Effects of ultrasound on disrupting metabolite profiles of Pseudomonas fluorescens biofilms cultured on the surface of lettuce

This study cultured Pseudomonas fluorescens biofilm on the surface of lettuce. Efficacy of ultrasound-based cleaning treatment on biofilm removal was evaluated at the intensity of 15.79 and 26.32 W/cm2. At the end of ultrasonic treatment, remained content of biofilm on the lettuce was significantly decreased with the increased intensity of ultrasound. Meanwhile, quality attributes of lettuce largely remained unchanged under the ultrasonic treatment at the highest intensity level. The total number of colonies and biofilm content were lowest, and quality attributes of the lettuce remained the highest when treated by ultrasound at the intensity of 26.32 W/cm2 compared to that treated by ultrasound at 15.79 W/cm2 and the untreated group during the cold storage from 0 to 8 d. Furthermore, non-targeted metabolomics revealed the influences of ultrasonic treatment on P. fluorescens metabolisms at different intensity levels. There are 27 metabolites differentially expressed in the ultrasound-treated bacteria, which were mainly involved in metabolisms of carbohydrate, amino acid, lipid, and nucleotide. The 15.79 W/cm2-ultrasonic treatment damaged exopolysaccharides, disrupted the energy level and genetic information of bacterial biofilm. However, the stress response of bacterial biofilm was induced simultaneously through accumulating the lipid metabolites. The ultrasonication at 26.32 W/cm2 significantly disrupted the metabolic level and osmotic pressure of bacterial biofilm and thereby, inhibited the synthesis of proteins and damaged genetic information. In turn, P. fluorescens accumulated UDP-Gal, ADPR and 4-PA at the end of ultrasound treatment at 26.32 W/cm2 as a mean of energy saving for self-protection. The differences in metabolic responses were mainly attributed to an extremely high, albeit momentary high temperature and strong oxidation generated by ultrasound.