Over time, cracks and damage occur in concrete. One way to repair this is by
introducing microbes into concrete during the building process. These would
heal the concrete if cracks appear. Which type of microbial species have been
found suitable in self-healing concrete.
Aspergillus sp.
Bacillus sp.
Methanococcus sp.
Dictyostelium sp.

The microbial species that have been found most suitable and are widely used in self‑healing concrete are Bacillus species, so the correct answer is Bacillus sp.

Correct option: Bacillus sp.

Bacillus species are the best‑studied and most commonly used microbes for self‑healing (bacterial) concrete because they are spore‑forming, highly resistant to alkaline pH, and capable of inducing calcium carbonate precipitation inside cracks. When mixed into concrete in the form of dormant spores along with a nutrient source (such as calcium lactate or urea‑based substrates), these bacteria remain inactive until a crack allows water and oxygen to enter. On activation, Bacillus cells metabolize the nutrient, produce carbon dioxide and change local chemistry so that calcium carbonate (biomineral) precipitates and fills the crack, sealing it and improving durability. Commonly researched species include Bacillus subtilis, B. sphaericus, B. pseudofirmus, and B. licheniformis for this purpose.​

Why other options are not suitable

Aspergillus sp.

Some fungi such as Aspergillus nidulans have been experimentally explored for fungal‑based self‑healing concrete and can promote calcium carbonate precipitation on concrete surfaces under highly alkaline conditions. However, fungal self‑healing concrete is still at an early research stage compared with bacteria‑based systems and is not the standard or classical example used in most self‑healing concrete questions, especially in competitive exams. Therefore, despite scientific interest in Aspergillus for future applications, Bacillus remains the established correct choice in this MCQ context.​

Methanococcus sp.

Methanococcus is an archaeal genus associated with methanogenesis in strictly anaerobic environments such as sediments and hydrothermal systems, where it converts substrates into methane under oxygen‑free conditions. These archaea are not adapted to the highly alkaline, oxygen‑exposed, and desiccating environment of concrete, and there is no standard application of Methanococcus in self‑healing concrete systems. They do not play a role in microbially induced calcium carbonate precipitation in civil engineering materials, so this option is incorrect for self‑healing concrete.​

Dictyostelium sp.

Dictyostelium species are social amoebae (slime molds) used as model organisms in cell biology and developmental studies, not as biomineralizing agents in construction materials. They are not known to form durable spores that survive high pH concrete, nor are they used to precipitate calcium carbonate for crack healing in cementitious systems. As a result, Dictyostelium has no practical or research‑based role in self‑healing concrete, making this option incorrect.​​

Introduction (SEO‑optimized)

Self‑healing concrete is an advanced construction material that uses microorganisms to repair cracks automatically and extend the service life of structures. Among all tested microbes, Bacillus species in self‑healing concrete are the most successful because their spores survive harsh alkaline conditions and precipitate calcium carbonate to seal micro‑cracks. Understanding why Bacillus is suitable, and why Aspergillus, Methanococcus, and Dictyostelium are not standard choices, is essential for civil engineering, biotechnology applications, and exam‑oriented questions.​

How Bacillus works in self‑healing concrete

When Bacillus spores and a mineral precursor (commonly calcium lactate or urea‑based nutrients) are embedded in concrete, they remain dormant for long periods until cracks open pathways for moisture. On contact with water, the spores germinate, grow, and metabolize the supplied nutrient, generating carbon dioxide and altering local ion concentrations, which leads to calcium carbonate precipitation inside the crack. The deposited CaCO₃ fills and seals the crack, reduces water ingress, and can partially restore mechanical properties such as compressive and tensile strength.​

Researchers have demonstrated that Bacillus‑based self‑healing concrete can heal cracks of several tenths of a millimetre, improve strength over time, and significantly reduce permeability compared with conventional concrete. Spore‑forming capacity, non‑pathogenic nature, and compatibility with carriers like lightweight aggregates or hydrogels make Bacillus species the preferred biological agents in practical and experimental self‑healing systems.​

Summary of options in exam context