Beta-lactams: the cell-wall builders
The beta-lactams are the workhorse drug class of antibacterial therapy, named for a four-membered beta-lactam ring at their core. They include the penicillins, the cephalosporins, plus carbapenems and monobactams. All of them bind and disable penicillin-binding proteins — the enzymes that cross-link peptidoglycan — so the growing bacterium can't finish its wall and bursts. Because human cells have no wall, beta-lactams are remarkably safe; their main hazard is drug allergy, not organ toxicity.
Cephalosporins are sorted into 'generations'. Roughly, each later generation trades some activity against gram-positive bacteria for more reach against gram-negative ones, and the newest cross into the brain to treat meningitis. This is the simplest example of expanding spectrum, the idea we unpack in guide 3.
The ribosome-blockers
Several classes work by jamming the bacterial ribosome, but they differ in how hard they hit and how toxic they are. Aminoglycosides (like gentamicin) bind the 30S subunit and are powerfully bactericidal against gram-negative bacteria — but they are also notably nephrotoxic and can damage hearing, so blood levels are often watched with therapeutic drug monitoring.
Macrolides (like azithromycin) bind the 50S subunit, stop the bug from growing, and are gentle enough to be a common choice for chest infections and for people allergic to penicillin. Tetracyclines also hit the ribosome and add cover against unusual organisms. As a family, ribosome-blockers tend to be more bacteriostatic — a distinction we make precise in the next guide.
Quinolones and how to read a class
Fluoroquinolones (like ciprofloxacin) inhibit DNA gyrase, an enzyme bacteria need to untangle DNA before copying it. They are orally well-absorbed, broad, and bactericidal — but their breadth and convenience led to overuse, and they carry rare but serious adverse effects on tendons and nerves, earning a black box warning in many countries.