Using EFM to probe the secrets of bacterial endospore survival strategies

Image: Electric EFM images of a single endospore and a of single bacterial cell under low (top) and high (bottom) relative humidity conditions. The increased contrast in the images at high relative humidities correlates with the moisture uptake by the endospore
and bacterial cell.)

They consist of a tight protein and lipid microcapsule enclosing the bacterial genetic content, and are designed to resist extremely harsh environmental conditions. Endospores can remain in a metabolically dormant state for decades in a variety of environments including air, soil or organic matter. Nevertheless, upon exposure to the appropriate conditions, they germinate back into vegetative bacterial cells within minutes, thus constituting an impressive survival strategy for living organisms.

The response of bacterial endospores to changes in environmental relative humidity (hygroscopicity) is among their more outstanding properties. Endospores are known to adsorb moisture from the environment, but their viability seems not to be compromised by doing so, which is believed to be related to the internal distribution of water in the endospores.

Gabriel Gomila’s Nanoscale Bioelectrical Characterization group, working together with IBEC Associate Researcher and UB professor Antonio Juárez, used Electrostatic Force Microscopy (EFM), a form of Atomic Force Microscopy, to measure the internal hydration properties – moisture uptake and internal distribution of water – of endospores under high relative humidity conditions, which is far more challenging and biologically relevant than the previous studies carried out by the group in dry or low humidity conditions.

“It has been amazing to observe how the endospore structure is able to preserve its core, where DNA is located, under low hydration conditions, irrespectively of the ambient relative humidity. This is certainly key in the endospore’s extraordinary survival abilities,” says Gabriel. “This same property is also at the basis of their extraordinary water responsive properties, which has led to research into energy-harvesting devices based on endospores that can generate electrical power from an evaporating body of water, as well as electromechanical tunneling graphene quantum dot-spore devices.”

The IBEC group’s results also demonstrate the sensitivity and potential of their EFM technique to accurately address the hygroscopic properties of small scale objects, which can be interesting for other nano-hygroscopic applications. Examples include the study of new water-responsive materials for energy harvesting, of humidity-dependent biological processes such as the production of mycotoxins, one of the largest food poisoning threats, or of aerosol nanoparticles relevant for atmospheric sciences.

Paper: Marc Van Der Hofstadt, Rene Fabregas, Ruben Millan-Solsona, Antonio Juarez, Laura Fumagalli & Gabriel Gomila (2016). Internal Hydration Properties of Single Bacterial Endospores Probed by Electrostatic Force Microscopy. ACS Nano, 10.1021/acsnano.6b06578

Partially supported by the FP7-funded Nanomicrowave project (FP7/People-2012-ITN 317116.EU).