Bioremediation seeks to enhance and exploit naturally occurring microorganisms which break down contaminants by using the contaminants as an energy source for growth.  In many cases, these microorganisms are already present at the site and require simple stimulation through the use of macro and micronutrients.  In most petroleum-based bioremediation approaches, the only additional step required is the addition of an oxygen source to stimulate facultative bacteria.  Some sites lack the necessary microorganism population to facilitate sustained bioremediation. In these instances, beneficial microorganism populations are artificially introduced, or inoculated.  After ensuring the environment is hospitable for inoculation through the inclusion of macro and micronutrients, inoculated microorganisms quickly multiply as the contaminants are utilized as a source of energy for their growth.  These microorganism populations die off as the available contaminants are consumed, returning the flora to its pre-inoculation populations.