The wide metabolic and physiological versatility of microorganisms can be used to degrade many pollutants. Bioremediation is the technological process whereby biological systems are harnessed to effect the clean-up of environmental pollutants. Nowadays, microbial systems are employed in bioremediation programmes, generally in the treatment of soils and waters contaminated with organic pollutants. There are instances where natural populations are not suitable for use in the remediation of polluted sites and therefore the utilization of genetically engineered microorganisms (GEMs) is being considered for in-situ bioremediation of contaminated ecosystems. The deliberate release of GEMs into the environment is a subject of considerable public concern. The potential risks associated with the release of GEMs into the environment has led to the construction of biological containment systems by which bacteria are killed in a controlled suicide process. Active biological containment systems usually consist of two different components, a killing element designed to induce cell death and a control element which modulates the expression of the killing function.