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theses

Microbial photoinactivation is a sensitizing process where a photosensitizer inactivates microorganisms by generating reactive oxygen species in the presence of light. Sodium copper chlorophyllin (Na-Chl) is a green dye approved by the Food and Drug Administration as generally recognized as safe (GRAS) and commonly used in dry beverage mixes. Na-Chl is a hydrophilic anionic photosensitizer, which is known to be less effective than cationic photosensitizer due to repulsive electrostatic force with the negatively charged membrane of the bacterial cell. Chitosan is a positively-charged antimicrobial polysaccharide. In this study, we investigated the synergistic effect of chitosan on photosensitization of Staphylococcus aureus and Escherichia coli O157:H7 by sodium copper chlorophyllin. Bacterial suspension in sterile water was incubated with different concentrations of Na-Chl prior to illumination with LED light at 400 nm to identify optimal concentration of Na-Chl for photosensitization. To examine the synergistic effect of chitosan, three different concentrations of chitosan were tested with the identified optimal concentration of Na-Chl. Three sample groups were tested: concurrent incubation of chitosan and Na-Chl prior to illumination, sequential incubation of chitosan first followed by addition of Na-Chl prior to illumination, and sequential incubation of Na-Chl first followed by addition of chitosan prior to illumination. We found that, in the experiment with S. aureus, the concurrent incubation group had a slight increase in log reduction compared to photosensitizer alone, whereas the sequential incubation with chitosan first reduced the effectiveness of photosensitizer. Interestingly, the sequential incubation with Na-Chl first followed by chitosan resulted in a synergistic effect, with an additional reduction of two log cycles compared to the photosensitizer alone. However, photosensitization had negligible killing effect on E. coli O157:H7. The results indicate that the intracellular localization of photosensitizer is important for the effectiveness of photosensitization, and that the ratio between chitosan and Na-Chl along with the sequence of treatment is important for the effectiveness of the hurdle system. Further studies are necessary to improve the effectiveness of photosensitization on gram-negative bacteria.

ETD_6238

M.S.

Includes bibliographical references

by Vi Khanh Dao

doi:10.7282/T3DB83QZ

ETD graduate

The author owns the copyright to this work.