Light Emitting Diodes as Alternative Light Sources: Effects of Ultraviolet Frequencies on Microbial Replication

Background: In the biomedical field the UV-C rays are known for altering the effect of replication on different microbial species. Most of the studies reported in bibliography use the UV-C lamps, as light sources, that have a broad spectral emission, on the contrary Light Emitting Diodes have specific wavelengths. The aim of our research is to test these innovative sources of light at different wavelengths on microorganism’s replication in different time. Methods: The present pilot study in which pre and post exposure phases were erformed at different LED sources. It was conducted in the months of January and February 2018 in the Environmental Hygiene Laboratory of the University of Siena. The tested LEDs had a power and the following wavelengths 276 and 279 nm (UV-C); 305,306 and 308 nm (UV-B); 343 and 345 nm (UV-A). For each of them, tests were carried out using: Enterococcus faecalis, Pseudomonas aeruginosa, Staphylococcus aureus, Escherichia coli and Bacillus subtilis. For each of these strains, bacterial suspensions were prepared. Aliquots of these were seeded in double in Petri dishes with selective medium for each microorganism chosen. Treatment involved illuminating exposed Petri dishes with a UV LED for 10-30 and 60 minutes and then incubated at 36°C for 24 hours. For each microorganism, four Petri dishes contaminated but not exposed to UV radiation were used as controls. They were also incubated in a thermostat at 36°C and then read at 24 hours. Results: We found differences in the replication of microorganisms between the different wavelengths. For UV-C there was a reduction between 97% and 100% already at 10 minutes. For UV-B we noted that the replication for all microorganisms was greater than the UV-C and time-related. For UV-A although with some uncertainties, in general, we still found a time-related reduction. Conclusion: The results show that the efficacy of the LED depends on exposure times, wavelengths and microbial species. There is evidence that monochromatic UV LED source, influence the microbial replication in different way. Such an eventuality opens innumerable hypotheses of study and possible innovative applications.