|Image: Kilauea Volcano|
by Brian Snelson
I have a few strange but true facts about spirochetes to share which you may not know. A few are ones I have shared here before - but most are not something about which I've already written. What you read here today may surprise you...
- Many people call Borrelia burgdorferi spirochetes Gram negative bacteria. However, Borrelia burgdorferi are not Gram-negative bacteria even if a Gram negative stain works on them:
"Borrelia were thought to be Gram negative because of their double membrane structure, but genetic analysis places them - along with other spirochetes - into a separate eubacterial phylum. Ultrastructural molecular and biochemical studies have emphasized the wide taxonomic gap between spirochetes and Gram-negative bacteria."
- From "The Genus Borrelia" by Melissa Caimano. Prokaryotes (2006) 7:235-293.
- Unlike Leptospira and Brachyspira, spirochetes in the Borrelia and Treponema genera appear to have acquired Phenylalanyl-tRNA synthetase (PheRS) genes from Archaea through horizontal gene transfer.  Borrelia and Treponema have Archaea genes.
- What are Archaea? It used to be that cells and organisms were labeled as being either eukaryotic (animal, plants, fungi, and protists) or prokaryotic (bacterial). In recent years, a subset of what seemed to be prokaryotic organisms called archaebacteria were shown to have a distinctly different genetic and molecular background than other bacteria. It was determined that these organisms were not bacteria and should become an entirely new domain, Archaea. Despite their differences, there is some evidence that Archaea may be more closely related to Eukaryotes than Prokaryotes.
- Somewhere along the line, an ancient Spirochaeta relative picked up genes from Archaea's order, Thermococcales. Borrelia and Treponema have close affinities with Thermococcus and Pyrococcus (not depicted on tree).
- The fascinating thing about this genetic relationship is that these genes come from organisms which are extremely thermophilic organisms. They are extremophiles - which means they can live in extreme environments. Thermophilic extremophiles thrive in hot environments such as volcanic vents and hot springs. That genes from extremophiles would end up in mesophilic organisms which thrive in lower temperatures - such as in mammalian and acarian hosts - seems surprising. The highest temperature Borrelia garinii can still grow in is around 41-42 C. That's not anywhere near the high temperatures in which one finds Archaean Thermococcales (often over 60 C, sometimes as high as 100 C).
- This all does seem really weird. But the reason why it isn't too far fetched to see genes from extremely thermophilic organisms in moderately warm Borrelia and Treponema is more easily understood once you know more about the wide diversity found within the genus Spirochaeta in general. A number of Spirochaeta species live in extreme environments and not just in humans, animals, or ticks. For example:
- S. halophila lives in a high salinity pond on the Sinai shore.
- S. thermophila lives in marine hot springs in New Zealand and Russia.
- S. americana lives in alkaline, hypersaline Mono Lake in California.
|Champagne Pool, Wai-O-Tapu, near Rotorua, New Zealand by Christian Mehlführer|
- When looking at a phylogenetic tree, Spirochaeta is at the base of the tree and Borrelia and Treponema branch off later. Based on this, the best assessment one can make about the gene transfer from Archaea to Spirochaeta is that the most recent common ancestor of Spirochaeta, Borrelia, and Treponema had to have been very similar to thermophilic Spirochaeta.
- My running joke on this is to imagine a pile of thermophilic Archaea and thermophilic Spirochaeta hanging out around a hot spring together, laughing, joking, and flirting. Before you know it, horizontal gene transfer occurs, and a new form of spirochete is born. (This would make for a good Far Side comic, I just know it.)
- As if having Borrelia acquire Archaea genes wasn't interesting enough, it's been thought that ProS prolyl-tRNA synthetase (BB402) was acquired from a eurkaryote.
- Treponema spirochetes have a symbiotic relationship with termites. These spirochetes help termites in breaking down cellulose in wood in the termites' guts. So it isn't just ticks which have a symbiotic relationship with spirochetes - termites have one, too.[1, 5]
- Borrelia burgdorferi survives on the equivalent of tick antifreeze in the tick's midgut inbetween tick blood feeding cycles. Borrelia burgdorferi prefers glucose when in the tick, but it will feast on glycerol instead. See: http://spirochetesunwound.blogspot.com/2011/10/lyme-disease-spirochete-feasts-on-tick.html
- Both Borrelia hermsii and Borrelia burgdorferi metabolize chitobiose and N acetyl-glucosamine, a nutrient of these spirochetes and the major constituent of chitin for the exoskeletons of ticks.
- Borrelia have most of the genes required for the enzymes which make up the mevalonate pathway - a metabolic pathway used by the bacteria for synthesis of isoprenoid precursors. Isoprenoids are very important compounds which are found in over 30,000 products from the three domains of life (Eukaryotes, Prokaryotes, and Archaea). One interesting proposal about how Borrelia has the genes required for these enzymes for this pathway is that they come from the genetic cenancestor - an ancestor which predates the split into the three domains.
- In Act II of Samuel Beckett's play, Waiting For Godot, one character, Estragon, curses at the other, Vladimir, by calling him, "Gonococcus! Spirochete!"
Spirochetes continue to hold surprises and mysteries for us all... both good and bad. Another interesting installment of strange spirochete facts could be posted here - probably not too far in the future.
1) Cheryl P Andam and J Peter Gogarten. Biased gene transfer and its implications for the concept of lineage. Biology Direct 2011, 6:47 doi:10.1186/1745-6150-6-47
2) Greenberg EP, Canale-Parola E: Spirochaeta halophila sp. n., a facultative anaerobe from a high-salinity pond. Arch Microbiol 1976, 110:185-19
3) Aksenova H, Rainey F, Janssen P, Zavarzin G, Morgan H: Spirochaeta thermophila sp. nov., an obligately anaerobic, polysaccharolytic, extremely thermophilic bacterium. Int J Syst Bacteriol 1992, 42:175-177
4) Hoover RB, Pikuta EV, Bej AK, Marsic D, Whitman WB, Tang J, Krader P: Spirochaeta americana sp. nov., a new haloalkaliphilic, obligately anaerobic spirochaete isolated from soda Mono Lake in California. Int J Syst Evol Microbiol 2003, 53:815-821.
5) Droge S, Frohlich J, Radek R, Konig H: Spirochaeta coccoides sp. nov., a novel coccoid spirochete from the hindgut of the termite Neotermes castaneus. Appl Environ Microbiol 2006, 72:392-397.
6) Tilly, K., Elias, A.F., Errett, J., Fischer, E., Iyer, R., Schwartz, I., et al. Genetics and regulation of chitobiose utilization in Borrelia burgdorferi. J Bacteriol 183: 5544–5553.
7) Jonathan Lombard and David Moreira. Origins and Early Evolution of the Mevalonate Pathway of Isoprenoid Biosynthesis in the Three Domains of Life. Mol Biol Evol 2011, 28 (1): 87-99. doi: 10.1093/molbev/msq177 http://mbe.oxfordjournals.org/content/28/1/87.full
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