Cells of magnetotactic bacteria and their magnetic inclusions have novel magnetic, physical, and perhaps optical properties that can and have been exploited in a variety of scientific, commercial, and other applications. The large number of reports on the applications of magnetotactic bacteria is enormous and thus we will only discuss some of the more interesting and significant ones. We direct the reader to reviews devoted to this subject (Lang and Schuler, 2006; Lang et al., 2007; Matsunaga and Arakaki, 2007).
In general, the amount of magnetite and magnetosomes from magnetotactic bacteria is relatively low especially considering the amount needed for specific applications. Thus, in order to produce enough cells, magneto-somes and magnetite crystals for these applications, cells must be grown in very large cultures where the conditions for growth and magnetite synthesis must be optimized.
Mass culture of a magnetotactic bacterium was first described using M. magneticum. Cells of this species were grown in a 1000-liter fermentor and the amount of magnetosomes recovered was 2.6 mg per liter of culture (Matsunaga et al., 1990). Different optimization experiments were conducted in fed-batch cultures of M. magneticum that did not result in a higher yield of magnetosomes or cells (Matsunaga et al., 1996, 2000a). Recombinant M. magneticum harboring the plasmid pEML was grown in a pH-regulated fed-batch culture system where the addition of fresh nutrients was feedback-controlled as a function of the pH of the culture (Yang et al., 2001a). The magnetosome yield was maximized by adjusting the rate of addition of ferric iron. Feeding ferric quinate at 15.4 mg/min resulted in a magnetosome yield of 7.5 mg/liter which may be the highest reported based on unit volume. Different iron sources and the addition of various nutrients and chemical reducing agents (e.g., L-cysteine, yeast extract, polypeptone) were shown to have a significant effect on magne-tosome yield by M. magneticum grown in fed-batch culture (Yang et al., 2001b).
A seemingly better control of the growth of Magnetospirillum species was achieved using an oxygen-controlled fermentor (Heyen and Schüler, 2003; Lang and Schüler, 2006). Three species were grown using this method, M. gryphiswaldense, M. magnetotacticum, and M. magneticum, and 6.3-, 3.3-, and 2.0-mg magnetite per liter per day were obtained from these species, respectively (Heyen and Schüler, 2003). The use of this system has resulted in the highest yields of cells and magnetite per unit of time reported thus far.
North-seeking cells of polar magnetotactic bacteria have been used to determine south magnetic poles in meteorites and rocks containing fine-grained (<1 mm) magnetic minerals (Funaki et al., 1989,1992). Harasko et al. (1993, 1995) investigated the applicability of magnetotactic bacteria for nondestructive domain analysis on soft magnetic materials. Cells of magnetotactic bacteria have also been used in medical applications. For example, they have been introduced to and phagocytized by granulocytes and monocytes which were then magnetically separated (Matsunaga et al., 198 ). Since cells of magnetotactic bacteria can be separated magnetically relatively easily, they may have potential in the area of bioremediation. The possibility of using magnetotactic bacteria in the removal of heavy metals and radio-nucleotides from waste water was discussed (Bahaj et al., 1993,1998a,b,c). Cells of the sulfate-reducing magnetotactic bacterium, D. magneticus, were used in cadmium recovery using magnetic separation (Arakaki et al., 2002). A very recently described application is the trapping of magnetotactic bacteria using a commercial magnetic recording head. This method may be useful in counting magnetotactic bacteria cells in water samples collected from the natural environment or to detect magnetically labeled bacteria or magnetosomes (Krichevsky et al., 2007).
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