Blue 470 nm light suppresses the growth of Salmonella enterica and methicillin-resistant Staphylococcus aureus (MRSA) in vitro

Abstract

Background and Objective Emerging evidence suggests that blue light can photo-inactivate some bacteria of clinical importance. Consequently, we tested the hypothesis that 470 nm light can suppress growth of two recalcitrant bacteria, MRSA and Salmonella. Materials and Methods We plated 5 × 106 and 7 × 106 CFU/ml USA300 strain of MRSA and 1 × 106 CFU/ml of Salmonella enterica serovars Typhimurium and Heidelberg. Plates were irradiated with 55, 110, 165 and 220 J/cm2 of blue light, incubated at 37C for 24 hours and colony counts determined. Results Compared with controls, blue light irradiation produced a significant dose-dependent reduction in the number of colonies formed by each bacterial strain (P < 0.001). Irradiation of 5 × 106 and 7 × 106 CFU/ml MRSA with 55 J/cm2 produced 92% (4.6 × 106 CFU/ml) and 86% (6 × 106 CFU/ml) inactivation respectively, while 110 and 220 J/cm2 suppressed each MRSA density 100%. Irradiation of Salmonella Typhimurium with 55 and 110 J/cm2 suppressed bacterial growth 31% (3.1 × 105 CFU/ml) and 93% (9.3 × 105 CFU/ml) respectively; while Salmonella Heidelberg was inhibited 11% (1.1 × 105 CFU/ml) and 84% (8.4 × 105 CFU/ml) respectively by the two fluences. Complete inactivation of each Salmonella strain was achieved using 165 or 220 J/cm2. Conclusion The observed inhibition of Gram-positive (MRSA) and Gram-negative (Salmonella) bacteria suggests the versatility of blue light in bacteria eradication, making it a viable intervention strategy for decontamination of food and environments that harbor such bacteria. Lasers Surg. Med. 47:595-601, 2015.