Spore-forming Bacillus subtilis vis-à-vis non-spore-forming Deinococcus radiodurans, a novel bacterium for self-healing of concrete structures: A comparative study

Abstract

Research on microbial concrete has predominantly considered spore-forming bacteria, such as from the Bacillus species, as they can survive under unfavourable conditions of high alkalinity and low water availability by forming spores. Consequently, when such conditions are prevalent, the microbial induced calcite precipitation of spore-forming bacteria is highly restricted, leading to reduced self-healing efficiency of the microbial concrete. Considering this fact, the current study aims to identify a bacterium that can contribute to the precipitation of calcite in a high alkaline, low water environment, which is very important in terms of real-life application of microbial concrete. In this paper, such a bacterium, namely, Deinococcus radiodurans, an extremophilic, non-spore-forming bacterium, is identified for use in microbial concrete. The crack healing ability of D. radiodurans in mortar is tested at three different cell concentrations, namely 103, 105 and 107 cells/ml, and the results are compared with the healing achieved by a well-established, spore-forming bacterium, Bacillus subtilis. Thereafter, the effect of each bacterium on the basic properties of microbial concrete, namely, compressive strength and water absorption, is evaluated and the results are compared. Finally, to study the precipitation patterns due to the addition of each bacterium at the considered cell concentrations under favourable and unfavourable conditions characterized by high and low water availability, a microstructure analysis is carried out at the surface and at the inner matrix of the mortar samples. The results indicate that D. radiodurans is a potential source of biomineralization as its inclusion can lead to substantial enhancement in the mechanical properties of microbial concrete, which is at par with the performance of B. subtilis in a favourable environment. Significantly, microbial concrete prepared with D. radiodurans can provide considerably improved mechanical properties as compared to B. subtilis in an unfavourable environment.