Examples are penicillin, tetracycline, and fluoroquinolones.
Originally, antibiotics were derived from natural sources, e.g., penicillin from molds, but many currently used antibiotics are semi-synthetic and modified with additions of synthetic chemical components. Some scientists reserve the term antibiotic for naturally produced substances and use antimicrobial to encompass both synthetic and natural forms. Resistant bacteria evolve as a result of using antibiotics. Antibiotics kill or inhibit susceptible bacteria and resistant ones survive. Susceptibility to one antibiotic does not mean susceptibility to all antibiotics.