Interfacial interactions between protective, surface-engineered shells and encapsulated bacteria with different cell surface composition

Surface-engineered encapsulation is a non-genetic method to protect living organisms against harsh environmental conditions. Different cell encapsulation methods exist, yielding shells with different interfacial-interactions with encapsulated, bacterial surfaces. However, the impact of interfacial-interactions on the protection offered by different shells is unclear and can vary for bacteria with different surface composition. Probiotic bacteria require protection against gastro-intestinal fluids and antibiotics. Here, we encapsulated two probiotic strains using ZIF-8 (zeolitic imidazolate framework) biomineralization (strong-interaction by coordinate-covalent bonding), alginate gelation (intermediate-interaction by hydrogen bonding) or protamine-assisted packing of SiO2 nanoparticles yielding a yolk-shell (weak-interaction across a void between shells and bacterial surfaces). The surface of probiotic Lactobacillus acidophilus was rich in protein, yielding a hydrophilic, positively-charged surface below and a negatively-charged one above pH 4.0. Probiotic Bifidobacterium infantis had a hydrophilic, uncharged surface, rich in polysaccharides with little proteins. Although amino groups are required for coordinate-covalent bonding of zinc and hydrogen bonding of alginate, both L. acidophilus and B. infantis could be encapsulated using ZIF-8 biomineralization and alginate gelation. Weakly, intermediately and strongly interacting shells all yielded porous shells. The strongly interacting ZIF-8 biomineralized shell made encapsulated bacteria more susceptible to antibiotics, presumably due to the cell wall damage already inflicted during Zif-8 biomineralization. Overall, weakly interacting yolk-shells and intermediately interacting alginate gels protected best and maintained probiotic activity of encapsulated bacteria. The impact of interfacial-interactions between shells and encapsulated bacteria on different aspect of protection described here, contributes to the further development of effective surface-engineered shells and its application for protecting bacteria.