Cryo-electron microscopy of an extremely halophilic microbe: technical aspects

Area of Science:

• Microbiology
• Structural Biology
• Biophysics

Background:

• Halophilic Archaea, such as Halobacteriaceae, require high salt concentrations (e.g., 4.3 M NaCl) for growth and cellular integrity.
• High salt levels impede structural studies, particularly electron microscopy, due to reduced contrast.
• Cryo-electron microscopy (cryo-EM) offers in situ visualization of cellular organization under near-native conditions.

Purpose of the Study:

• To develop and evaluate strategies for preparing and imaging halophilic Archaea using cryo-electron microscopy.
• To overcome the challenges posed by high salt concentrations for structural analysis of halophilic microbes.
• To enable in situ structural investigations of Halobacterium salinarum under near-physiological salt conditions.

Main Methods:

• Adaptation of Halobacterium salinarum to reduced salt concentrations (down to 3 M NaCl) via dialysis.
• Optimization of vitrification parameters for high-salt-containing cells.
• Focused-ion-beam (FIB) micromachining for thinning cryo-fixed cells.
• Cryo-electron microscopy (cryo-EM) for high-resolution imaging of intact cells.

Main Results:

• Vital Halobacterium salinarum cells were successfully maintained at 3 M NaCl after adaptation and dialysis.
• Optimized vitrification and FIB milling enabled the preparation of suitable cryo-EM samples from high-salt cells.
• High-contrast, in situ structural details of halophilic Archaea were obtained using cryo-EM.

Conclusions:

• Structural studies of halophilic Archaea in situ are feasible using cryo-electron microscopy.
• The developed methods allow for the investigation of cellular organization under high-salt conditions.
• This approach opens new avenues for understanding the structure-function relationships of extremophilic microorganisms.