Marker and reporter genes: illuminating tools for environmental microbiologists.

Fluorescent and luminescent marker and reporter genes provide easily detectable phenotypes to microbial cells and are therefore valuable tools for the study of microorganisms in the environment. Although these tools are becoming widely adopted, there are still issues that remain to be solved, such as the dependence of the reporter output on the physiological status of the cell. Eventually it might be the use of marker and reporter genes themselves that will contribute towards better understanding of the physiological status of specific microbial populations in nature.