Flow cytometry (including fluorescence activated cell sorting) is another method to quantify gfp-labelled bacteria. This approach allows the detection and quantitation of individual fluorescent cells within a population since the flow cytometer measures and analyzes the optical properties of hundreds of single cells per second passing through a focused laser beam. It can also quantify fluorescence intensity of various groups of gfp-labelled microorganisms. Tombolini et al. (1997) showed that a pseudomonas bacteria chromosomally labelled with one copy of gfp had a lower fluorescence intensity than an E. coli strain containing multiple copies of a gfp-plasmid. The authors also observed an increase in the fluorescence signal per cell during starvation. They suggested that this was due to an increase in gfp concentration in the cells as a result of cell size reduction during starvation.