Master Thesis Defense by Mrs Maria-Ekaterini Lironi

Title MSc «Water disinfection methods for bacterial inactivation and study of damages in subcellular level»

Wednesday  21 October  2020, at: 10:30, Venue: https://tuc-gr.zoom.us/j/86708297129?pwd=SzB6TkJrZXpKS3lCenVZQXVtT20yUT09

Examination Committee

• Associate Professor        Danae Venieri (advisor)
• Professor Evan Diamadopoulos
• Assistant Professor  Nikolaos Xekoukoulotakis

Abstract

Due to the increasing pressure on the environment and the threat of some key resources, such as water, from microbial pathogens, the development of alternative, economical and efficient disinfection techniques, is of high importance. In this context, in the present study, it was investigated the disinfection potential of photocatalysis and ozonation, regarding the inactivation rates of three reference bacteria (Escherichia coli, Pseudomonas aeruginosa & Bacillus cereus), which are considered important for public health. Also, we studied the damages in the subcellular level in terms of disinfection effects on cellular components, namely, lipids, membrane and proteins. The commercially available P25 catalyst was used at concentrations of 25 - 100 mg/L for the photocatalytic experiments, while during ozone treatment the applied ozone dose ranged from 2,40 to 19,99 mg /L. The results confirmed the effectiveness of both disinfection techniques. The increase in catalyst concentration led to higher inactivation rates, with Gram-negative E. coli and P. aeruginosa being inactivated by 7-8 Logs in 20 min, while Gram-positive B. cereus exhibited a 4 Log reduction in 60 min of treatment, despite its lower initial concentration (105 CFU/mL). Ozonation proved to be more effective as a disinfection method of water samples compared to UV-A photocatalysis, as a complete microbial decay was achieved within 15 min for Gram-negative bacteria, while for B. cereus the same reduction was achieved within 30 min. Photoreactivation of E. coli was observed, however, the recorded levels were low. Regarding the subcellular level, disinfection techniques were accompanied by lipid peroxidation, which developed exponentially during treatment. The ONPG (ortho-nitrophenyl-β-D-galactopyranoside) hydrolysis assay showed alterations in the course of treatment, indicating the increase in cell membrane permeability, allowing the free efflux of cytoplasm into the outer solution. The SDS PAGE (Sodium dodecyl sulfate – polyacrylamide gel electrophoresis) analysis showed that in the course of treatment the protein pattern did not alter significantly, with generally high levels of homology between the samples of the intact cells and the samples after each treatment.