Genetically Engineered Magnetotactic Bacteria Enable the Synthesis of Enlarged Magnetite Crystals with Enhanced Coercivity

Magnetotactic bacteria synthesize magnetite nanoparticles with diameters of 30–40 nm within intracellular organelles called magnetosomes, which exhibit well‐defined size, high crystallinity, and inherent biocompatibility, making them attractive for biomedical and nanotechnological applications. Because coercivity increases with particle size within the single‐domain range, enlarging magnetosome crystals through genetic engineering represents a promising strategy for enhancing their magnetic performance. Here, we report an inducible overexpression strategy targeting the mms6 and mamGFDC operons in a high‐yield magnetosome‐producing Magnetospirillum magneticum AMB‐1 strain. Overexpression of either operon drives a reproducible and tunable enlargement of magnetite crystal diameter, approximately 1.6‐ to 2.1‐fold larger than the uninduced condition and the wild type. The enlarged crystals display well‐faceted cubo‐octahedral morphology and single‐crystal character. Vibrating sample magnetometry reveals that the mamGFDC ‐overexpressing strain exhibits an approximately 1.2‐fold increase in coercivity relative to the uninduced condition, while saturation magnetization remains unchanged. These findings establish that operon‐level overexpression in an optimized genetic background provides precise, tunable control over magnetite crystal size and magnetic properties, advancing the rational design of high‐performance biogenic magnetic nanomaterials.