Magnetosomes are intracellular structures produced by magnetotactic bacteria, which comprise magnetic nanoparticles surrounded by lipid bilayer membrane. They have attracted much attention for biotechnological applications. This is due to a series of appealing properties summarized below that are not usually found in chemically synthesized nanoparticles. (i)

The magnetosomes are magnetic nanoparticles, which possess a narrow size distribution and uniform morphology when the magnetotactic bacteria are cultivated in optimum conditions, i.e., essentially using a low oxygen concentration (varied between 0.25 and 10 mbar, Heyen and Schüler, 2003) during the growth. In these conditions, the magnetosome size distribution can be as small as ~10 nm with magnetosome sizes typically lying between 45 and 55 nm for the most commonly studied species of magnetotactic bacteria (AMB-1 and MSR-1) (Bazylinski and Frankel, 2004; Taylor and Barry, 2004).

Depending on the type of application, different types of suspensions containing either magnetotactic bacteria or isolated magnetosomes can be prepared. For example, for targeting tumors, it has been suggested to use suspensions of living magnetotactic bacteria, which are administered intravenously and are naturally attracted by the anoxic environment of the tumor (Benoit et al., 2009). However, the use of living magnetotactic bacteria for medical applications will unlikely be accepted by regulatory agencies (FDA in the USA, EMA in Europe) and this review focuses on magnetosomes that will more likely be accepted for clinical trials. For treating cancers using magnetic hyperthermia, it has been suggested to use suspensions containing chains of magnetosomes (chains of magnetic nanoparticles) extracted from magnetotactic bacteria (Alphandéry et al., 2011a,b, 2012a, 2013). For other applications, the magnetosomes that have been used have been isolated from magnetotactic bacteria and treated to remove biological material surrounding them. They have then been coated with lipids for stabilization (Yoshino et al., 2007). To prepare suspensions containing living whole magnetotactic bacteria, AMB-1, MSR-1, or MS-1 strains can be purchased from the ATCC or DSMZ culture collection with a growth protocol, which is provided. A TEM (transmission electron microscopic) image of a typical whole magnetotactic bacterium containing several chains of magnetosomes is presented in Figure 1A. Among the different strains of magnetotactic bacteria, MSR-1 has achieved the highest yield of magnetosome production (170 mg/L/day) (Zhang et al., 2011), and therefore seems to be a promising strain for medical applications, which require a large quantity of magnetosomes. To obtain suspensions containing extracted chains of magnetosomes such as those shown in Figure 1B, the latter can be isolated from magnetotactic bacteria using either sonication (Taoka et al., 2006; Sun et al., 2008b; Alphandéry et al., 2011b), a treatment with sodium hydroxide (Philipse and Maas, 2002), with a French press (Grünberg et al., 2004; Matsunaga et al., 2007; Xiang et al., 2007), or with a pressure homogenizer (Guo et al., 2011; Tang et al., 2012). Different methods have also been suggested to purify the suspension of magnetosomes after extraction involving either magnetic separation of the magnetosomes from the cellular debris (Grünberg et al., 2001; Alphandéry et al., 2011b, 2012a), a treatment with proteinase K to remove surface proteins (Guo et al., 2011), phenylmethylsulfonyl fluoride to inhibit the activity of the protease, DNase I to remove DNA (Sun et al., 2007). The most commonly used method of sterilization for magnetosome suspensions is gamma rays (Guo et al., 2011). The magnetosomes can be stabilized in water. Before being administered to human, the suspensions of magnetosomes also need to be characterized. The biological material surrounding the magnetosomes can be characterized using chromatography, infrared spectroscopy, SDS Page, and mass spectroscopy (Grünberg et al., 2004). TEM can be used to measure the size of the magnetosomes and to verify the high level of crystallinity of the magnetosome core. Magnetic measurements could also be carried out to detect the Verwey transition, which would reveal the presence of magnetite in the magnetosomes (Alphandéry et al., 2008). Finally, it is also possible to obtain suspensions of individual magnetosomes isolated from magnetotactic bacteria (Figure 1C), in which most of the biological material has been removed by heating the suspensions of magnetosomes during 5 h at 90°C in the presence of 1% SDS (Alphandéry et al., 2011b, 2012a).

The cytotoxicity of bacterial magnetosomes was found to depend strongly on the type of cell tested. For Hepatoma H22 cells (liver cancer cells) and promyelocytic HL60 cells (leukemia cells), the bacterial magnetosomes began to show signs of cytotoxicity at relatively low magnetosome concentrations of 9 μg/mL (Sun et al., 2010). On the other hand, for fibroblasts and breast cancer cells, such as EMT-6 cells or MDA-MB-231 cells, the magnetosomes were not cytotoxicity below ~1 mg/mL (Xiang et al., 2007; Sun et al., 2010; Alphandéry et al., 2011b). The cytotoxicity was also found to depend on the incubation time (Alphandéry et al., 2008). Indeed, for magnetosome concentrations higher than 1 mg/mL, cytotoxicity was found to be low when the incubation time was shorter than 24 h. On the other hand, for magnetosome concentrations lower than 1 mg/mL, cytotoxicity remained low even for longer incubation times than 24 h. The cytotoxicity of the magnetosomes seems to be comparable to that of superparamagnetic iron oxide nanoparticles (SPION), which were found to be cytotoxic above a concentration of 100 μg/mL (Karlsson et al., 2008; Ankamwar et al., 2010; Singh et al., 2010). The acute toxicity of the bacterial magnetosomes was also studied. It was found that for an intravenous administration of bacterial magnetosomes, rats could survive for a maximum quantity of magnetosomes administered of 480 mg/kg (Liu et al., 2012). This quantity of nanoparticles is higher than that of 135 mg/kg found for SPION (Liu et al., 2012). The immunotoxicity of the magnetosomes was also reported. For that, 1 mg of a suspension of bacterial magnetosomes (magnetic nanoparticles) was administered in the ears of rabbits and rabbit temperatures were monitored (Sun et al., 2010). It was found that rabbit temperatures did not increase following the administration of the suspensions of magnetosomes. This result together with other findings presented in reference (Sun et al., 2010) support the idea that the magnetosomes are not pyrogenic when they are prepared in the specific conditions described in Sun et al. (2010).

Regarding the biodistribution of the magnetosomes, different situations can arise. The magnetosomes could be metabolized by the organism and transformed into free iron. They may also remain in the form of crystallized nanoparticles. In both cases, free iron or crystallized magnetosomes can either accumulate in the organism or be eliminated in the feces or urines. In order to understand the biodistribution of the magnetosomes, the latter have been mixed with proteins from bovine pancreas that simulate the activity of lysosomes. It was observed that the magnetosomes were degraded by the proteases after 28 days, which suggests that lysosomes degrade the magnetosomes (Liu et al., 2012). Magnetosomes have also been administered intravenously in rats (Sun et al., 2009) and mice (Liu et al., 2012), and it was found that they end up in the lysosomes of liver and spleen (Sun et al., 2009; Liu et al., 2012). These results suggest that the reticuloendothelial system removes the magnetosomes from bloodstream and that the magnetosomes are then transformed into free iron (Sun et al., 2009). In this study (Sun et al., 2009), the magnetosomes were neither found in the feces nor in the urine or rats. However, in other studies, magnetosomes have been found in the feces of rats following intratumoral administration (Alphandéry et al., 2011b), or intravenous administration, as observed by TEM observation of the feces (unpublished results obtained by us). We can conclude that after being administered in the organism, a portion of the magnetosomes is most probably transformed into free iron while another portion remains in a crystallized form and is eliminated in the feces. However, more studies are necessary to better understand the magnetosome biodistribution.

Magnetosomes can be used for other applications, for example, to detect nucleotide polymorphism, which is useful to diagnose diseases such as cancer, hypertension, or diabetes, to separate cells or to detect DNA (Arakaki et al., 2008). To separate cells, magnetic beads or SPION have been tested. However, these two types of magnetic materials present drawbacks. Magnetic beads are large and hence prevent cells from dividing and proliferating correctly. SPION on the other hand are only weakly magnetic due to their unstable magnetic moment at physiological and room temperatures, which makes them poorly efficient to separate cells. By contrast, the magnetosomes are of smaller sizes than the magnetic beads and are more strongly magnetic than the SPION due to their ferrimagnetic properties. This makes them ideal candidates for applications in cell separation (Arakaki et al., 2008). The magnetosomes have also been used for immunoassays, for example, to detect small molecules such as pollutant, hormones, or toxic detergents. These molecules have been attached to the magnetosome surface using antibodies that specifically bind to them. The complex formed by the magnetosomes and these molecules has then been detected (Arakaki et al., 2008). Finally, magnetosomes have been used to extract DNA. For that, they have been modified and covered with layers of aminosilanes that link DNA. The complex formed by the magnetosomes and DNA has been bound to a magnetic column and DNA has been collected by elution with a phosphate buffer (Arakaki et al., 2008).

In conclusion, we have presented in this review several medical applications of the magnetosomes and we have also briefly described a few methods that can be used to prepare the magnetosomes for these applications.

The author declares that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.