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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 (shown 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 image above on the left 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 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, the black region is the bacterium and the gray filaments are the flagella that the bacteria use to propel themselves. 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. Please visit his homepage for more information.

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.

References:

Reviews:

Waterfield, N.R., T. Ciche and D. Clark. 2009. Photorhabdus and a host of hosts. Annu. Rev. Microbiol. 63:557-574.

Richards, G.R. and H. Goodrich-Blair. 2009. Masters of conquest and pillage: Xenorhabdus nematophila global regulators control transitions from virulence to nutrient acquisition. Cell Microbiol. 11:1025-1033.

Herbert, E.E. and H. Goodrich-Blair. 2007. Friend and foe: the two faces of Xenorhabdus nematophila. Nat Rev Microbiol. 5:634-646.

Goodrich-Blair, H. 2007. They've got a ticket to ride: Xenorhabdus nematophila-Steinernema carpocapsae symbiosis. Curr Opin Microbiol. 10:225-230.

Forst S, Dowds B, Boemare N, Stackebrandt E. 1997. Xenorhabdus and Photorhabdus spp.: bugs that kill bugs. Annu. Rev. Microbiol. 51:47-72.

Forst, S. and K. Nealson. 1996. Molecular biology of the symbiotic-pathogenic bacteria Xenorhabdus spp. and Photorhabdus spp. Microbiol. Rev. 60:21-43.

Selected Research Articles:

Ciche, T.A., K.S. Kim, B. Kaufmann-Daszczuk, K.C. Nguyen and D.H. Hall. 2008. Cell Invasion and Matricide during Photorhabdus luminescens Transmission by Heterorhabditis bacteriophora Nematodes. Appl. Environ. Microbiol. 74:2275-2287.

Cowles, C.E. and H. Goodrich-Blair. 2008. The Xenorhabdus nematophila nilABC genes confer the ability of Xenorhabdus spp. to colonize Steinernema carpocapsae nematodes. J. Bacteriol. 190:4121-4128.

Heungens, K., C. E. Cowles and H. Goodrich-Blair. 2002. Identification of Xenorhabdus nematophila genes required for mutualistic colonization of Steinernema carpocapsae nematodes. Mol. Microbiol. 45: 1337-1353.

Ciche, T. A., S. B. Bintrim, A. R. Horswill and J. C. Ensign. 2001. A phosphopantetheinyl transferase homolog is essential for Photorahabdus luminescens to support growth and reproduction of the entomopathogenic nematode Heterorhabditis bacteriophora. J. Bacteriol. 193:3117-3216.

Vivas, E. I. and H. Goodrich-Blair. 2001. Xenorhabdus nematophilus as a Model for Host-Bacterium Interactions: rpoS Is Necessary for Mutualism with Nematodes. J. Bacteriol. 2001 183: 4687-4693.

Govaidan, A. and A. Lanois. 2000. flhDC, the flagellar master operon of Xenorhabdus nematophilus: requirement for motility, lipolysis, extracellular hemolysis, and full virulence in insects. J. Bacteriol. 182:107-115.

Volgyi, A. A. Fodor, S. Forst. 2000. Inactivation of a novel gene produces a phenotypic variant cell and affects the symbiotic behavior of Xenorhabdus nematophilus. Appl. Environ. Micorbiol. 66:1622-1628.

Tabatabai, N. and S. Forst. 1995. Molecular analysis of the two-component genes, ompR and envZ, in the symbiotic bacterium Xenorhabdus nematophilus. Mol. Microbiol. 17:643-652.