Imagine a bacterium that lives as both a symbiont in one organism and a pathogen in another. One organism needs this bacteria in order to survive, whereas other organisms may die once infected by this particular bacterium. Meet Xenorhabdus nematophilus. A rod-shaped, facultative anaerobic bacteria it is a gram-negative member of the family Enterobacteriaceae. Although very similar to other Enterobacteriaceae, X. nematophilus is of a bigger size and does not reduce nitrates to nitrites. Also, although other members of Xenorhabdus might luminescence, this particular bacterium does not. Additionally it has no catalase activity.
X. nematophilus lives a very unique life cycle. The bacteria reside in the intestines of juvenile nematodes of the genus Steinernema. These nematodes are terrestrial and reside in soil. When nematodes come upon soil dwelling insect larvae, they will crawl into either the spiracles or the mouth. Once there the nematodes penetrate the insect hemocoele and X. nematophilus is released from the gut of the nematode. It is here that X. nematophilus reproduces and also begin to alter the insect tissues into products that can be utilized by the nematode. During the time that the insect is infected X. nematophilus produces both exo- and endo- toxins. These toxins are harmful to the host insect, and eventually kill the insect. Once the insect is dead, both X. nematophilus and the nematode will continue living off of the dead insect until all nutrients are gone. From here both organisms are off to find a new host.
X. nematophilus exists in two different forms. Although the two forms can be easily distinguished by observing colonies that are formed, the two forms must be injected into insect larvae to observe the differences. The primary form provides better conditions than the secondary for nematode reproduction when injected with axenic nematodes. The secondary form is the most stable of the two. The primary easily converts to the secondary form in vivo.
Since X. nematophilus kills soil residing insects, people have begun to analyze the possibility of using these particular bacteria as an insecticide. X. nematophilus could possibly help kill off the insects the effect crops and fields, without damaging the surrounding areas since the bacteria would only be affecting the insect itself. Furthermore, scientists are also considering adding some type of bioluminescence to X. nematophilus since this is one member of Xenorhabdus that does not bioluminescence. If the bacteria could possibly give off light, it would be possible to trace the path the bacteria makes through the insect to see exactly how it is killing the insect. Being able to do this would help shine light on the field of bacterial insecticides.
Xenorhabdus nearly 400 years ago when a physician named Aldrovandi recorded various nematode diseases concerning bacteria. It was not until the mid-1970’s that Poinar recorded the first documentation involving a bacterium that lives mutualistically with a nematode. Since then, new members have been discovered that live symbiotically with various species of nematodes.
X. nematophilus is a very unique bacterium that lives as a symbiont with nematodes, but as a parasite in insects. The bacterium is essential in killing insects and also for the development of juvenile nematodes. Since X. nematophilus easily kills insects, various organizations are looking into the possibility that these particular bacteria might usefully serve as insecticides. The future looks very positive for X. nematophilus.
Sources• Khandelwal, P. Applied and Environmental Microbiology. Insecticidal Activity Associated with the Outer Membrane Vesicles of Xenorhabdus nematophilus. April 2003. http://aem.asm.org/cgi/content/full/69/4/2032. April 1, 2004.
• Xiu-fen, Y. Journal of Agricultural University of Hebei. Inhibition of metabolites from Xenorhabdus nematophilus against Phtytophthora infestants. April 2001. http://www.cipotato.org/GILB/Pubs/proceedings_easa/yanxiufen%2814%29.pdf . April 2, 2004.
• Thomas, G. and Poinar, G. Symbiotic Bacteria. Xenorhabdus gen. nov., a Genus of Entomopathogenic, Nematophilic Bacteria of the Family Enterobacteriaceae. http://kbn.ifas.ufl.edu/biology/XENOPHOT.htm. April 1, 2004.
• Goodrich-Blair, H. Research Description. http://www.loci.wisc.edu/molbio/profiles/Goodrich_BlairHeidi.html. April 1, 2004.*Disclaimer - This report was written by a student participaring in a microbiology course at the Missouri University of Science and Technology. The accuracy of the contents of this report is not guaranteed and it is recommended that you seek additional sources of information to verify the contents.
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