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Dulbecco’s modified Eagle’s medium (HyClone, Logan, UT), and serial dilutions were similarlymadeusing TSB. The 4 test organisms were C. difficile spores (BI strain), a MRSA strain (ATCC 43300), a VRE strain (ATCC strain 51299), and a clinical isolate of MDR Acinetobacter baumannii. Rodac plate templates were drawn on the Formica sheet and inoculated with 10–15 µLofa104 dilution of test organisms suspended in TSB, producing an estimated inoculum of 100–500 test organisms. After inoculation, each surface was allowed to air dry for 10 minutes after inoculation. Once dry, the test For- mica sheets were exposed to the disinfecting light and triplicate samples were collected with Rodac plates containing Dey-Engley Neutralizing Agar after 0, 1, 3, 5, 6, 7, 24, 48, and 72 hours. These plates were then incubated based on the test organism being studied (aerobically at 37oC for 48 hours for bacteria and anaerobically at 37oC for 48 hours for C. difficile) in an AnaeroPack anaerobic gas generator (Anaeropack, Mitsubishi Gas Chemical, Tokyo, Japan). After incubation, the colony-forming units (CFU) of the test organisms on each plate were quantified. Each template area was sampled only once. Surfaces were maintained at ambient room temperature and relative humidity. A control Formica sheet was placed in an adjacent area but not exposed to the HINS light to accommodate the expected natural in vitro die-off of vegetative bacteria. Triplicate samples were collected with Rodacs at the same test times as the test surfaces. Two experimental runs were con- ducted for all time points.
Statistical methods
We fit a mixed-effects negative binomial model to the data using the R statistical software8 and the lme4 package.9 We modeled the “blue” light as augmenting the “white” light. Both linear and squared time variables were included in the model to account for any nonlinear effects. The full model began with a 3-way inter- action of treatment × bacteria × time, and hypotheses were tested using likelihood ratio tests of progressively nested models. A P value<.05 was considered significant.
Results
A 3-way interaction was significant (χ2=265.5; df=12; P<.001), indicating that the effect of the type of light treatment differed with different combinations of test organisms and time. The treatment (ie, both blue and white light) had significantly dif- ferent rates of pathogen killing over time for all 4 organisms: Acinetobacter (χ2=117.2; df=4; P<.001), MRSA (χ2=80.5; df=4; P<.001), VRE (χ2=150.4; df=4; P<.001), and C. difficile (χ2=25.8; df=4; P<.001). We also performed individual tests of the interactions between
the white (vs the control) and time, and blue (vs white) and time. Both types of light treatments were associated with more rapid decreases in observed bacterial counts over time with all 4 organisms with 1 exception, the use of white light had no effect on C. difficile compared to control (Fig. 1). Specifically, the number of CFUs on test Rodac plates decreased over time for Acineto- bacter with the white light (χ2=95.7; df=2; P<.001) and the blue light (χ2=16.6; df=2; P<.001); for MRSA, for both white (χ2=31.7; df=2; P<.001) and blue (χ2=29.9; df=2; P<.001); and for VRE, for both white (χ2=7.1; df=2; P<.029) and blue (χ2=138.5; df=2; P<.001). However, white was not superior to control for C. difficile (χ2=2.6; df=2; P=.20), but the use of blue light increased killing of C. difficile (χ2=23.9; df=2; P<.001).
William A. Rutala et al Table 1 lists the earliest hour by which our statistical model
predicted a sustained reduction in the number of CFUs by a given percentage. Overall, the model demonstrates enhanced inactiva- tion of pathogens with the “blue” and “white” light.
Discussion
The use of light disinfection technology for continuous disinfec- tion of the healthcare environment has been proposed by various investigators.5–7 The use of disinfecting lights, if effective, could augment the episodic disinfection (eg, daily) that occurs in patient rooms or care areas by preventing or reducing the microbial regrowth on surfaces following disinfection, and by reducing the microbial level due to recontamination. These light sources are thought to be safe for surfaces and for humans,7 although there has been limited human experience. We demonstrated that the “blue” and “white” light significantly
reduced the 3 vegetative test bacteria; and “blue” light yielded lower counts of C. difficile spores after 72 hours.Whether the level of these reductions are sufficient to reduce healthcare-associated infections remains uncertain, and the question requires further study. This study was a preliminary evaluation. Future studies will
need to consider cost-effectiveness, multiple types of surfaces (eg, porous vs nonporous surfaces, stainless steel) with taxonomically diverse pathogens (eg, norovirus, Enterobacteriaceae) to include spores, use areas (eg, operating room), and the ability of the technology to continuously reduce the overall bioburden in inpatient and outpatient care areas and reduce HAIs. A separate issue is the acceptance of continuous light (ie, 24 hours) by patients and staff. If shorter durations of continuous light expo- sure are deemed necessary, the level of decontamination achieved by use during times when the patient is awake (eg, ~16 hours per day) needs further study. In addition, future studies should include rechallenging the surfaces with additional contamination (eg, every 4–6 hours). Given that environmental surfaces in a patient’s room are often not thoroughly disinfected and that recontamination occurs rapidly, it is important to develop either methods of continuous disinfection or a germicide with persistant antimicrobial effectiveness.
Acknowledgments. Kenall Manufacturing loaned UNC Hospitals the high- intensity, narrow spectrum light fixtures for the study period. They also allowed UNC Hospitals to use the NIST calibrated spectroradiometer to measure surface irradiance. Kenall had no role in the design, conduct, analysis, or publication of this study.
Financial support. No financial support was provided relevant to this article.
Conflicts of interest. Drs Rutala and Weber are consultants for PDI in 2017–2018 and consultants for Clorox in 2012–2016. Dr Weber is a consultant for Germitec. Dr Rutala has received an honorarium from Kenall.
References
1. Weber DJ, Rutala WA, Anderson DJ, Chen LF, Sickbert-Bennett EE, Boyce JM. Effectiveness of UV devices and hydrogen peroxide systems for terminal room decontamination: focus on clinical trials. Am J Infect Control 2016;44:e77–e84.
2. Rutala WA, Weber DJ. Monitoring and improving the effectiveness of surface cleaning and disinfection. Am J Infect Control 2016;44: e69–e76.
3. Stiefel U, Cadnum JL, Eckstein BC, Guerrero DM, Tima MA, Donskey CJ. Contamination of hands with methicillin-resistant Staphylococcus aureus after contact with environmental surfaces and after contact
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