Classification and Chemical Structure
The foundation of the macrolide molecule is a macrocyclic lactone ring attached to sugar moieties. In pharmacology, these drugs are classified according to two main principles:
- By origin: natural and semisynthetic antibiotics.
- By lactone ring size: depending on the number of carbon atoms in the core, macrolides are divided into 14-membered, 15-membered, and 16-membered rings.
Mechanism of Antimicrobial Action
Macrolides exert a bacteriostatic effect—they halt bacterial growth and reproduction by interfering with protein synthesis.
- Molecular target: the antibiotic binds to a specific ligand within the ribosomal exit tunnel on the 50S subunit of the bacterial ribosome.
- Site of action: inhibition occurs at the translocation step.
- Pathogenesis: the macrolide physically blocks the movement of the nascent peptide chain from the A-site to the P-site of the ribosome. This disrupts the elongation of peptide chains, terminating protein assembly.
Note: despite their primary bacteriostatic effect, at high concentrations macrolides can act bactericidally against certain pathogens: pneumococci, group A $\beta$-hemolytic streptococci, as well as the bacteria causing pertussis and diphtheria.
Spectrum of Activity and Clinical Application
The spectrum of macrolides is largely similar to that of benzylpenicillin. They are active against gram-positive (streptococci, staphylococci) and gram-negative cocci (gonococci, meningococci), as well as Haemophilus influenzae, borrelia, Treponema pallidum, and Helicobacter pylori.
The main advantage of this group is their ability to penetrate host cells effectively. This makes them powerful weapons against intracellular pathogens: chlamydia, legionella, ureaplasma, and mycoplasma.
Main indications for use:
- Respiratory tract and ENT organs: streptococcal tonsillopharyngitis, pertussis, diphtheria, and "atypical" pneumonia.
- Skin and soft tissues: erysipelas, scarlet fever.
- Specific infections: chlamydial infections, mycoplasmal infections, osteomyelitis, oral cavity infections.
- Gastroenterology: inclusion in H. pylori eradication regimens for peptic ulcer disease.
- Mycobacteriosis: therapy and prevention of infections caused by Mycobacterium avium (clarithromycin and azithromycin are used, presumably inhibiting protein synthesis in these bacterial organelles).
- Prophylaxis: year-round rheumatic fever prophylaxis in patients with penicillin allergy.
Resistance and Environmental Influences
The efficacy of macrolides depends directly on tissue acidity. In an inflammatory focus, the pH typically drops. In such an acidic environment, drug ionization increases, leading to inactivation. Optimal conditions for antibiotic activity require a pH above 5.
Bacteria actively develop resistance to macrolides, and resistance genes are frequently plasmid-mediated. Four microbial defense mechanisms are recognized:
- Enzymatic inactivation: production of esterases that hydrolyze the drug. This pathway is particularly characteristic of enterococci.
- Target modification: alteration of the ribosomal binding site structure via chromosomal mutations or methylation. The production of methylase enzymes is the primary resistance mechanism in gram-positive flora.
- Impaired transport: bacteria either decrease cell membrane permeability or upregulate active drug efflux pumps.
- Cross-resistance (MLS phenotype): if a bacterium constitutively expresses methylase, it becomes invulnerable not only to macrolides but also to structurally distinct drugs with a similar mechanism of action—lincosamides (clindamycin) and group B streptogramins.