Magnetomechanical Detachment of Bacterial Biofilms Using Anisotropic Magnetic Iron Oxide Nanochains

Bacterial biofilms attach to various surfaces and represent an important clinical and public health problem, as they are highly recalcitrant and are often associated with chronic, nonhealing diseases and healthcare-associated infections. Antibacterial agents are often not sufficient for their elimination and have to be combined with mechanical removal. Mechanical forces can be generated by actuating nonspherical (anisotropic) magnetically responsive nanoparticles in a rotating magnetic field. We have thus prepared anisotropic superparamagnetic nanochains in the size range of 0.5-1 μm by magnetically assembling several iron oxide nanoparticle clusters and coating them with a layer of silica with different shell morphologies: smooth, moderately rough, and highly rough. The silica surface was additionally functionalized with carboxylic groups to increase colloidal stability. The efficacy of the nanochains in biofilm removal was studied systematically with three different model nonpathogenic bacterial species Escherichia coli, Lactococcus lactis, and Pseudomonas fragi; two different magnetic field strengths; two stirring speeds; and two treatment durations. All bacterial species were engineered to express fluorescent proteins to enable quantification of biofilm removal by colony-forming unit count and fluorescence measurements. Nanochains removed >90% of Gram-negative E. coli and P. fragi with a stronger magnetic field, and <90% of Gram-positive L. lactis with a weaker magnetic field. Surface roughness of nanochains, duration, and stirring speed also affected removal, but the effect could not be generalized. In contrast to their effects on biofilms, the functionalized nanochains showed no toxicity to Caco-2 intestinal epithelial cells, regardless of whether magnetomechanical force was employed or not. In summary, we demonstrated that remotely controlled spatial movement of nanoparticles can generate sufficient mechanical forces to disperse attached biofilms while retaining safety in an epithelial cell model.