The emergence and spread of antimicrobial resistance highlights the urgent need for new antibiotics. Organoarsenicals have been used as antimicrobials since Paul Ehrlich's salvarsan. Recently a soil bacterium was shown to produce the organoarsenical arsinothricin. We demonstrate that arsinothricin， a non-proteinogenic analog of glutamate that inhibits glutamine synthetase， is an effective broad-spectrum antibiotic against both Gram-positive and Gram-negative bacteria， suggesting that bacteria have evolved the ability to utilize the pervasive environmental toxic metalloid arsenic to produce a potent antimicrobial. With every new antibiotic， resistance inevitably arises. The gene， widely distributed in bacterial arsenic resistance () operons， selectively confers resistance to arsinothricin by acetylation of the α-amino group. Crystal structures of ArsN1 -acetyltransferase， with or without arsinothricin， shed light on the mechanism of its substrate selectivity. These findings have the potential for development of a new class of organoarsenical antimicrobials and ArsN1 inhibitors.