A STUDY OF BACTERIAL ADAPTATION TO ULTRAVIOLET LIGHT IN ULTRAPURE WATER SYSTEMS

Abstract

UV radiation is an essential unit operation in ultra pure water treatment systems forits efficiency in treating microbes and oxidizing organics. This research investigated allfactors that affect the ability of UV light to deactivate and kill microorganisms. Morespecifically, bacterial growth conditions and the UPW stimulation time were investigated.The two major UV lights sources, UV254 and UV 185, were compared. The resultingdeath rates of bacteria living in UPW indicate that UV 185 is very effective in killingbacteria, and even more powerful than UV254, providing there is no mass transferlimitations.The effect of UV light on bacteria surviving in UPW system was also investigated.Mutants with different phenotypes were found with both strains of bacteria used in thisresearch. Partial 16s rRNA test precluded the possibility of contamination. Fortunately,two types of mutation happened. One was sustainable across microbial generations whichsuggest a genetic variation. The other one may be from alteration in gene expressioncontrolled at the transcriptional level, since the strain readily reverted to the originalphenotype. Compared with original or wild type cells, mutants are more resistant to UVlight, indicating that bacteria in UPW have the ability to adjust and adapt to UV treatment.Fourier transform infrared spectroscopy was used to explore bacterial surface andstructural differences existing between the original and mutant bacteria. The goal was tounderstand some of the cell structure changes that occur after exposure to UV light. FTIRturned out to be a useful tool in this exploration, indirectly supplying information that themutant strain used in this research had a differently organized cell membrane, and a moreadhesive cell-wall. This observation is consistent with the well-known protectivemechanism for bacteria living in aquatic environment to form biofilms. The spectra alsoindicated a more stable protein structure in the mutant. Furthermore, the indication of anincrease in ribose-5-phosphate in the mutant suggested an increase in ability to repairnucleic acids. Overall, the FTIR spectra did give some indirect evidences in explainingthe higher UV resistant property of the mutant. One may find more help from FTIRspectra in the future, so that a breakthrough point in completely eliminatingmicroorganisms in UPW may be achieved based on the structural and componentchanges indicated by FTIR.