Wednesday, September 30, 2015

A bacterium that loves hot water, eating sugar, and bending itself into balls

Dictyoglomus thermophilum is a bacterium found in hot springs all over the world. It specifically enjoys aquatic places where there isn't any oxygen, it's not acidic, and the temperature is between 73 and 78 degrees Celsius (thermophilum = heat-loving). Turns out a hot spring is actually a pretty okay place for a strict anaerobe (organism poisoned by oxygen) to hang out, in part because hot water contains less dissolved oxygen. Hot springs also tend to be filled with lots of tasty organic matter for anaerobes to inefficiently grab energy from without using oxygen (aka fermentation).

A festive scene at the Tsuetate Hot Spring in Japan, where D. thermophilum was first isolated (Source)

Dictyoglomus thermophilum makes its living by fermenting carbohydrates. These include the plant polysaccharides starch, pectin, xylan, and cellulose, which the bacterium accesses by secreting an array of specialized enzymes to break them apart into smaller sugars (oligosaccharides). These products are then taken up by cells for use as a source of carbon and energy.

Enzymes secreted by D. thermophilum are of interest to industry folks because they're stable under relatively high temperatures and so can be used in hot processes without needing to spend a bunch of energy/time to cool things off. For example, a xylanase (breaks apart xylan, a polysaccharide found in plant cell walls) produced by the bacterium can be used instead of chlorine bleach to increase the brightness of hot wood pulp being turned into paper. Another heat-stable enzyme made by D. thermophilum is a β-glucosidase with the potential to be used to manufacture alkyl β-D-glucosides. These glucose derivatives are non-ionic surfactants added to foods, drugs, and detergents to inhibit microbial growth and stabilize emulsions. Apparently the β-glucosidase is so stable it can work in boiling water!

Dictyoglomus basically means net ball, referring to the ability of members of this genus to form large globe-shaped aggregates (up to several 100 μm in diameter) consisting of as many as a hundred threadlike (0.4-0.6 μm wide x 5-25 μm long) cells. These cell-filled spheres ("rotund bodies") are enclosed by an outer envelope connecting together a net-like surface layer of cells, essentially creating a super-cell. This weird multicellular arrangement is thought to facilitate the processing of polysaccharides, since it permits a bunch of cells to collectively cut down on the number of enzymes they need to produce (they can share them beneath the outer envelope, which also keeps them nearby). It also makes it easier for cells to take up oligosaccharides after they're produced.

It's been suggested the spherical shape of the aggregates are the result of Dictyoglomus species adapting to life in an undisturbed water column, since the spheres wouldn't be as stable in soil or sediment environments. Supporting their importance to this way of life, similar balls are formed by related members of the genera Aquifex and Thermus with a shared propensity toward hot spring residence.


References

Hoppert M, Valdez M, Enseleit M, Theilmann W, Valerius O, Braus GH, Föst C, Liebl W. 2012. Structure-functional analysis of the Dictyoglomus cell envelope. Systematic and Applied Microbiology 35(5):279-290.

Saiki T, Kobayashi Y, Kawagoe K, Beppu T. 1985. Dictyoglomus thermophilum gen. nov., sp. nov., a chemoorganotrophic, anaerobic, thermophilic bacterium. International Journal of Systematic Bacteriology 35(3):253-259. [Full text]

Te'o VSJ, Cziferszky AE, Bergquist PL, Nevalainen KM. 2000. Codon optimization of xylanase gene xynB from the thermophilic bacterium Dictyoglomus thermophilum for expression in the filamentous fungus Trichoderma reesei. FEMS Microbiology Letters 190(1):13-19. [Full text]

Zou ZZ, Yu HL, Li CX, Zhou XW, Hayashi C, Sun J, Liu BH, Imanaka T, Xu JH. 2012. A new thermostable β-glucosidase mined from Dictyoglomus thermophilum: Properties and performance in octyl glucoside synthesis at high temperatures. Bioresource Technology 118:425-430.

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