Desiccated E coli: Hibernation in salt

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  • Published: Aug 1, 2014
  • Author: David Bradley
  • Channels: Raman
thumbnail image: Desiccated E coli: Hibernation in salt

Dendritic salt

Dried biosaline patterns formed by the interaction of Escherichia coli cells with common salt. / Credit: J. M. Gómez-Gómez

A serendipitous discovery by the team working on the Raman instrument for the ExoMars rover mission reveals an interaction between microorganisms and salt in which Escherichia coli cells added to a droplet of salt water left to dry out manipulate the process of sodium chloride crystallization to create structures within which they can "hibernate". When the desiccated is rehydrated the bacteria are revived.

The team observed no biosaline patterns if the droplet contained only salt and again no pattern if there were only bacteria without salt. Both salt and bacteria had to be present to generate these intriguing structures.

Escherichia coli is widely studied as an easy to culture model for countless processes and cellular behaviours in molecular biology and biomedicine. But, as well known to biologists as this bacterium is, no one had previously reported what the microorganism can do when it is essentially trapped in a single drop of salt water. According to the latest study by researchers in Spain, E coli has the ability to create impressive biomineralogical patterns in which it shelters itself when the droplet evaporates and can emerge effectively unscathed when water is added to these saline structures.

"It was a complete surprise, a fully unexpected result, when I introduced E. coli cells into salt water and I realised that the bacteria had the ability to join the salt crystallisation and modulate the development and growth of the sodium chloride crystals," explains biologist José María Gómez.

Domestic discovery

"Thus, in around four hours, in the drop of water that had dried, an impressive tapestry of biosaline patterns was created with a complex three-dimensional architecture," he adds. The initial finding was apparently made at home with a simple light microscope although the result was subsequently confirmed in Laboratory of BioMineralogy and Astrobiological Research (LBMARS) at the University of Valladolid-CSIC, Spain. The team used scanning electron microscopy (SEM) and energy dispersive X-ray spectrometry (EDS) analyses to show that the biosaline formations are organized in a two-layered characteristic 3D architectural morphology.

Other researchers had spotted similar patterns created from saline solutions and isolated proteins, but these were assumed to arise through molecular recognition, self-assembly or crystallization rather than being directed by a living thing. This is the first time that scientists have reported how a living bacterial cell can engineer the crystallisation of sodium chloride. The resulting structures are highly branched, dendritic and fractal in appearance.

"The most interesting result is that the bacteria enter a state of hibernation inside these desiccated patterns, but they can later be revived simply by rehydration," adds Gómez. The finding may well have important implications for the field of astrobiology wherein researchers hope to understand nature's processes with a view to understanding how putative life forms elsewhere in the universe might arise and survive in extraterrestrial conditions. "Given the richness and complexity of these formations, they may be used as biosignatures in the search for life in extremely dry environments outside our own planet, such as the surface of Mars or that of Jupiter’s satellite, Europa," Gómez says.

Historical precedent

It is, of course, no coincidence that Gómez recognized this significance of the discovery given that the LBMARS laboratory is one of the participants in the development of the Raman RLS instrument for the ExoMars rover, a mission of the European Space Agency (ESA) that will send a probe to the red planet in 2018. The new finding offers another clue as to how the probe might look for signs of ancient and long-extinct life on Mars. Gómez adds that, "the patterns observed will help calibrate the instrument and test its detection of signs of hibernation or traces of Martian life." The next step is how to explain the bacterial control of sodium chloride crystallisation observed. Conversely, it will be important to learn how the physical chemistry of the sodium chloride itself drives the bacteria to create these complex 3D structures. "We propose that these E. coli biosaline drying patterns represent an excellent experimental model for understanding different aspects of anhydrobiosis phenomena in bacteria as well as for revealing the mechanisms of bacterially induced biomineralization, both highly relevant topics for the search of life in extraterrestrial locations," the team reports.

"This [work] is a tribute to scientists such as the Spaniard Santiago Ramón y Cajal and the Dutch scientist Anton van Leeuwenhoek, who also worked from home with their own microscopes," Gómez adds.

At this moment I am working with other salts (I have discovered amazingly that the morphology of the drying pattern depends strongly on the kind of salt used in the preparation of saline solution (e.g., NaCl versus NaBr). In addition, we are exploring with Raman spectroscopy the fingerprinting generated for each different biomineralogical biosaline pattern," Gómez told SpecteoscopyNOW.

Related Links

Astrobiol 2014, 14, online: "Drying Bacterial Biosaline Patterns Capable of Vital Reanimation upon Rehydration: Novel Hibernating Biomineralogical Life Formations"

Article by David Bradley

The views represented in this article are solely those of the author and do not necessarily represent those of John Wiley and Sons, Ltd.

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