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Biogenic magnetite is a fascinating example of how nature can generate functional magnetic nanostructures. Inspired by the magnetic bacteria, an attempt is made to mimic their magnetic properties, rather than their structures, to create living magnets at room temperature. The non‐magnetic probiotic bacteria Lactobacillus fermentum and Bifidobacteria breve are used as bioplatforms to densely arrange superparamagnetic nanoparticles on their external surfaces, thus obtaining the artificial magnetic bacteria. Magnetic probiotic bacteria can be produced by using superparamagnetic maghemite nanoparticles assembled at their surfaces. They present a collective ferromagnetic phase at room temperature. The blocking temperature of these maghemite nanoparticles increases more than 100 K when assembled at the artificial magnetic bacteria.

We have undertaken a magnetic study on the oral biodistribution and biodegradation of nude maghemite nanoparticles of 10 nm average size (MNP) and probiotic bacteria, Lactobacillus fermentum, containing thousands of these same nanoparticles (MNP-bacteria). Using AC magnetic susceptibility measurements of the stomach, small intestine, cecum and large intestine obtained after rat sacrifice, and iron content determination by ICP-OES, we have monitored the biodistribution and biodegradation of the maghemite nanoparticles along the gastrointestinal tract, after oral administration of both MNP and MNP-bacteria. The results revealed that the amount of magnetic nanoparticles accumulated in intestines is sensibly higher when MNP-bacteria were administered, in comparison with MNP. This confirms our initial hypothesis that the use of probiotic bacteria is a suitable strategy to assist the magnetic nanoparticles to overcome the stomach medium, and to achieve their accumulation in intestines. This finding opens doors to different applications. Since iron absorption in humans takes place precisely in the intestines, the use of MNP-bacteria as an iron supplement is a definite possibility. We have actually illustrated how the administration of MNP-bacteria to iron-deficient rats corrects the iron levels after two weeks of treatment.

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Abstract. "Two-in-one" magneto-optical bacteria have been produced using the probiotic Lactobacillus fermentum ® for the first time. We took advantage of two features of bacteria to synthesize this novel and bifunctional nanostructure: their metal-reducing properties, to produce gold nanoparticles, and their capacity to incorporate maghemite nanoparticles at their external surface. The magnetooptical bacteria survive the process and behave as magnets at room temperature.