A novel method was developed for isolating magnetic bacteria without magnetic collection using magnetotaxis. The method consists of incubation of sediments, enrichment of bacteria in the medium, isolation of enriched bacteria by colony formation, and optimization of conditions for growth and synthesis of magnetic particles. The water column above natural sediment, incubated at 25°C under dim light, and containing many species of bacteria, was employed as the inoculum. Collection of magnetic bacteria using magnets was not carried out. Ferric quinate was used as the main iron source in the liquid isolation medium. Due to iron sulfide precipitation, formation of black crystals was observed in the enriched culture of magnetic bacteria. Magnetic bacteria were purified by colony formation from enriched cultures which formed the black crystals. Culturing condition was optimized by addition of appropriate nutrients which behave as electron accepters or donors. This method allows isolation of non-motile and non- or weakly-magnetotactic bacteria, which would not accumulate in the presence of an applied magnetic field. A sulphate-reducing magnetic anaerobe, RS-1, which is weakly magnetotactic, was isolated by this method. In addition, the successful isolation of RS-1 by this method suggests the presence of magnetic bacteria which exist in a non-magnetic state in sediments.
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