The Philosopher's Bacterium

Picture the mediaeval alchemist, standing before a smoking crucible, adding ingredients and performing the requisite incantations. Eventually, he allows the crucible to cool so that he might examine the results. Has he succeeded? Has he achieved his goal of extracting pure gold from the base earth? Alas, he has not*, because he is only human, and he lacks the powers of a Geovibrio.

*For which we should perhaps be grateful. To quote King Midas in an episode of Concrete Cow, "I really should have foreseen the inflationary effect of all that new gold on our economy".

Deferribacter desulfuricans, photographed by K. Takai.

Geovibrio ferrirudecens is a species belonging to the 'phylum' Deferribacteres, an assemblage of Gram-negative bacteria known as yet from only a small number of species. However, the known species are all quite ecologically disparate, so we may have only scratched the surface of deferribacterial diversity. All are anaerobic organotrophs, utilising various organic compounds as electron donors in respiration*. Species vary in their temperature preferences from mesophily (growing at 'room temperature') to moderate thermophily (up to about 60°C). Cells may be immobile rods, or they may be motile with a single polar flagellum (bipolar flagella have been observed in Denitrovibrio acetiphilus, indicating reproduction by budding). Cells of Geovibrio, at least, can occur in chains as well as individually. Known habitats of Deferribacteres include water in oil reservoirs for Deferribacter thermophilus (Garrity & Holt 2001), Japanese hot springs for Calditerrivibrio nitroreducens (Iino et al. 2008), hydrocarbon-contaminated soils for Geovibrio ferrireducens (Garrity & Holt 2001), and the epithelial lining of a mouse intestine for Mucispirillum schaedleri (Robertson et al. 2005). With regard to this last genus, it stands as a good example of how poor is our understanding of real bacterial diversity that a significant component of the intestinal flora of a major model organism was not identified until 2005! When people express surprise to me that there are still new species of organism to be described in the world, I inform them that, at the microbial level, there are almost certainly undescribed species living on you right now.

*As do we, of course. If you remember your high school biology, you'll recall that human respiration reacts glucose and oxygen to produce carbon dioxide and water. In this reaction, glucose is the electron donor while oxygen is the electron receptor.

Geovibrio ferrireducens with periplasmic gold deposits, from Kashefi et al. (2001).

Species of Deferribacteres also vary in their preferred electron receptors. Calditerrivibrio nitroreducens, as indicated by its species name, reduces nitrate, converting it into ammonium. The very recently described Seleniivibrio woodruffii utilises selenium and arsenic compounds (Rauschenbach et al. 2013; arsenate-respiring bacteria can be significant to public health as they convert arsenates to more soluble arsenites, causing arsenic to leach into groundwater). Deferribacter thermophilus and Geovibrio ferrireducens each received part of their names because they grow off ferric iron (Fe³⁺), converting it into ferrous iron (Fe²⁺). Ferrous iron is much more soluble than ferric iron, so iron-reducing bacteria can cause corrosion to iron structures. Both these taxa may also use other electron acceptors. Deferribacter can grow off manganese(IV) or nitrate. Geovibrio doesn't reduce either of those substances, but it has been shown that it can use hydrogen as an electron donor to reduce soluble gold(III) to elemental gold, which becomes deposited in the periplasmic space between the cell membranes (Kashefi et al. 2001). Geovibrio is only one of a number of bacteria that can pull off this trick, and it has been bacteria may have been responsible for the creation of many of the world's gold deposits. When the alchemist of our opening paragraph was struggling with his proto-chemistry, he should have been examining his biology.

REFERENCES

Garrity, G. M., & J. G. Holt. 2001. Phylum BIX. Deferribacteres phy. nov. In Bergey’s Manual of Systematic Bacteriology, 2nd ed., vol. 1. The Archaea and the Deeply Branching and Phototrophic Bacteria (Boone, D. R., R. W. Castenholz & G. M. Garrity, eds) pp. 465-471. Springer.

Iino, T., T. Nakagawa, K. Mori, S. Harayama & K. Suzuki. 2008. Calditerrivibrio nitroreducens gen. nov., sp. nov., a thermophilic, nitrate-reducing bacterium isolated from a terrestrial hot spring in Japan. International Journal of Systematic and Evolutionary Microbiology 58: 1675-1679.

Kashefi, K., J. M. Tor, K. P. Nevin & D. R. Lovley. 2001. Reductive precipitation of gold by dissimilatory Fe(III)-reducing Bacteria and Archaea. Applied and Environmental Microbiology 67 (7): 3275-2279.

Rauschenbach, I., V. Posternak, P. Cantarella, J. McConnell, V. Starovoytov & M. M. Häggblom (in press, 2013) Seleniivibrio woodruffii gen. nov., sp. nov., a selenate- and arsenate-respiring bacterium in the Deferribacteraceae. International Journal of Systematic and Evolutionary Microbiology.

Robertson, B. R., J. L. O’Rourke, B. A. Neilan, P. Vandamme, S. L. W. On, J. G. Fox & Adrian Lee. 2005. Mucispirillum schaedleri gen. nov., sp. nov., a spiral-shaped bacterium colonizing the mucus layer of the gastrointestinal tract of laboratory rodents. International Journal of Systematic and Evolutionary Microbiology 55: 1199-1204.