Fiber–Microbe Interactions in Bio-Concrete: Influence of Natural Fibers on Bacterial Viability, MICP Efficiency Mechanisms, Challenges, and Future Directions

Concrete is inherently susceptible to cracking, which compromises durability and accelerates structural deterioration. Microbially induced calcium carbonate (CaCO3) precipitation (MICP)-based self-healing concrete has emerged as a sustainable strategy to autonomously repair cracks; however, its efficiency is often limited by challenges related to bacterial survival, nutrient availability, and uncontrolled crack geometry. In this context, the integration of natural fibers presents a promising yet underexplored approach to enhance microbial performance within cementitious systems. This study critically examines the interactions between natural fibers (e.g., coir and sisal) and microbial activity in bio-concrete, with a focus on their influence on bacterial viability, MICP efficiency, and crack-healing performance. Natural fibers are hypothesized to act as multifunctional components by serving as internal moisture reservoirs, providing surface sites for bacterial attachment, and regulating crack width through bridging mechanisms. These combined effects can promote localized microenvironments conducive to bacterial activation and calcium carbonate precipitation. The paper synthesizes current knowledge on fiber-reinforced concrete and bio-concrete systems and proposes a conceptual framework describing fiber–microbe interaction mechanisms, including moisture retention, interfacial transition zone (ITZ) modification, and enhanced nucleation of CaCO3. Key challenges such as fiber degradation in alkaline environments, variability in natural fiber properties, and long-term durability of hybrid systems are also discussed. Finally, future research directions are outlined, emphasizing the need for systematic experimental validation, microstructural characterization, and optimization of hybrid fiber–microbial systems. This work provides new insights into the synergistic role of natural fibers in bio-concrete and establishes a foundation for developing more efficient, durable, and sustainable self-healing construction materials.