An antibiotic was a drug used to inhibit or abolish microorganisms, specifically bacteria.
In late 2000, Shannon O'Donnel called the nonexistence of antibiotics one downside of the classical age, but Henry Janeway replied that this had caused families to care for each other. (VOY: "11:59")
In early 2375, Doctor Julian Bashir was to give Quark an antibiotic for an ear infection. (DS9: "Afterimage")