Magnetic resonance imaging (MRI) is a non-invasive imaging modality used in longitudinal cell tracking. Previous studies suggest that MagA, a putative iron transport protein from magnetotactic bacteria, is a useful gene-based magnetic resonance contrast agent. Hemagglutinin-tagged MagA was stably expressed in undifferentiated embryonic mouse teratocarcinoma, multipotent P19 cells to provide a suitable model for tracking these cells during differentiation. Western blot and immunocytochemistry confirmed the expression and membrane localization of MagA in P19 cells. Surprisingly, elemental iron analysis using inductively-coupled plasma mass spectrometry revealed significant iron uptake in both parental and MagA-expressing P19 cells, cultured in the presence of iron-supplemented medium. Withdrawal of this extracellular iron supplement revealed unexpected iron export activity in P19 cells, which MagA expression attenuated. The influence of iron supplementation on parental and MagA-expressing cells was not reflected by longitudinal relaxation rates. Measurement of transverse relaxation rates ( R 2 * and R 2 ) reflected changes in total cellular iron content but did not clearly distinguish MagA-expressing cells from the parental cell type, despite significant differences in the uptake and retention of total cellular iron. Unlike other cell types, the reversible component R 2 ′ ( R 2 * ‒ R 2 ) provided only a moderately strong correlation to amount of cellular iron, normalized to amount of protein. This is the first report to characterize MagA expression in a previously unrecognized iron exporting cell type. The interplay between contrast gene expression and systemic iron metabolism substantiates the potential for diverting cellular iron toward the formation of a novel iron compartment, however rudimentary when using a single magnetotactic bacterial gene expression system like m agA . Since relatively few mammalian cells export iron, the P19 cell line provides a tractable model of ferroportin activity, suitable for magnetic resonance analysis of key iron-handling activities and their influence on gene-based MRI contrast.

Bacteria constitute a significant part of the biomass of the human microbiota, but their interactions are complex and difficult to replicate outside the host. Exploiting the superior resolution of magnetic resonance imaging (MRI) to examine signal parameters of selected human isolates may allow tracking of their dispersion throughout the body. We investigated longitudinal and transverse MRI relaxation rates and found significant differences between several bacterial strains. Common commensal strains of lactobacilli display notably high MRI relaxation rates, partially explained by outstanding cellular manganese content, while other species contain more iron than manganese. Lactobacillus crispatus show particularly high values, 4-fold greater than any other species; over 10-fold greater signal than relevant tissue background; and a linear relationship between relaxation rate and fraction of live cells. Different bacterial strains have detectable, repeatable MRI relaxation rates that in future may enable tracking of their persistence in the human body for enhanced molecular imaging.