Thermal quenching of fluorescence of freshwater, planktonic bacteria

This study aims to determine the thermal quenching properties of pure bacterial cultures as a means of aiding the development of fluorescence measurement in natural waters. The bacterium Pseudomonas aeruginosa was isolated from the urban River Tame, Birmingham, UK, and planktonic bacteria were grown in sterile, sealed glass jars, in 100 mL of sterile growth media at 37 °C for a maximum of 24 h. Samples were taken at T = 6 h and at T = 24 h, and thermal fluorescence quenching measured at 5 °C increments between 10 and 45 °C over 30 min. 3D excitation–emission matrix (EEM) plots were generated from the fluorescence analyses over time. It was found that the fluorescence of a microbial culture was significantly thermally quenched, but the results were dependent on the fluorophore type and the stage of the bacterial growth curve. Quenching was sometimes non-linear, presumably due to fluorophore production exceeding thermal quenching during the growth phase of the bacteria. Thermal quenching has the potential to allow us to confirm the importance of microbes in fluorescence signals by the non-linear response to increasing temperature, and to utilise the thermal fluorescence quenching properties of molecules to differentiate between fluorophores present during bacterial growth.