Microscopic analysis towards rhamnolipid-mediated adhesion of Thiobacillus denitrificans: A QCM-D study

Rhamnolipid was proved to increase the abundance of Thiobacillus denitrificans in the mixotrophic denitrification biofilm while its microscopic mechanism remains to be explored. Effect of rhamnolipids on deposition of macromolecular substances and adhesion of Thiobacillus denitrificans at room (20 °C) and low temperature (10 °C) were systematically investigated by the quartz crystal microbalance with dissipation monitoring (QCM-D) for the first time. Results showed that low concentration of rhamnolipids (20-80 mg/L) could promote the deposition of macromolecular substances by reducing hydraulic repulsion force, with the maximum deposition amount increased by 4.28 times than that of the control at room temperature. Deposition amount of microorganisms could be improved by increasing its concentration at room temperature while it didn't work at low temperature. Meanwhile, low temperature could significantly inhibit adhesion of Thiobacillus denitrificans (p < 0.05) and deposited layers under low concentration of rhamnolipids were generally rigid, resulting in the negative feedback effect on the microorganisms' adhesion. While high concentration of rhamnolipids (120-200 mg/L) could regulate the biofilm from rigid to viscoelastic and significantly promote the initial adhesion of Thiobacillus denitrificans on SiO₂ surface (p < 0.05). This study demonstrated the microscopic mechanism of rhamnolipids on the initial biofilm formation, that is, the reduction of hydration repulsion force was responsible for the enhanced deposition of macromolecules while the regulation of biofilm properties was account for the promoted adhesion of Thiobacillus denitrificans.