Bioengineering Magnetic Nanoparticles in Magnetotactic Bacteria

Magnetic nanoparticles (MNPs) are in great demand in diverse modern technologies such as high-density data storage, electromagnetic shielding, and various applications for biomedicine including targeted drug delivery or hyperthermia. Compared to bulk magnets, MNPs shows unique and interesting properties, which largely dependent on their size, shape or crystal composition. The strong interest in the filed is to produce uniform-sized MNPs with desired properties required for each technological application. However, chemical synthesis of structurally perfect MNPs with narrow size distributions still remains a challenge [1]. In contrast, magnetic nanoparticles biomineralized by natural organisms are known to show narrow size distributions, high crystal perfections that could be biosynthesized under environment-friendly reaction conditions.

Our goal is to design and produce MNPs with various properties by genetically engineering Magnetospirillum magneticum AMB-1, a bacteria that can biosynthesize chains of magnetic nanoparticles (called magnetosomes). Magnetosomes are composed of magnetite (Fe₃O₄) nanoparticles with remarkably narrow size/shape distribution. By modifying bacterial MNPs from bottom-up and retaining its natural ability to produce them with narrow size distributions, we envision the production of “designer magnetic nanoparticles” which is otherwise difficult to achieve. 

Results

Characterizing genetic parts for magnetotactic bacteria. Magnetospirillums are microaerobic alphaproteobacteria with 6-hour doubling time, and culturing this bacteria is known to be difficult due to several reasons, including the cell’s genomic instability and their fastidious and microaerobic mode of growth. Several efforts have been reported to optimize and develop a genetic system for this strain [2]. We have engineered and characterized basic genetic parts for M. magneticum to further expand the genetic toolkit. First, we have characterized stable plasmids in this strain, which became useful to further characterize following genetic parts. Second, we have constructed and characterized several constitutive promoters which spanned 25-fold range of expression, and inducible promoters that showed high dynamic range, up to 260-fold.

Tuning the magnetic properties of magnetosome-MNPs. For many magnetic applications, maximized magnetic moment per particles is one of the ideal properties of MNPs. This could be achieved by several strategies such as: enlarging their size up to the single-domain limit, elongating their shape, or doping heavy metals such as cobalt or nickel. To explore these possibilities, several studies have reported their effort to change the size or composition of bacterial MNPs [3]. We used the genetic tools that we have constructed to tune the expression of magnetosome-related genes, such as mamGFDC or mms6 cluster, to explore the landscape of expression level vs size/shape relationship. In the presentation, we will also discuss our attempts to decorate the surface or modify the chemical composition of MNPs.

References

1) Prozorov T et al. Mater Sci Eng R 74, 133–172 (2013).
2) Schultheiss D & Schuler D. Arch Microbiol 179, 89–94 (2003), Okamura Y et al. Appl Environ Microbiol 69, 4274–4277 (2003).
3) Staniland S et al. Nat Nanotech 3, 158–162 (2008), Tanaka M et al. J Biol Chem 286, 6386–6392 (2011).