Teredinibacter turnerae

Wood found at the bottom of an ocean eaten by shipworms Courtesy of Dr. Grace Lim-Fong. Picture taken at the Zoological Museum of Keil

Characteristics of the symbiont/pathogen

Teredinibacter turnerae is a member of the proteobacterium family, and is a gram-negative rod with a single polar flagellum used for mobility. This organism is primarily marine, although certain variants have been found to live in fresh water streams. T. turnerae optimally grows at 30-35 degrees Celsius, pH 8-8.5, .3M NaCl, and in elevated concentrations of Ca2+/Mg2+, which reflects its primary existence in sea water. [2] The genome of T. turnerae has been completely sequenced, containing 5,192,641 base pairs with over 100 genes implicated in the digestion of plant polysaccharides. [2] It is closely related to Saccharophagus degradans, another bacterium that is capable of digesting cellulose. Unlike S. degradans, which digests primarily aquatic polysaccharides, T. turnerae encodes genes that are primarily specialized in the degradation of terrestrial polysaccharides. [4] T. turnerae is culturable, and furthermore exhibits antibiotic production, making it a potentially medically important microbe. [6]

Characteristics of the host

The host of T. turnerae, Lyrodus pedicellatus, are commonly known as "shipworms" because they digest wood immersed in salt water, are small salt water clams with small shells. Even though they resemble worms, they are not actually worms, but mollusks. There are 300 species of Teredindiae, the shipworm family, which all have an organ known as Deshayes. Deshayes is a special organ where the digestion of cellulose occurs, which is made possible by symbiotic T. turnerae.[6]

"Lyrodus pedicellatus (shipworm)" [1]

Host-Symbiont Interaction

The symbiosis between T. turnerae and L. pedicellatus is assumed to be a mutualism because there is no direct evidence supporting that T. turnerae benefits from the relationship.

T. turnerae has at least 2 enzymes important for L. pedicellatus. T. turnerae has cellulase which breaks down cellulose, the carbohydrate found in wood. Evidence supports that T. turnerae thrives in the Deshayes of the shipworm. T. turnerae also performs nitrogen fixation via the enzyme nitrogenase that converts nitrogen into ammonia. Nitrogen fixation gives L. pedicellatus a usable source of nitrogen.[6]

Studies have shown that T. turnerae are endosymbionts and are faculative endosymbionts. This is due to the fact that T. turnerae have not lost genes of its core metabolism, nor has it lost genes implicated in defense against bacteriophages, and has not accrued harmful mutations. This suggests that T. turnerae can live outside of L. pedicellatus, although there has been no evidence that it does as it has never been isolated from any environment other than that of the gills of L. pedicellatus.[4]

Molecular Insights into the Symbiosis

Recently there have been studies focusing on the important enzyme used by L. pedicellatus to digest cellulose. CelAB (cellulase), is the enzyme used by shipworms to break down cellulose, and is found to have an optimum pH of 6 and temperature of 42 degrees Celsius.[6][7] This enzyme has been shown to bind cellulose and chitin, and can degrade multiple polysaccharides. This enzyme has come to light in recent years due to the interest in industrial cellulases, which would be used to break the primary constituent of all plant matter (cellulose) into sugars that can then be used to produce the biofuel ethanol. [7]

"Genome of T. turnerae"

Ecological and Evolutionary Aspects

It is assumed that the host had engulfed T. turnerae. Focusing on the genome of T. turnerae, there have been no deletions suggesting that they do not have to be completely dependent on L. pedicellatus. However, there has been no known evidence suggesting that T. turnerae is a free-living organism like S. degradans. Even though no evidence has been found, it makes sense to suggest that T. turnerae can live in a free-living environment because it has all the genes necessary to survive and thrive outside L. pedicellatus.[4]

Recent Discoveries

There has been a recent discovery in the genome of T. turnerae that has not been found anywhere else. Approximately 7% of the genome has metabolite pathways that have been found to have antibiotic properties similar to Streptomyces which leads to a benefit in medical research to kill of pathogens in humans.[4]

More evidence is starting to prove that there is more than one type of symbiont communities that is harbored in the Deshayes of L. pedicellatus. Focusing on diversity as a factor, research has determined that there are four different types of communities of symbionts found in L. pedicellatus. These symbionts had ribotypes very close to S. degradans. This could mean that maybe the four ribotypes rely on each other to help the shipworm function with digestion of wood. Each might have special genes that encode for one thing and when those genes are on, all of the ribotypes together are needed to produce the enzymes necessary for L. pedicellatus.[4]

Another reason why there are more than one ribotype is because there may be multiple symbiosis such as bacterium-bacterium and bacterium-animal. This means that one of the bacteria uses something from the other and then when that combination occurs, the host benefits as well by the enzymes produced. The bacterium-bacterium symbiosis may help promote growth of bacterium communities suggesting that there maybe a true symbiosis between the host and the bacterium.[3]

References

[[2]Distel, D., Morrill, W. MacLaren-Toussaint, N., Franks, D., Waterbury, J. 2002. Teredinibacter turnerae gen. nov., sp. nov., a dinitrogen-fixing, cellulolytic, endosymbiotic gamma-proteobacterium isolated from the gills of wood-bring molluscs. International Journal of Systematic and Evolutionary Microbiology 52:2261-2269.]

[[3]Distel, D., Beaudoin, D., Morrill, W. 2002. Coexistence of multiple proteobacterial endosymbionts in the gills of wood-boring bivalve Lyrodus pedicellatus (Bivalvia:Terdinidae). American Society for Microbiology 68:6292-6299.]

[[4]Yang, J., Madupu, R., Durkin, A., Ekborg, N., Pedamallu, C., Hostetler, J., Radune, D., Toms, B., Henrissat, B., Coutinho, P., Hanora, A., Schmidt, E., Haygood, M., Posfai, J., Benner, J., Madinger, C., Nove, H., Anton, B., Chaudhary, K., Foster, J., Holman, A., Kumar, S., Lessard, P., Luyten, Y., Slatko, B., Wood, N., Wu, B., Teplitski, M., Mougous, J., Ward, N., Eisen, J., Badger, J., Distel, D. 2009. The complete genome of Teredinibacter turnerae T7901: An intercellular endosymbiont of marine wood-boring bivalves (shipworms) PLoS ONE 4:6085.]

[[5]Luyten, Y., Thompson, J., Morrill, W., Ploz, M., Distel, D. 2006. Extensive variation in intercellular symbiont community composition among members of a single population of the wood-boring bivalve Lyrodus pedicellatus. Applied and Environmental Microbiology 72:412-417.]

[[6]Trindade-Silva, A., Machado-Ferreira, E., Senra, M., Vizzoni, V., Yparraguirre, L., Leoncini, O., Soares, C. 2009. Physiological traits of the symbiotic bacterium Teredinibacter turnerae isolated from the mangrove shipworm Neoteredo reynei. Genetics and Molecular Biology 32:572-581.]

[[7]Ekborg, N., Morrill, W., Burgoyne, A., Li, L., Distel, D. 2007. CelAB, a multifunctional cellulase encoded by Teredinibacter turnerae T7902, a culturable symbiont isolated from the wood-boring marine bivalve Lyrodus pedicellatus. Applied and Environmental Microbiology 73:7785-7788.]

Edited by [Ahron Flowers], students of Grace Lim-Fong