A biomarker, or molecular marker, or reporter gene is defined as a DNA sequence introduced into organisms. It confers a distinct genotype or phenotype to enable monitoring in a given environment. Molecular markers such as: LacZ (-galactosidase), xylE (catechol 2,3-dioxygenase), lux (bacterial luciferase), luc (insect luciferase), phoA (alkaline phosphatase), gusA and gurA (-glucuronidase), gfp (green fluorescent protein), bla (-lactamase) and antibiotic or heavy metals resistance genes are widely used in genetically engineered (GEMs) microorganisms research. These genes are involved in the detection and enumeration of GEMs after their introduction into the environment. Molecular markers, especially lux and gfp, are widely used in the creation of whole-cell based biosensors which are commonly used for the examination of toxicity of environmental pollutants.