Role of disinfectants and pipe materials on bacterial adhesion onto biofilms

Biofilms are ubiquitous in aquatic environments. Biofilms have been shown to attract and harbor pathogens such as P. aeruginosa and Legionella pneumophila in premise plumbing system. The fact that biofilms can protect attached bacterial cells from disinfectants raises rudimentary questions regarding interactions of bacterial cells with biofilm surfaces. Consequently, the main objectives of this study were to: 1) investigate the mechanisms that govern E. coli S17, E. coli 14f and Legionella cells adhesion on clean PVC, copper and biofilms; 2) examine the role of disinfectants on biofilms structure and subsequent effect on bacterial adhesion. Mechanisms of three strains of bacteria attachment on biofilms grown on PVC and copper surfaces were investigated. Biofilms were grown in CDC reactors using different types of feed water such as groundwater, monochloramine-treated groundwater, dechlorinated tap water and tap water. Biofilm physical structure was characterized at micro- and meso-scales using Scanning Electron Microscopy, Optical Coherence Tomography and Confocal Laser Scanning Microscopy. On clean PVC, copper and biofilms surfaces, the adhesion of three bacterial strains was found to increase as a function of ionic strength. However, on established biofilms, the adhesion was independent of solution chemistry. It rather had a positive correlation with biofilm roughness. Adhesion of every bacterial strain had found to increase on rougher biofilms than smoother ones. Besides normally grown biofilms, disinfectants were also introduced into feed water. After 3 months of exposure to monochloramine, aged groundwater biofilms became smoother. This smooth biofilm surface discouraged bacterial cells from adhering. Even though disinfectant can alter biofilm surface roughness, it did not seem to change the average thickness of well-established biofilms. Besides monochloramine, free chlorine from tap water was able to eradicate thin biofilms from the pipe surface leading to lower adhesion of bacterial cells. The effect of bacteria surface hydrophobicity on bacterial adhesion was also investigated by starving Legionella cells in Newmark Groundwater. Starved cells exhibited more hydrophobicity and adhered more on hydrophobic PVC surfaces than fresh cells. Conversely, adhesion of starved cells on copper surfaces was lower than fresh cells due to incompatibility of hydrophobicity between bacterial cells and copper surfaces.