IT IS A PRIMARILY BACTERIOSTATIC COMPOUND, BUT MAY BE CIDAL AT HIGHER CONCENTRATIONS AND IN ACID URINE ITS ACTIVITY IS ENHANCED AT LOWER PH. IT INHIBITS MANY GRAM NEGATIVE BACTERIA, BUT DUE TO DEVELOPMENT OF RESISTANCE, ACTIVITY IS NOW RESTRICTED LARGELY TO E.COLI. IT ANTAGONIZES THE ACTION OF NALIDIXIC ACID. NITROFURANTOIN IS REDUCED BY BACTERIAL FLAVOPROTEINS TO REACTIVE INTERMEDIATES WHICH INACTIVATE OR ALTER BACTERIAL RIBOSOMAL PROTEINS AND OTHER MACROMOLECULES. AS A RESULT OF SUCH INACTIVATIONS, THE VITAL BIOCHEMICAL PROCESSES OF PROTEIN SYNTHESIS, AEROBIC ENERGY METABOLISM, DNA SYNTHESIS, RNA SYNTHESIS, AND CELL WALL SYNTHESIS ARE INHIBITED. THE BROAD-BASED NATURE OF THIS MODE OF ACTION MAY EXPLAIN THE LACK OF ACQUIRED BACTERIAL RESISTANCE TO NITROFURANTOIN, AS THE NECESSARY MULTIPLE AND SIMULTANEOUS MUTATIONS OF THE TARGET MACROMOLECULES WOULD LIKELY BE LETHAL TO THE BACTERIA.