Penicillin is derived from Penicillium mold and is one of the older antibiotics. The main antibacterial spectrum of penicillin, including gram-positive cocci and positive bacilli, is less effective in treating gram-negative bacilli. Gram-positive cocci, including hemolytic streptococci, Staphylococcus aureus, and Staphylococcus epidermidis, but also meningococcus and gonococcus. Penicillin is also therapeutic for syphilis caused by the spirochete syphilis. Gram-positive bacilli for tetanus bacillus, diphtheria bacillus, and anthrax bacillus infections can be treated with penicillin. Skin tests need to be done before penicillin is used because penicillin can cause death from severe allergic reactions.