Bacillus pumilus is a bacterium commonly found across the globe, yet it possesses the ability to withstand conditions lethal to almost all other life forms. This dual nature makes it a subject of intense scientific interest, both for practical applications on Earth and for implications in the search for life beyond our planet.
Characteristics and Natural Habitat
Bacillus pumilus is a Gram-positive, rod-shaped bacterium belonging to the genus Bacillus. It is aerobic, requiring oxygen to grow, and its defining feature is its ability to form endospores.
It is routinely isolated from soil, where it plays a role in nutrient cycling, but it is also found in marine water, deep-sea sediments, and the air. While generally considered non-pathogenic to humans, its hardiness has led to strains being identified as opportunistic contaminants in industrial and clean-room settings.
Mechanisms of Extreme Survival
The extreme survival of B. pumilus is due to its ability to undergo sporulation, converting the active cell into a dormant endospore. This process involves forming a highly dehydrated core containing the bacterial DNA and ribosomes, encased in a protective layer of peptidoglycan called the cortex. A rigid, multi-layered protein coat surrounds the cortex, acting as the spore’s primary shield against environmental stressors.
This dormant architecture provides resistance to heat, desiccation, and chemical disinfectants such as hydrogen peroxide. The spore is also highly resistant to radiation, including intense ultraviolet (UV) light and gamma radiation.
Specific strains, such as SAFR-032, isolated from spacecraft assembly facilities, show a high tolerance for UV irradiation and oxidative stress. Space-exposed spores have even developed first-generation cells with elevated resistance to UVC radiation compared to their ground-based counterparts.
Applications in Biocontrol and Biotechnology
The natural capabilities of B. pumilus have been harnessed for applications in agriculture and industrial biotechnology. In agriculture, the bacterium is utilized as a natural biocontrol agent to protect crops against disease. It actively suppresses various plant pathogens, including fungi and bacteria, by producing antagonistic compounds. The biocontrol mechanism involves the secretion of lipopeptides and hydrolytic enzymes such as chitinase and cellulase, which break down the cell walls of fungal pathogens.
Beyond pathogen suppression, B. pumilus acts as a plant growth promoter by contributing to soil fertility. It achieves this by producing phytohormones, such as indole-3-acetic acid. It also fixes atmospheric nitrogen and solubilizes soil phosphorus, making these nutrients available for plant uptake.
In industrial settings, B. pumilus is a valuable source for the commercial production of specific enzymes. Its ability to secrete enzymes like proteases and amylases, which break down proteins and starches, is highly valued. These enzymes are incorporated into various manufacturing processes, including the formulation of biological detergents and industrial cleaning agents.
Role in Astrobiology and Planetary Protection
The exceptional endurance of B. pumilus has made it a model organism for astrobiology research and planetary protection. Hardiest strains, such as the SAFR-032 isolate from the Jet Propulsion Laboratory (JPL) Spacecraft Assembly Facility, are used to evaluate the potential for terrestrial life to survive on other celestial bodies. Researchers use this strain in experiments simulating the harsh conditions found on Mars, including low pressure, low temperature, and high radiation levels.
Planetary protection protocols prevent the biological contamination of solar system bodies by terrestrial spacecraft, and B. pumilus directly informs these guidelines. Spores of strain SAFR-032 were sent to the International Space Station (ISS) and exposed to real space conditions for 18 months. The spores showed a high survival rate (85% to 100%) when protected in a dark, simulated Martian atmosphere. This ability necessitates rigorous sterilization procedures for missions intended to land on potentially habitable worlds.