Local Optical Temperature Measurements Around Magnetosomes Within Single Bacteria To Study Size And Geometry Effects On Heating

Using the temperature sensitive of the fluorescence of GFP, we record the local temperature inside magnetotactic bacteria. These bacteria use magnetosomes - membrane and protein enclosed magnetite crystals - for navigating along the earth magnetic field lines. Upon purification, suspensions of magnetosomes are found to be highly effective heaters while subject to radio frequency magnetic field, yielding larger Specific Absorption Rates (SAR) than most of the common ferrofluids, making magnetosomes ideal candidate for application in magnetic field induced cell stimulation or hyperthermia cancer treatment. In the wild-type bacteria the magnetosomes are aligned in well-ordered chains, but mutants are available with different spatial arrangement or crystal size. We demonstrate that GFP fused to a magnetosome protein MamC may function as a molecular-scale temperature probe, and show that wild type magnetosomes raise the local temperature around the magnetosome membrane significantly, upon application of radio frequency magnetic field. The heating is confined to the vicinity of the magnetosomes, as the mutant strain producing the same magnetosomes but expressing a cytosolic GFP does not show heating of the entire cytosol. This approach enables us to study the effect of different spatial arrangement and nano-crystal size on the RF magnetic field induced heating directly in the individual bacteria, and to screen for more efficiently heating mutations. Finite element simulation is performed to understand the relation between spatial arrangement of magnetosomes and their heating efficiency.