Boromycin is a bacteriocidal polyether-macrolide antibiotic. It was initially isolated from the Streptomyces antibioticus, and is notable for being the first natural product found to contain the element boron. It is effective against most gram-positive bacteria, but is ineffective against gram-negative bacteria. Boromycin kills bacteria by negatively affecting the cytoplasmic membrane, resulting in the loss of potassium ions from the cell. Boromycin has not been approved as a drug for medical use.