macrolide

Macrolides are a family of antibiotics built around a large ring-shaped molecule, with erythromycin, clarithromycin and azithromycin as the familiar members. They are go-to drugs for many respiratory and atypical infections, and a common alternative for people who cannot take penicillin.

Macrolides work by binding the 50S subunit of the bacterial ribosome and blocking the elongation of the protein chain, so the bacterium can no longer make the proteins it needs. This usually stops growth rather than killing outright, making macrolides largely bacteriostatic, though the effect can become bactericidal at high concentrations against susceptible organisms. They reach high levels inside cells, which suits them to intracellular pathogens like Legionella, Mycoplasma and Chlamydia.

Two practical points matter. First, several macrolides, especially clarithromycin and erythromycin, inhibit the liver enzyme CYP3A4 and can dangerously raise the levels of many co-administered drugs. Second, macrolides can prolong the QT interval on the heart's electrical trace, a risk worth weighing in patients with cardiac vulnerability. Azithromycin is gentler on both fronts and has an unusually long tissue half-life that allows very short courses.