Resazurin rapid screening for antibacterial activities of organic and inorganic nanoparticles: Potential, limitations and precautions

Nanoparticles have been used as antibacterial agents in several products. To optimize their effectiveness, synthesis processes and particle modifications have been developed, creating the need for a rapid screening method to investigate their potencies. Owing to the opacity and insolubility of nanoparticles, a classical method to determine antibacterial activity-such as the minimum inhibitory concentration (MIC), which relies on turbidimetry-might not apply to them. In this study, we demonstrate the potential of a dye (resazurin)-based assay as an indicator of bacterial growth to rapidly screen the antibacterial activities of both organic and inorganic nanomaterials against both gram-negative (E. coli) and gram-positive (S. aureus) bacteria. The results indicate that the resazurin-based assay successfully determine the MIC of organic lipid nanocarriers, and several inorganic nanoparticles. However, the use of resazurin require a precaution for nanoparticles with photocatalytic properties, which may cause dye degradation at higher concentrations. In this study, resazurin bleaching was observed at approximately >50 mg/ml of TiO2. In summary, the modified MIC assay with resazurin can evaluate antibacterial activity of nanomaterials, whose turbidity interferer conventional MIC assay. This modification conserves an advantage of MICs assay which are simple and reliable. This would be useful for screening of antibacterial nanomaterials.