), confocal laser scanning microscopy (Eberl et al. 1997) or enumerated by flow cytometry (Tombolini et al. 1997). A further advantage of GFP, over other biomarkers, is the fact that no other energy source or substrate addition is required, other than oxygen during initial formation of the chromophore. Therefore, the GFP biomarker holds tremendous promise for elucidation of specific bacterial numbers and their behaviour, colonization, distribution, interaction, and movement in situ in a diversity of environmental sample types. Additionally, mutations have been introduced into the GFP gene in order to produce fluorescence signals with altered properties (Heim et al. 1994, Delagrave et al. 1995, Crameri et 1996, Heim and Tsien 1996). For example, one of the mutants (P4) is a