Biocoatings: Painting bacteria on surfaces

Background: Biocoatings are nanoporous polymer materials which encapsulate bacterial cells with carbohydrates as osmoprotectants. Here, we optimised biocoatings to offer a favourable environment for the metabolic activity of bacteria. Methods: E. coli were used as a model organism and mixed with the colloidal polymer particles (i.e. synthetic latex), inorganic nanoparticles and different carbohydrates. Films were casted and dried to create a coalesced latex film and finally rehydrated to re-establish bacterial metabolism. The toxicity of the sterile latices to the bacteria was tested by using the colourimetric redox indicator resazurin. Visualisation of the bacteria inside the biocoatings was performed by confocal laser scanning microscopy (CLSM) and scanning electron microscopy (SEM). Results: We introduced halloysite (clay nanotubes) to create nanoporosity, which created voids in the structure that will permit gas exchange. The biocoatings were tested in liquid and rehydrated states with resazurin to find the most promising composition ensuring bacterial viability. Rehydrated biocoatings were visualised by CLSM by tracking the constitutively expressed yellow-fluorescent protein (YFP) for viable cells and the membrane exclusion dye propidium iodide for dead cells. The structure of the biocoatings appeared to be unaffected by freeze-drying compared to chemical fixation. Following this fixation, SEM allowed the observation of the organisation of the latex polymers, halloysite and bacteria. Conclusions: The biocoatings were highly porous thanks to halloysite. E. coli survived the film formation process. Next, we will use E. coli and cyanobacteria to achieve higher efficiency for a variety of applications e.g. pollutant degradation, solar energy harvesting and carbon recycling.