Effects of ultra violet and visible light on bacterial survival / by Natasha Vermeulen.

Abstract

The optical dose in mJ/cm[superscript]2 for causing mortality of Escherichia coli, using wavelengths between 230-375 nm in the UV and 400-532 nm in the mid-visible spectral regions, has been determined in this study. Currently, the mercury 254 nm UV line or UV flash lamp sources are used in most optical equipment designed for bactericidal action. However, little is known about the bactericidal effect of UV at longer than 254 nm and visible light radiation between 400-532 nm. An accurate knowledge of the bactericidal dose versus wavelength is of central importance in the optimal choice of light source for a given application. Now that UV diodes and lasers emitting in the 280 to 340 nm range are entering the developmental phase, they can be added to the list of existing UV flash lamps, discharge lamps and high intensity visible lasers for use in bactericidal applications. E. coli cell suspensions at about 1x10[superscript]8 CFU/ml were exposed to various dosages of radiation between 230-532 nm and the survival cell densities were determined by drop-plating. Radiation between 260 to 280 nm in the UV region was most efficient in killing the E. coli cells. In addition, significant mortality of E. coli was observed when the cell suspensions were exposed to visible light at 458 and 488 nm. Based on the E. coli survival data at various wavelengths and dosages, we constructed a predictive equation to estimate the survival of E. coli when exposed to a known dosage of radiation at a specific wavelength. Log(S/So) = - (1.089 x 10[superscript]7 e[superscript]-0.0633γ)D Where S = survivor cell density (CFU/ml), So = initial cell density (CFU/ml), D = Radiation dose (mJ/cm[superscript]2), γ = wavelength of radiation (nm). It was also determined that E. coli cells had an absorption peak between 260-280 nm which coincided with the UV spectrum that had the highest killing capacity for E. coli. The FTIR spectrums of UV-treated and untreated E. coli showed that there was a decrease of C-O-C and C-O-P bondings after the UV treatment, indicating the destruction of the glycan backbone of peptidoglycan and phosphodiester backbone of nucleic acids, respectively. There was also an increase in protein content in the UV-treated samples because there was a significant increase in the amide bonds. This increase may be a stress response mechanism of the E. coli cells exposed to the UV treatment. The UV-treated cells also showed an increase in the amount of CH[subscript]2 stretching of fatty acids, indicating a change in membrane structure of the UV treated cells. Furthermore, UV-treated samples showed an increase of hydroxyl functional group, an indication of an increase of reactive oxidation species (ROS) in the E. coli cells.