Acid tolerant bacterium Bacillus amyloliquefaciens MBNC retains biocontrol efficiency against fungal phytopathogens in low pH.

Soil pH conditions have important consequences for microbial community structure, their dynamics, ecosystem processes, and interactions with plants. Low soil pH affects the growth and functional activity of bacterial biocontrol agents which may experience a paradigm shift in their ability to act antagonistically against fungal phytopathogens. In this study, the antifungal activity of an acid-tolerant soil bacterium Bacillus amyloliquefaciens MBNC was evaluated under low pH and compared to its activity in neutral pH conditions. Bacterial supernatant from 3-day-old culture (approximately 11.2 × 108 cells/mL) grown in low pH conditions was found more effective against fungal pathogens. B. amyloliquefaciens MBNC harboured genes involved in the synthesis of secondary metabolites of which surfactin homologues, with varying chain length (C11-C15), were identified through High-Resolution Mass Spectroscopy. The pH of the medium influenced the production of these metabolites. Surfactin C15 was exclusive to the extract of pH 4.5; production of iturinA and surfactin C11 was detected only in pH 7.0, while surfactin C12, C13 and C14 were detected in extracts of both the pH conditions. The secretion of phytohormones viz. indole acetic acid and gibberellic acid by B. amyloliquefaciens MBNC was detected in higher amounts in neutral condition compared to acidic condition. Although, secretion of metabolites and phytohormones in B. amyloliquefaciens MBNC was influenced by the pH condition of the medium, the isolate retained its antagonistic efficiency against several fungal phyto-pathogens under acidic condition.