Bioemulsifier production by a halothermophilic Bacillus strain with potential applications in microbially enhanced oil recovery

A halothermotolerant Gram-positive spore-forming bacterium was isolated from petroleum reservoirs in Iran and identified as Bacillus licheniformis sp. strain ACO1 by phenotypic characterization and 16S rRNA analysis. It showed a high capacity for bioemulsifier production and grew up to 60°C with NaCl at 180 g l⁻¹. The optimum NaCl concentration, pH and temperature for bioemulsifier production were 4% (w/v), 8.0, and 45°C, respectively. Although ACO1 did not utilize hydrocarbons, it had a high emulsifying activity (E ₂₄ = 65 ± 5%) on different hydrophobic substrates. Emulsification was optimal while growing on yeast extract as the sole carbon source and NaNO₃ as the nitrogen source. The efficiency of the residual oil recovery increased by 22% after in situ growth of B. licheniformis ACO1 in a sand-pack model saturated with liquid paraffin.