Figure 1.

The path to single-molecule detection of proteins inside living bacterial cells. A look at the evolution of imaging bacterial proteins using fluorescent protein fusions is shown. GFP was first developed as a biological probe for gene expression and was used on bacterial populations. Soon thereafter, fluorescence microscopy was focusing on single bacterial cells (10) as well as the subcellular distribution of proteins because there was adequate spatial resolution to do this. In 2006, it became possible to visualize single fluorescent protein fusions (using the Venus-YFP variant (23)) in cells with only a few copies of the protein of interest, and in 2008, the single-molecule detection capability was combined with photoactivation and tracking to study proteins of any copy number inside living bacterial cells (both nonactivated (P) and activated (FP) proteins are represented). To see this figure in color, go online.