Mapping Bacterial Surface Layers Affinity to Polyelectrolytes Through the Building of Hybrid Macromolecular Structures

A novel hybrid sandwich-like supramolecular structure (polyelectrolyte multilayer/S-layer/polyelectrolyte multilayer/S-layer) has been built by combining polyelectrolyte multilayer deposition and self-assembly of isolated SbpA proteins from Bacillus sphaericus CCM2177. Neutron reflectometry measurements were used to confirm the formation of an S-layer on negative poly(styrene sulphonate) (PSS) terminated multilayers, further adsorption of cationic poly(allylamine hydrochloride) polyelectrolyte on the exposed S-layer surface, and final polyelectrolyte multilayer deposition. Surface topography investigations with atomic force microscopy showed that: (i) the two dimensional structure of the S-layer is similar to those found in bacteria, (ii) cationic poly(allylamine hydrochloride) adsorbs on the bacterial protein side that faces the aqueous media, and (iii) anionic poly(styrene sulphonate) does not adsorb on the S-layer surface. Mechanical stability studies on recrystallized S-layers on anionic negative poly(styrene sulphonate) reveal that loads of 20 nN are able to unfold the S-layer protein. A second adsorption of SbpA monomers on top of a structure composed of polyelectrolyte multilayer/S-layer/polyelectrolyte multilayer led to the formation of S-layers patches mechanically stable for loads up to 9 nN. This hybrid polymer-protein supramolecular complex has permitted to elucidate the nature of the affinity of the bacterial cell surface protein to polyelectrolytes.