Scanning surface potential microscopy of spore adhesion on surfaces

The adhesion of spores of Bacillus anthracis – the cause of anthrax and a likely biological threat – to solid surfaces is an important consideration in cleanup after an accidental or deliberate release. However, because of safety concerns, directly studying B. anthracis spores with advanced instrumentation is problematic. As a first step, we are examining the electrostatic potential of Bacillus thuringiensis (Bt), which is a closely related species that is often used as a simulant to study B. anthracis. Scanning surface potential microscopy (SSPM), also known as Kelvin probe force microscopy (KPFM), was used to investigate the influence of relative humidity (RH) on the surface electrostatic potential of Bt that had adhered to silica, mica, or gold substrates. AFM/SSPM side-by-side images were obtained separately in air, at various values of RH, after an aqueous droplet with spores was applied on each surface and allowed to dry before measurements. In the SSPM images, a negative potential on the surface of the spores was observed compared with that of the substrates. The surface potential decreased as the humidity increased. Spores were unable to adhere to a surface with an extremely negative potential, such as mica.

Graphical abstractSSPM surface potential images of Bt spores on grounded gold substrate at (A) 0%, (B) 40%, and (C) 80% RH.Figure optionsDownload full-size imageDownload as PowerPoint slideHighlights► AFM and SSPM surface potential map for Bt spores adhered to silica, mica, or gold at various relative humidity (RH). ► Surface potentials of both Bt spore and silica substrate decreased as the RH increased. ► Bt spores could not adhere to extremely negative surfaces, such as mica. ► Surface potentials of Bt spores on Au have a trend similar to Bt on silica.