Antibacterial Activity of Positively and Negatively Charged Hematite (<i>α</i>-Fe₂O₃) Nanoparticles to <i>Escherichia coli</i>, <i>Staphylococcus aureus</i> and <i>Vibrio fischeri</i>

• Main Authors: Svetlana Vihodceva, Andris Šutka, Mariliis Sihtmäe, Merilin Rosenberg, Maarja Otsus, Imbi Kurvet, Krisjanis Smits, Liga Bikse, Anne Kahru, Kaja Kasemets
• Format: Article
• Language: English
• Published: MDPI AG 2021-03-01
• Series: Nanomaterials
• Subjects: hematite; <i>α</i>-Fe₂O₃ nanoparticles; hydrothermal synthesis; surface charge; antibacterial; nano-bio interactions
• Online Access: https://www.mdpi.com/2079-4991/11/3/652
• ISSN: 2079-4991

In the current study, the antibacterial activity of positively and negatively charged spherical hematite (<i>α</i>-Fe₂O₃) nanoparticles (NPs) with primary size of 45 and 70 nm was evaluated against clinically relevant bacteria <i>Escherichia coli</i> (gram-negative) and <i>Staphylococcus aureus</i> (gram-positive) as well as against naturally bioluminescent bacteria <i>Vibrio fischeri</i> (an ecotoxicological model organism). <i>α</i>-Fe₂O₃ NPs were synthesized using a simple green hydrothermal method and the surface charge was altered via citrate coating. To minimize the interference of testing environment with NP’s physic-chemical properties, <i>E. coli</i> and <i>S. aureus</i> were exposed to NPs in deionized water for 30 min and 24 h, covering concentrations from 1 to 1000 mg/L. The growth inhibition was evaluated following the postexposure colony-forming ability of bacteria on toxicant-free agar plates. The positively charged <i>α</i>-Fe₂O₃ at concentrations from 100 mg/L upwards showed inhibitory activity towards <i>E. coli</i> already after 30 min of contact. Extending the exposure to 24 h caused total inhibition of growth at 100 mg/L. Bactericidal activity of positively charged hematite NPs against <i>S. aureus</i> was not observed up to 1000 mg/L. Differently from positively charged hematite NPs, negatively charged citrate-coated <i>α</i>-Fe₂O₃ NPs did not exhibit any antibacterial activity against <i>E. coli</i> and <i>S. aureus</i> even at 1000 mg/L. Confocal laser scanning microscopy and flow cytometer analysis showed that bacteria were more tightly associated with positively charged <i>α</i>-Fe₂O₃ NPs than with negatively charged citrate-coated <i>α</i>-Fe₂O₃ NPs. Moreover, the observed associations were more evident in the case of <i>E. coli</i> than <i>S. aureus</i>, being coherent with the toxicity results. <i>Vibrio fischeri</i> bioluminescence inhibition assays (exposure medium 2% NaCl) and colony forming ability on agar plates showed no (eco)toxicity of <i>α</i>-Fe₂O₃ (EC₅₀ and MBC > 1000 mg/L).