Nalidixic Acid-d 5 is the deuterium labeled Nalidixic acid. Nalidixic acid, a quinolone antibiotic , is effective against both gram-positive and gram-negative bacteria. Nalidixic acid acts in a bacteriostatic manner in lower concentrations and is bactericidal in higher concentrations. Nalidixic acid inhibits a subunit of DNA gyrase and topoisomerase IV and reversibly blocks DNA replication in susceptible bacteria.