Antibiotic is a chemotherapeutic agent with activity against microorganisms such as bacteria, fungi or protozoa. Antibiotics can be classified as bactericidal or bacteriostatic. Bactericidals kill bacteria directly and bacteriostatics prevent them from dividing.

Anti-bacterial antibiotics are used for treatment or prevention of bacterial infection. They are not active against viruses, fungi, or other microbes. Anti-bacterial antibiotics can be categorized based on their target specificity: "narrow-spectrum", which target particular types of bacteria, such as Gram-negative or Gram-positive bacteria, while "broad-spectrum" antibiotics affect a wide range of bacteria.