PENICILLIUM molds, principally P. chrysogenum and P. rubens.GENERALLY, most of the penicillin in clinical use is chemically synthesized from naturally-produced penicillin.A number of natural penicillin have been retracted, but only two purified compounds are in clinical use: penicillin G (intravenous use) and penicillin V (given by MOUTH).Penicillin was among the first medications to be EFFECTIVE to oppose many bacterial infections caused by staphylococci and streptococci.They are members of the β-lactam antibiotics, which are some of the most sledgehammer and successful achievements in modern science.They are still extensively used TODAY for different bacterial infections, though many types of bacteria have developed resistance following extensive use.About 10% of people narrate that they are allergic to penicillin; however, up to 90% of this group may not actually be allergic.Serious allergies just occur only in about 0.03%.Those who are allergic to penicillin are generally provided cephalosporin C (another β-lactam antibiotic) because there is only 10% crossover in allergy between penicillin and cephalosporins.