Abstract EANA2025-91

Life without water is far from being an option for most eukaryotic species. Although many bacteria, known as xerophiles, are capable of proliferating in extreme desiccation conditions, such an environment is responsible for considerable physiological damage. 

Indeed, low water potential is often considered the biggest life-threatening abiotic stress. Water plays an important role in all the metabolic processes of bacterial cells. Its virtually non-existent bioavailability therefore implies an overall reduction in metabolic activity, leading in most cases to cell death. 

The existing literature on the duration of xerophilic resistance to desiccation offers a sometimes reductive view of the real capabilities of these extremophiles.  It was against this backdrop that the 500-Year Microbial Experiment was designed [1,2] with the aim of exploring the resistance of xerotolerant bacteria to prolonged desiccation beyond the human scale. To do this, two xerophilic species, Bacillus subtilis and Chroococcidiopsis sp. were subjected to desiccation in silica gel beads in sealed flasks in 2014, and then their viability will be tested regularly for 500 years. 

Prior to the experiment, the liquid culture of Chroococcidiopsis sp. used was found to be contaminated with a red isolate. This finding was ignored in the belief that this contaminant would not survive such extreme desiccation. However, the first results after 10 years of desiccation were recently revealed. Surprisingly, the survival of the contaminant was verified, and sequencing enabled the contaminant species to be identified as Methylobacterium sp. 

Dormancy and sporulation are typical adaptive responses to desiccation, allowing xerotolerant microorganisms to endure drought by entering a metabolically inactive state. Some bacteria, such as Methylobacterium sp., resist desiccation in a vegetative state through various strategies, such as the production of protective molecules and mechanisms for repairing cellular components [3]. However, the lack of studies on Methylobacterium sp. potential resistance to desiccation led us to ask what resistance mechanisms enabled it to withstand this stress vegetatively for 10 years.  

In this study we carried out a non-targeted metabolomic analysis of Methylobacterium sp. cultures at different times of exposure to desiccation to obtain a comprehensive chronology of metabolomic changes during this osmotic and mechanical stress. 

A remodelling of energy metabolism was revealed, marked by a preferential use of pathways that consume less energy than the Krebs cycle, such as amino acid catabolism. This reorientation suggests an energy-saving strategy in favour of the development of adaptive responses. Various metabolites associated with these responses, including antioxidants such as pigments and vitamins, osmoprotectants such as sugars and small amino acids, and a reconfiguration of membrane composition, testify to a fine metabolic adaptation in the face of marked osmotic stress.

 This initial study is a starting point for more targeted analyses to validate our hypotheses. This research is part of a wider project aimed at understanding the mechanisms that enable life to persist in extreme environments.

[1] Cockell, Charles. ‘A 500-Year Experiment’. Astronomy & Geophysics 56, no. 1 (2015): 1.28-1.29. https://doi.org/10.1093/astrogeo/atv028.

[2] Ulrich, Nikea, Katja Nagler, Michael Laue, Charles S. Cockell, Peter Setlow, and Ralf Moeller. ‘Experimental Studies Addressing the Longevity of Bacillus Subtilis Spores – The First Data from a 500-Year Experiment’. PLOS ONE 13, no. 12 (2018): e0208425. https://doi.org/10.1371/journal.pone.0208425.

[3] Lebre, Pedro H., Pieter De Maayer, and Don A. Cowan. ‘Xerotolerant Bacteria: Surviving through a Dry Spell’. Nature Reviews Microbiology 15, no. 5 (2017): 285–96. https://doi.org/10.1038/nrmicro.2017.16.