Are the soft, liquid-like structures detected around bacteria by ambient dynamic atomic force microscopy capsules?

Area of Science:

• Microbiology
• Biophysics
• Surface Science

Background:

• Bacterial capsules are crucial virulence factors, but their imaging is difficult.
• Atomic force microscopy (AFM) has been proposed for imaging delicate bacterial capsules.
• Previous studies suggested AFM could detect liquid-like substances as bacterial capsules.

Purpose of the Study:

• To investigate the capacity of AFM for imaging bacterial capsules.
• To explore the nature of the liquid-like substances observed around staphylococcal strains using AFM.
• To determine the origin and mechanism of formation of these observed structures.

Main Methods:

• Utilized atomic force microscopy (AFM) to image three staphylococcal strains.
• Employed phosphate buffer and water as suspending liquids.
• Conducted extensive imaging and chemical analysis, including experiments with nonliving particles.

Main Results:

• AFM visualized liquid-like substances around staphylococcal strains, similar to previously reported bacterial capsules.
• Chemical analysis and experiments with nonliving particles indicated the suspending liquid (buffer) is central to substance formation.
• The phenomenon was reproduced with nonliving particles, refuting a biological origin.

Conclusions:

• The observed liquid-like structures are not bacterial capsules but result from the deliquescence of buffer components (e.g., K(2)HPO(4), CaCl(2), HEPES).
• This deliquescence mechanism explains the similarity of results across different bacterial strains and suspending liquids.
• The biological/biomedical implications of these ultrasmall liquid amounts wrapping microorganisms warrant further discussion.