Xerotolerance: A New Property in <i>Exiguobacterium</i> Genus

The highly xerotolerant bacterium classified as <i>Exiguobacterium</i> sp. Helios isolated from a solar panel in Spain showed a close relationship to <i>Exiguobacterium sibiricum</i> 255-15 isolated from Siberian permafrost. Xerotolerance has not been previously described as a characteristic of the extremely diverse <i>Exiguobacterium</i> genus, but both strains Helios and 255-15 showed higher xerotolerance than that described in the reference xerotolerant model strain <i>Deinococcus</i> <i>radiodurans</i>. Significant changes observed in the cell morphology after their desiccation suggests that the structure of cellular surface plays an important role in xerotolerance. Apart from its remarkable resistance to desiccation, <i>Exiguobacterium</i> sp. Helios strain shows several polyextremophilic characteristics that make it a promising chassis for biotechnological applications. <i>Exiguobacterium</i> sp. Helios cells produce nanoparticles of selenium in the presence of selenite linked to its resistance mechanism. Using the <i>Lactobacillus</i> plasmid pRCR12 that harbors a cherry marker, we have developed a transformation protocol for <i>Exiguobacterium</i> sp. Helios strain, being the first time that a bacterium of <i>Exiguobacterium</i> genus has been genetically modified. The comparison of <i>Exiguobacterium</i> sp. Helios and <i>E. sibiricum</i> 255-15 genomes revealed several interesting similarities and differences. Both strains contain a complete set of competence-related DNA transformation genes, suggesting that they might have natural competence, and an incomplete set of genes involved in sporulation; moreover, these strains not produce spores, suggesting that these genes might be involved in xerotolerance.