Abstract
BackgroundAntimicrobial violet-blue light in the region of 405 nm is emerging as an alternative technology for hospital decontamination and clinical treatment. The mechanism of action is the excitation of endogenous porphyrins within exposed microorganisms, resulting in ROS generation, oxidative damage and cell death. Although resistance to 405 nm light is not thought likely, little evidence has been published to support this. This study was designed to establish if there is potential for tolerance development, using the nosocomial pathogen Staphylococcus aureus as the model organism.MethodsThe first stage of this study investigated the potential for S. aureus to develop tolerance to high-intensity 405 nm light if pre-cultured in low-level stress violet-blue light (≤1 mW/cm2) conditions. Secondly, the potential for tolerance development in bacteria subjected to repeated sub-lethal exposure was compared by carrying out 15 cycles of exposure to high-intensity 405 nm light, using a sub-lethal dose of 108 J/cm2. Inactivation kinetics and antibiotic susceptibility were also compared.ResultsWhen cultured in low-level violet-blue light conditions, S. aureus required a greater dose of high-intensity 405 nm light for complete inactivation, however this did not increase with multiple (3) low-stress cultivations. Repeated sub-lethal exposures indicated no evidence of bacterial tolerance to 405 nm light. After 15 sub-lethal exposures 1.2 and 1.4 log10 reductions were achieved for MSSA and MRSA respectively, which were not significantly different to the initial 1.3 log10 reductions achieved (P = 0.242 & 0.116, respectively). Antibiotic susceptibility was unaffected, with the maximum change in zone of inhibition being ± 2 mm.ConclusionsRepeated sub-lethal exposure of non-proliferating S. aureus populations did not affect the susceptibility of the organism to 405 nm light, nor to antibiotics. Culture in low-level violet-blue light prior to 405 nm light exposure may increase oxidative stress responses in S. aureus, however, inactivation still occurs and results demonstrate that this is unlikely to be a selective process. These results demonstrate that tolerance from repeated exposure is unlikely to occur, and further supports the potential development of 405 nm light for clinical decontamination and treatment applications.
Highlights
Antimicrobial violet-blue light in the region of 405 nm is emerging as an alternative technology for hospital decontamination and clinical treatment
Determining potential for tolerance development when cultured under different lighting conditions Cultivation in different lighting conditions methicillin-sensitive S. aureus (MSSA) was inoculated in 100 ml nutrient broth (NB; Oxoid, UK) and cultivated under different lighting conditions: (i) complete darkness; (ii) normal laboratory fluorescent lighting, approximately 200 Lux; and (iii) 3 levels of low-intensity 405 nm light: 0.15, 0.5 & 1 mW/cm2 (measured using a radiant power meter and photodiode detector (LOT Oriel, USA))
The sensitivity of MSSA to high-intensity 405 nm light returns to a similar level of that when cultivated in complete darkness alone, with an average 0.7 × 101 Colony forming units (CFU)/ml population surviving following a dose of 216 J/cm2, which is not significantly different to the average 0.3 × 101 colony forming units per millilitre (CFU/ml) surviving population which had been grown in darkness (P = 0.230)
Summary
Antimicrobial violet-blue light in the region of 405 nm is emerging as an alternative technology for hospital decontamination and clinical treatment. There is global concern surrounding antibiotic resistant organisms, such as methicillin-resistant Staphylococcus aureus (MRSA) These organisms negatively impact on healthcare systems by causing infections which are harder to treat due to reduced antibiotic choices, resulting in longer hospital stays and increased mortality of patients [1]. To reduce transmission of these pathogenic organisms, new technologies are being developed to aid environmental decontamination and clinical treatment. One such antimicrobial technology is 405 nm light. Violet-blue light in this region photo-excites intracellular porphyrins within microorganisms, producing a range of reactive oxygen species (ROS) which cause oxidative damage and cell death [2,3,4]. It has recently been reported that 405 nm light can have a synergistic antimicrobial effect with common chlorinated disinfectants, providing further support for its beneficial use for environmental decontamination [19]
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