Effect of D-Tyrosine on Bacterial Biofilms: Mechanisms and Potential Applications in Membrane Biofouling Control
Doctor of Philosophy
Biofouling leads to diverse detrimental effects in water treatment and distribution systems. D-Tyrosine can be produced by a variety of bacteria and inhibits formation as well as triggers disassembly of biofilms thus has been proposed for biofouling control applications. The impact of D-tyrosine in different biofilm formation stages in G+ and G- bacteria was studied, and a non-monotonic correlation between D-tyrosine concentration and biofilm inhibition effect was revealed. D-Tyrosine inhibited attachment and biofilm formation in Pseudomonas aeruginosa and Bacillus subtilis with an effective concentration of 5 nM. Extracellular protein was decreased in P. aeruginosa biofilms, but increased in those of B. subtilis. Exopolysaccharides production by P. aeruginosa was increased at low concentrations and reduced at high concentrations while no impact was found in B. subtilis. These results suggest that distinct mechanisms are involved at different D-tyrosine concentrations and they may be species specific. Further investigation of the biofilm related gene expression in P. aeruginosa indicated repression of quorum sensing genes at high (200 µM) and low (5nM) effective concentrations but not at non-inhibitive concentrations (1 µM). Lipopolysaccharides production was reduced and the genes were down regulated by D-tyrosine at 5 nM but not at 200 µM. The efflux pump, flagella and racemase genes were also repressed by 5 nM D-tyrosine. Efflux pump is closely related to quorum sensing while how flagella and racemase are affected is unclear. At low concentration, D-tyrosine may serve as a signal molecule to regulate LPS production and quorum sensing, biofilm formation is inhibited through which. At micromolar level, the biofilm inhibition effect decreased with D-tyrosine concentration and could possibly be attributed to the repression of quorum sensing. D-Tyrosine was incorporated onto a nanofiltration membrane using FAU type zeolite nanoparticles covalently bound to the membrane surface as carriers to develop a long-lasting environmentally friendly anti-biofouling membrane. The initial P. aeruginosa cell attachment and subsequent biofilm formation were inhibited. D-tyrosine was gradually released from the membrane in ultrapure water for 5 days and the membrane retained its anti-biofouling capability for 6 days. The membrane alleviated flux decline and irreversible cell adhesion on membrane surface in dead-end filtration.