Microbial Encapsulation with Polyethylene-Glycol Based Hydrogel Materials for Protection Against Environmental Stress Factors

Many emerging applications in microbial biotechnology require that microorganisms function under harsh or unfavorable environments. These environments may contain chemical toxins, reactive oxygen species, limited nutrients, extremes in pH and temperature, or exogenous microorganisms, among other stress factors. Microbial electrochemical cells (MxCs), which use electroactive biofilms of anode respiring bacteria (ARB) for current production, are often limited by loss of ARB electrochemical activity due to these stress factors. This inhibits the use of novel MxC systems in applications related to water treatment, sub-surface remediation, remote sensing, and self-powered bio-batteries. In this work, we report the use of polyethylene-glycol (PEG) based hydrogel materials for encapsulation, stabilization, and toxin protection of ARB biofilms in a MxC system.<br/><br/>Hydrogels were formed using PEG divinyl sulfone and PEG tetrathiol precursor molecules. These molecules couple together through based-catalyzed Michael addition reactions between thiol and vinyl sulfone end groups, generating highly crosslinked hydrogel networks with well-controlled, tunable mesh sizes. Precursor molecules were mixed together over anodic biofilms using an established dip coating procedure. The resulting hydrogels were resistant to acid or base hydrolysis over a range of pH values (pH = 3 to 10), appeared stable over ARB biofilms, and did not compromise biofilm viability over a 30-day trial period. The electrochemical activity of hydrogel-coated ARB biofilms was compared to uncoated ARB biofilms in a side-by-side manner, and it was found that the hydrogel coated anodes displayed higher current density under oligotrophic environments, higher current densities during exposure to high concentrations of ammonium toxin, and faster recovery after an ammonium shock. Hindered diffusion of ammonium ions through the hydrogel was identified as the cause for improved ARB performance during and after the ammonium shock.<br/><br/>These findings demonstrate the novel use of hydrogel materials to protect EAB from sharp chemical gradients that commonly occur during a chemical shock. Because hydrogel pore size can be modulated by controlling the molecular weights of the PEG precursor molecules, and catalytic functionality can be added to the hydrogel, these coatings can be further engineered to provide selective control of mass transport to functional microbes for their protection in a variety of applications. On-going work investigates enzymatic modification of these hydrogels to protect encapsulated bacterial cells from other chemical toxins for bioproduction and biomedicine applications.