Bioremediation not produce any other toxic compounds.The microorganisms

Bioremediation can be applied in two ways ex situ and in situ. Ex situ is when the contaminants are transferred from contaminated site to somewhere else for treatment while in situ is the treatment of contaminants at the contaminated site. Ex situ is more expensive than in situ bioremediation ( Sitinder et al, 2006 ). Another technique used is biosimulation that involves the providing of suitable environment for the microorganisms already present at the contaminated site to speed up their remediation process.An eligibility criteria must be met by the microorganisms selected for the process of Bioremediation that is: The microorganisms selected must not be pathogens of plants, animals or human beings.The microorganisms must not produce any other toxic compounds.The microorganisms must have additional features that enable them to adopt to the new environment. The microorganisms are supposed to rapidly degrade or convert the targeted contaminants into non bioavailable,harmless substitute.Our main focus in this paper will be specifically heavy metals contamination and their remediation using microorganisms. Heavy metals: There is not a universally acceptable definition for heavy metals every author define heavy metals in his respective context however heavy metals can be broadly defined based on the successive criteria (1) density, (2) atomic number, (3) chemical properties. Heavy metals are characterized as elements having density greater than 5g/cm3 (five times denser than water).Heavy metals are elements having atomic number greater than 20 ( Raskin et al, 1994 ) except alkaline earth metals, alkaline metals, actinides and lanthanides.Heavy metals can form complexes due to their variable valency ( Nieboer and Richardson, 1980 ).The most common toxic heavy metals found at polluted sites, in order of their abundance are lead (Pb), chromium ( Cr),  Arsenic (As), Zinc (Zn), cadmium (Cd), Copper (Cu) and Mercury (Hg) ( USEPA, 1996 ) which can be taken up by plants and microbes ( Miller, 1996 ). Heavy metals having relatively high density are toxic even at low concentration ( Iram et al, 2013 ) The presence of toxic metals in soil can severely inhibit the biodegradation of organic contaminants, Environment contaminated with heavy metals pose serious ecological and health risks such as As, Cd, Pb and Hg cause serious health conditions such as heart problems, cancer, liver damage, cardiovascular and neurological diseases, fatal damage to the central nervous system, hypophosphatemia and sensory disturbances ( Scragg, 2006; NSC, 2009 ) The problem of mercury (Hg) contamination came into international focus after high levels of methylmercury (CH3)2Hg was discovered in shellfish and fish in Japan (Minamata Bay) that resulted in hundreds of deaths and thousands of poisonings.The Center for Disease Control (CDC) and Agency for Toxic Substances and Disease Registry (ATSDR) estimate that in the united states of America, approximately 10% of children have high lead (Pb) levels in blood, greater than 10  g/dl, a potentially toxic level. Heavy metals are generally thought to have no known biological function however new research (data) suggests that some heavy metals have limited biological use. For example, As we know Cadmium (Cd) is a toxic non-essential heavy metal, though some marine phytoplankton can substitute cadmium (Cd), cobalt (Co) and zinc (Zn) for one another to manage the normal function of enzymes whenever the required element is unavailable or limiting.