Penicillin G (benzylpenicillin) molecule. Computer artwork showing the molecular structure of penicillin G. Penicillin G is a beta-lactam antibiotic used for the treatment of a number of bacterial infections. It is on the World Health Organization's List of Essential Medicines, the most important medications needed in a basic health system. Atoms are represented as spheres: carbon (black), hydrogen (white), nitrogen (dark-blue), oxygen (red), sulphur (yellow) and bonds between the atoms by cylinders.