BackgroundMagnetotactic bacteria have long intrigued researchers because they synthesize intracellular nano-scale (40-100 nm) magnetic particles composed of Fe3O4, termed magnetosomes. Current research focuses on the molecular mechanisms of bacterial magnetosome formation and its practical applications in biotechnology and medicine. Practical applications of magnetosomes are based on their ferrimagnetism, nanoscale size, narrow size distribution, dispersal ability, and membrane-bound structure. However, the applications of magnetosomes have not yet been developed commercially, mainly because magnetotactic bacteria are difficult to cultivate and consistent, high yields of magnetosomes have not yet been achieved.ResultsWe report a chemostat culture technique based on pH-stat feeding that yields a high cell density of Magnetospirillum gryphiswaldense strain MSR-1 in an auto-fermentor. In a large-scale fermentor, the magnetosome yield was significantly increased by adjusting the stirring rate and airflow which regulates the level of dissolved oxygen (DO). Low concentration of sodium lactate (2.3 mmol l-1) in the culture medium resulted in more rapid cell growth and higher magnetosome yield than high concentration of lactate (20 mmol l-1). The optical density of M. gryphiswaldense cells reached 12 OD565 nm after 36 hr culture in a 42 L fermentor. Magnetosome yield and productivity were 83.23 ± 5.36 mg l-1 (dry weight) and 55.49 mg l-1 day-1, respectively, which were 1.99 and 3.32 times higher than the corresponding values in our previous study.ConclusionsCompared to previously reported methods, our culture technique with the MSR-1 strain significantly increased cell density, cell yield, and magnetosome yield in a shorter time window and thus reduced the cost of production. The cell density and magnetosome yield reported here are the highest so far achieved with a magnetotactic bacteria. Refinement of this technique will enable further increase of cell density and magnetosome yield.

Magnetotactic bacteria synthesize magnetic particles called magnetosomes that cause them to orient to their external magnetic fields. However, the physiological significance and other possible functions of these magnetosomes have not been explored in detail. In this study, we have investigated the biological functions of magnetosomes with respect to their ability to scavenge reactive oxygen species (ROS) in Magnetospirillum gryphiswaldense MSR-1. To assess the changes in ROS levels under different conditions, cells were cultured under aerobic or micro-aerobic conditions in medium containing high and low amounts of iron. To ensure that the observed results were not due to nonspecific interactions, reactions were carried out using a mutant deficient in synthesizing magnetite (mamO-deficient mutant), its complementary strain or the wild-type MSR-1. We observed that the levels of intercellular ROS under micro-aerobic conditions with high-iron medium were much higher when the non-synthetic Fe(3) O(4) crystals mutant Mu21-415 was employed for the assay, compared with the wild-type or complementary strain, or when conditions were aerobic with low-iron medium. These results indicated that magnetosomes function in the scavenging of intracellular ROS. Furthermore, we have demonstrated that the magnetosomes exhibit peroxidase-like properties, by using the earlier reported in vitro horseradish peroxidase assay for artificial magnetic nanoparticles. In addition to possessing peroxidase-like activity, the magnetosomes also exhibited a more enzymatic kinetic response, suggesting that proteins on the membranes of the magnetosomes likely contribute to the enzymatic activity. This is the first study to demonstrate that magnetosomes play an important role in decreasing or eliminating ROS.

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