Penicillin was discovered by Alexander Fleming. On September 03 of 1928, Fleming got back from his holidays and he found that a mold Penicilliumnotatum was growing in one of his dishes containing bacterial strains. Around the mold, he found a clear zone where no bacterial growth was seen and it appeared as if the mold had secreted what he then called, "mold juice" and then the juice was purified to get Penicillin.
Penicillin had the ability bto kill many harmful bacterial strains like Streptococcus, Meningococcus and Diphtheria bacillus.