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Although trace elements such as Iron is an essential nutrient of organisms, it is a toxic substance at high concentration for their growth and also change their metabolic patterns. In order to find out the influence of Iron on gram negative and gram-positive bacteria cells growth, the present investigation has been undertaken. In the series of experiments, B.cereus and E.coli incubated in NB-medium with and without Iron for 5 hours at 37?C. the result showed that addition 5.6 and 11.2 mg/li Fe+² or Fe+³ in medium (Inhibit) growth of B.cereus, but 11.2 mg/li Fe (II) or Fe (III) Inhibits of E.coli growth (P<0.001). These finding suggested that Iron is one of the necessary elements for microorganisms growth, but on the other hand Iron at high concentration has an toxic effect on their growth. B.cereus is more sensitive to toxic dose than E.coli.