Abstract: Methods and compositions for using magnetosomes as cellular contrast agents and markers for magnetic resonance imaging are provided. Certain methods involve synthesizing magnetosomes in a cell as directed by a nucleotide construct comprising an exogenous polynucleotide sequence, wherein the magnetosome serves as a contrast agent or marker for magnetic resonance imaging. Methods of synthesizing and isolating magnetosomes for introduction into immune-matched cells within a tissue or subject for use as a contrast agent or marker for magnetic resonance imaging are also provided. Also provided are methods for stably transfecting cells to express a polypeptide that drives or modulates magnetosome production in the cell, cells produced by such methods and methods for their isolation, transgenic animals comprising at least one eukaryotic cell produced by such methods, and vectors and delivery systems for the transfection of such cells.