Isolation and Characterization of Biosurfactant-Producing Bacteria from Garlic Farmland Soil and Evaluation of Antimicrobial Activity

Garlic suffers from frequent pathogenic diseases, seriously affecting its yield and quality. Biosurfactants have been reported with high antimicrobial activity and wide antimicrobial spectrum. Biosurfactant-producing bacteria existing in garlic farmland soil are expected to be used in control of garlic diseases. In this study, strain FA1 having high yield of biosurfactants was isolated from garlic farmland soil. Combined with morphological analysis and its 16S rDNA sequences analysis, the strain FA1 was identified as Pseudomonas aeruginosa. HPLC-MS analysis showed that biosurfactants produced from P. aeruginosa FA1 were presented rhamnolipids, mainly Rha-C₈-C₁₀, Rha-Rha-C₈-C₁₀, Rha-C₁₀-C₁₀ and Rha-Rha-C₁₀-C₁₀. The proportion of mono-rhamnolipids and di-rhamnolipids was about 1 : 1. P. aeruginosa FA1 can produce 7.96 g/L of rhamnolipids. The FA1 rhamnolipids can decrease the air-water surface tension to 27.5 mN/m with a critical micelle concentration of 50 mg/L. Through the agar diffusion method, P. aeruginosa FA1 exhibited high antimicrobial activity with diameters of inhibition zones larger than 20 mm to both the garlic potential pathogenic bacteria and fungi. Rhamnolipids produced from P. aeruginosa FA1 induced the potential pathogens to generate higher level of intracellular Reactive Oxygen Species (ROS), thus, exerting antimicrobial effects. P. aeruginosa FA1 can highly produce rhamnolipids biosurfactants and exhibit high antimicrobial effect to the garlic potential pathogens. This study verified that indigenous biosurfactant-producing bacteria from garlic farmland soil were feasible and effective to control garlic potential pathogens.