nematode intestine
electron micrograph
This symbiosis involves three different players: the nematode, Steinernema carpocapsae, the symbiotic bacterium Xenorhabdus nematophilus and the insect that nematode infects. While the bacteria are mutualistic symbionts inside the nematode, the same bacteria are pathogens inside the insect. This shows how one bacterium can have two different life styles and either help or harm the host animal.

The nematode has different developmental stages, one them is the infective juvenile stage (second image on the right). This stage of the nematode is soil-dwelling and non-feeding; it's sole purpose is to locate and infect a larval stage insect. 
Inside of this insect the nematodes reproduce. These nematodes carry the symbiotic X. nematophilus symbiont bacterium in a specialized intestinal organ known as the vesicle. The nematode requires the bacterium to efficiently kill and reproduce inside insect hosts. The first image on the right shows the infective juvenlie nematode intestine 
(extruded from the body) stained with crystal violet. The prurple, rod-shaped bacteria are apparent in the anterior portion of the intestine. This image was taken with a Nikon Eclipse TE300 inverted microscope at X,1000 magnification.

The third image on the right side shows Galleria mellonella (wax worm) larvae. The one on the right is a living insect, 
the one on the 
left has been infected and killed by the S. carpocapsae-X. nematophilus symbiotic pair. Note the change in color after death of the insect that is due to a pigment produced by the bacterium.

In this electron micrograph of a single X. nematophilus cell (last picture on the right), the black region is the bacterium and the gray filaments are the flagella that the bacteria use to propel the
mselves. These bacteria are capable of switching their lifestyle. In one host they are pathogens and in the other host they are mutualistic symbionts. 
Identifying the factors that allow the bacteria to recognize the host environment and regulate the gene expression accordingly represents an exciting avenue of research.

Investigators who study the Xenorhabdus/Photorhabdus and Steinernema/Heterorhabditis symbioses

Steven Forst's group is at the University of Wisconsin at Milwaukee. 

Heidi Goodrich-Blair's group works on both the symbiotic and pathogenic aspects of this symbiosis at the University of Wisconsin at Madison. For more information please visit her homepage.