Antibacterial drugs are derived from bacteria or molds or are synthesized de novo. Technically, “antibiotic” refers only to antimicrobials derived from bacteria or molds.

Antibiotics have many mechanisms of action, including the following:

Inhibiting cell wall synthesis

Increasing cell membrane permeability

Interfering with protein synthesis, nucleic acid metabolism, and other metabolic processes (eg, folic acid synthesis)

Antibiotics sometimes interact with other drugs, raising or lowering serum levels of other drugs by increasing or decreasing their metabolism or by various other mechanisms.
The most clinically important interactions involve drugs with a low therapeutic ratio (ie, toxic levels are close to therapeutic levels). Also, other drugs can increase or decrease levels of antibiotics.