|Pantoea agglomerans and Pseudomonas syringae bacteria|
Here's some information about the bacteria and how the images were taken:
Pantoea agglomerans and Pseudomonas syringae are two bacterial species that live in association with plants: the former as a harmless inhabitant of plant leaves and the latter as a pathogen that can colonize the inside part of the plants. Because it is not easy to visualize these bacteria in their natural environment (the surface of plant leaves), it is common to use fluorescently-tagged strains. I discussed this type of research in a previous post.
In this case, P. agglomerans expressed the green fluorescent protein (GFP), while P. syringae expressed the red fluorescent protein (DsRed). Hence the colors that are displayed in the pictures are only pseudo-colors produced by the image analysis software. There's a good reason for that: I take black & white pictures! The red and green fluorescence were recorded sequentially, as we have to use different filters to capture one or the other. What is presented is a merged image, a reconstruction of the scene based on two distinct pictures. In contrast, the grayscale image is a phase contrast image that captured the two species simultaneously.