Cora Norrbom, “Using Light to Kill Bacteria with Photodynamic Inactivation”
Mentor: Troy Skwor, Biomedical Sciences
Poster #98

The CDC has identified carbapenem- resistant Enterobacterales, which includes Escherichia coli, as an urgent threat to healthcare due to a strong resistance against last-line antibiotics. One alternative to combating multi-drug resistant bacteria is photodynamic inactivation (PDI). This treatment employs a chemical known as a photosensitizer, which upon exposure to light in the presence of oxygen, produces reactive oxygen species that can kill bacteria. Previous work in our laboratory used the metalloporphyrin, PdT4, with 405nm light and it was very effective killing 99.999% of Gram- positive methicillin-resistant Staphylococcus aureus (MRSA) in 11 seconds. However, this treatment was not as efficient against Gram-negative bacteria.  Our objective was to test the PDI effect on E. coli using a new porphyrin 5,10,15-tri(N-methyl-4-pyridyl)20(N-C14H29)-4-pyridyl)porphin (C14), which has a C14 alkyl chain off of one pyrrole ring in contrast to PdT4, with or without incorporating free Pd⁺² (PdC14) and irradiating with a hand-held 405nm light-diode. We first acquired the absorbance spectrum of the various porphyrins:  TMPyP, PdT4, C14, and PdC14 to determine the effect of the Pd⁺² addition on the soret peak, also known as the absorbance peak in the blue wavelength region. The soret peak of the PdC14 was shifted 5nm to the left compared to the C14 alone, and only 2nm to the right of PdT4. This was determined to ensure that the porphyrins would be excited with the 405 nm irradiance. To assess bactericidal effects by the photodynamic process, E. coli cultures were pre-incubated for 5 minutes with porphyrins, PdT4 and PdC14, then exposed to varying doses of irradiation. Serial dilutions of the treatments were performed and plated on tryptic soy agar with subsequent incubation overnight at 35℃. After counting the number of bacteria from each treatment, PdC14 killed 99.9% more E. coli compared to PdT4 after 5J/cm² of 405nm light. In conclusion, these findings identified PdC14 as a more effective photosensitizer against Gram- negative bacteria with 405nm irradiance compared to PdT4.