Immobilization and Monitoring of Clostridium carboxidivorans and Clostridium kluyveri in Synthetic Biofilms

The growing need for sustainable biotechnological solutions to address environmental challenges, such as climate change and resource depletion, has intensified interest in microbial-based production systems. Synthetic biofilms, which mimic natural microbial consortia, offer a promising platform for optimizing complex metabolic processes that can convert renewable feedstocks into valuable chemicals. In this context, understanding and harnessing the interactions between co-immobilized microorganisms are critical for advancing bioprocesses that contribute to circular bioeconomy goals. In this study, we investigated the viability and metabolic activity of <i>Clostridium carboxidivorans</i> and <i>Clostridium kluyveri</i> within a synthetic, dual-layered biofilm composed of agar hydrogel. This setup compartmentalized each bacterial species. Embedding the bacteria in a structured biofilm offers numerous opportunities for bioproduction, but the inability to monitor cell growth or movement within the immobilization matrix limits process insights. To address this, we adapted a fluorescence in situ hybridization (FISH) protocol, enabling precise, species-specific visualization of bacterial distribution and growth within the gel matrix. Batch processes with the dual-layered biofilm in anaerobic flasks, designed with a metabolic advantage for <i>C. kluyveri,</i> revealed distinct growth dynamics. <i>C. kluyveri</i> exhibited significant metabolic activity, forming clusters at low initial cell concentrations and converting ethanol and acetate into 1-butyrate and 1-hexanoate, indicating viability and cell growth. <i>C. carboxidivorans</i> remained evenly distributed without significant growth or product formation, suggesting that while the cells were viable, they were not metabolically active under the experimental conditions. Both bacterial species were confined to their respective compartments throughout the process, with <i>C. kluyveri</i> showing enhanced substrate conversion at higher initial cell densities in the hydrogel. The pH drop throughout the batch experiment likely contributed to incomplete substrate consumption, particularly for <i>C. kluyveri</i>, which thrives within a narrow pH range. These findings highlight synthetic biofilms as a promising platform for optimizing microbial interactions and improving bioprocess efficiency, especially in applications involving complex metabolic exchanges between co-immobilized microorganisms. Further research will focus on applying conditions to support the growth and metabolic activity of <i>C. carboxidivorans</i> to explore spatial dynamics of bacterial migration and cooperative relationships in the synthetic biofilm.