Fluorescent bacteria under the microscope

Pantoea agglomerans and Pseudomonas syringae bacteria

Some time ago I made experiments growing two bacterial species on a gel surface, using fluorescence to distinguish between them. Since some of these pictures looked nice to me, I decided to share them here!

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.

The bacteria grew on the surface of a nutrient gel medium, and they are at a stage where many of them form a single layer of cells on the gel surface (every area where you can distinguish individual bacteria). When the fluorescence intensity is higher and the details are blurred, it means that several layers of bacteria are sitting on top of each other.

The two bacterial species are morphologically distinct: Pseudomonas syringae form longer rods than Pantoea agglomerans. The fluorescence greatly enhances the distinction between the two, and this is useful in studies that aim at studying the proportion and distribution of different bacterial species in natural environments.