Evaluation of toxicity of heavy metals for Escherichia coli growth
Iron is an essential nutrient for living agents due to its noticeable activity in electron transport reactions in biological systems, but its insolubility and reactivity lead to problems of poor availability and toxicity, respectively (Andrews, 1998). Due to insolubility of this element at physiological pH all living agents have involved to use iron transport systems and storage proteins. Bacteria elaborate and secrete high-affinity extracellular ferric chelators (siderophores) and many of them have ferrous iron transporter, to soluble iron prior to transport (Andrews et al., 2003, Köster, 2001). They may posses other iron transport systems (Velayudhan, et al., 2000, Marlovits et al., 2002). Furthermore, it was demonstrated that extracellular iron is not the only source of available iron and many bacteria deposit intracellular reserves of this nutrient within iron storage proteins. These iron stores can then be used to enhance growth when external iron supplies are restricted (Andrews, 1998). However, a relationship between concentration of iron and microbial infection was seen by many investigators using experimental studies on human and laboratory animal. These revealed that pathogens often use low environmental iron levels as a signal for the induction of virulence genes. For example, induction of exotoxins and proteases by many bacteria such as enterohaemorrhagic E. coli (Shiga-like toxin I) which affected the bacterial virulence (Litwin and Calderwood, 1993, Calderwood and Mekalanos, 1987). In other hand, high concentration of iron is extremely toxic and may implicate to enhance bactericide effects of antimicrobial agent or noxious substances (Gelvan, 1997, Chamnongpol, 2002).