserve drug.

Spectrum of antimicrobial action

Spectrum of activity is the number of pathogen species sensitive to a specific drug.

• Narrow spectrum: drugs are active against a small number of microorganisms. For example, penicillins, macrolides, and lincosamides act predominantly on Gram-positive bacteria. Monobactams and polypeptides, conversely, target Gram-negative flora.
• Broad spectrum: drugs affect many different species of bacteria simultaneously, covering both the Gram-positive and Gram-negative spectrum.

Class and exam questions

What else to study on this topic

• Classification and spectrum of activity of 1st–4th generation cephalosporins.
• 5th generation cephalosporins, anti-anaerobic cephalosporins, and cephamycins.
• Mechanisms of bacterial resistance to antibiotics.
• Specifics of using antituberculosis drugs (INH and PAS derivatives).
• Pharmacology of antifungals and antiprotozoal agents.

Other topics in the "Additional Topics" section

Prepared by WhiteCoat Pro faculty in accordance with the medical school curriculum. For educational purposes only.