A method for producing high purity iron oxide nanoparticles using nanoparticle-producingcells, including: a) a pre-growth step that includes amplifying thenanoparticle-producingcell(s) in a pre-growth and/or fed-batch medium/media, and b) a growth stepthat includesamplifying the nanoparticle-producing cell(s) originating from the pre-growthstep in agrowth and/or fed-batch medium/media, wherein the pre-growth and/or growthand/or fed-batchmedium/media comprise(s), per kilogram or liter of pre-growth and/or growthand/orfed-batch medium/media: i) no more than 0.005 gram of yeast extract, and ii)no more than 0.001gram of CMR agent selected from boric acid and nitrilotriacetic acid, whereinthe fed-batchmedium when it is present is a medium that supplements the pre-growth and/orgrowthmedium/media, and wherein more nanoparticles are produced in the growth stepthan in thepre-growth step.
