Ammonium polyoxomolybdate modulates antibiotic activity against resistant Staphylococcus aureus and Escherichia coli

This study investigated the antibacterial and potentiating activity of ammonium polyoxomolybdate tetrahydrate (NHMO; (NH4)6Mo7O24·4H2O; 1235.86 g/mol), particularly in combination with ampicillin against Staphylococcus aureus and Escherichia coli strains. Spectroscopic characterization by attenuated total reflectance Fourier transform infrared spectroscopy (ATR-FTIR) and microdilution tests with resazurin were used. NHMO showed no direct antibacterial activity, with a minimum inhibitory concentration (MIC) ≥1024 μg/mL (≈0.83 mM). However, when used at a subinhibitory concentration (MIC/8, 128 μg/mL; ≈0.10 mM) with antibiotics initially tested at 1024 μg/mL, NHMO reduced the MIC of ampicillin by approximately 50% for both bacterial strains. Sulbactam, a known β-lactamase inhibitor, showed superior results, confirming the presence of enzymatic resistance. NHMO also demonstrated a resistance-reversal effect in β-lactamase-producing S. aureus strains, lowering the MIC of ampicillin from 20.16 to 16 μg/mL in K4100 and from 322.54 to 256 μg/mL in K4414. Ciprofloxacin did not show relevant modulation, whereas gentamicin potentiation was observed against S. aureus 10. These findings indicate that NHMO can enhance the activity of antibiotics, particularly ampicillin and gentamicin, in resistant bacterial strains. Its action in β-lactamase-related assays suggests that it is a promising candidate in the search for alternatives to combat antimicrobial resistance.