Penicillins are a group of antibiotics originally obtained from Penicillium moulds, principally P. chrysogenum and P. rubens. Most penicillins in clinical use are chemically synthesised from naturally-produced penicillins. A number of natural penicillins are discovered but only two purified compounds are in clinical use such as penicillin G and penicillin V. Penicillins were among the first medications to be effective against many bacterial infections caused by staphylococci and streptococci. They are members of the β-lactam antibiotics, which are some of the most powerful and successful achievements in modern science. They are still widely used today for different bacterial infections, though many types of bacteria have developed resistance following extensive use.