Iron oxide nanoparticles functionalized with nisin for rapid inhibition and separation of Alicyclobacillus spp.

Alicyclobacillus acidoterrestris can spoil various fruit juices, and tools to reduce the impact of this contaminant are urgently needed. Here, we describe a novel therapeutic approach in which iron oxide nanoparticles (IONPs) were conjugated with the known antibiotic nisin (IONPs–nisin). The size, crystalline nature, magnetic characteristics and surface charge of the nanoparticles were characterized with transmission electron microscopy (TEM), X-ray diffraction (XRD), vibrating sample magnetometry (VSM) and Zetasizer Nano ZS (Zeta). The average diameter of the IONPs was 30 ± 7 nm. The TEM and XRD data indicated that the IONPs were very spherical magnetic particles with a single crystalline phase. The VSM study showed that the IONPs were superparamagnetic with a saturation magnetization (MS) of about 33 emu g−1. The IONPs had a negative charge of around −32.8 mV at pH 7.0. The optimal preparation of IONPs–nisin composites was at pH 7.0 with a 2 : 1 (IONPs : nisin) ratio for 1 h. FT-IR data confirmed conjugation of IONPs to nisin, and the composites retained 86.69% of their antimicrobial activity after storage for 7 days at 4 °C and 83.46% after storage for 7 days at 25 °C. The products retained their antibacterial activity after treatment at pH 2 to 11. The minimum inhibitory concentration (MIC) of IONPs–nisin composites against A. acidoterrestris DSM 3923 was 2.5 mg mL−1 of AAM medium (pH 4.2). Finally, we used the CFDA-SE leakage assay and scanning electron microscopy (SEM) to show that the IONPs–nisin inhibited target strains via pore formation in the membrane—this is the same as with nisin. These composites retained the features of both nisin and IONPs. The composites were easily separated from the sample via a magnet after being deployed as antimicrobials. To the best of our knowledge, this is the first report of an IONP–nisin composite with significant implications to the food safety community. These products might be useful tools for controlling spoilage due to A. acidoterrestris.