Blue light eliminates community-acquired methicillin-resistant Staphylococcus aureus in infected mouse skin abrasions

Abstract

Background and objective: Bacterial skin and soft tissue infections (SSTI) affect millions of individuals annually in the United States. Treatment of SSTI has been significantly complicated by the increasing emergence of community-acquired methicillin-resistant Staphylococcus aureus (CA-MRSA) strains. The objective of this study was to demonstrate the efficacy of blue light (415 ± 10 nm) therapy for eliminating CA-MRSA infections in skin abrasions of mice.

Methods: The susceptibilities of a CA-MRSA strain (USA300LAC) and human keratinocytes (HaCaT) to blue light inactivation were compared by in vitro culture studies. A mouse model of skin abrasion infection was developed using bioluminescent USA300LAC::lux. Blue light was delivered to the infected mouse skin abrasions at 30 min (acute) and 24 h (established) after the bacterial inoculation. Bioluminescence imaging was used to monitor in real time the extent of infection in mice.

Results: USA300LAC was much more susceptible to blue light inactivation than HaCaT cells (p=0.038). Approximately 4.75-log10 bacterial inactivation was achieved after 170 J/cm(2) blue light had been delivered, but only 0.29 log10 loss of viability in HaCaT cells was observed. Transmission electron microscopy imaging of USA300LAC cells exposed to blue light exhibited disruption of the cytoplasmic content, disruption of cell walls, and cell debris. In vivo studies showed that blue light rapidly reduced the bacterial burden in both acute and established CA-MRSA infections. More than 2-log10 reduction of bacterial luminescence in the mouse skin abrasions was achieved when 41.4 (day 0) and 108 J/cm(2) (day 1) blue light had been delivered. Bacterial regrowth was observed in the mouse wounds at 24 h after the blue light therapy.

Conclusions: There exists a therapeutic window of blue light for bacterial infections where bacteria are selectively inactivated by blue light while host tissue cells are preserved. Blue light therapy has the potential to rapidly reduce the bacterial load in SSTI.

FIG. 1.

Hematoxylin and eosin (H&E) stained histology of representative BALB/c mouse skin. (A) Intact skin. (B) Partial thickness skin abrasion. Scale bars: 20 μm.

FIG. 2.

In vitro cell survival curves of USA300 LAC::lux and human keratinocyte (HaCaT) cells in response to blue light irradiation. Bars: standard deviation.

FIG. 3.

Transition electron microscopy (TEM) images of (panel A) untreated and (panel B) blue light treated methicillin-resistant Staphylococcus aureus (MRSA) cells. Arrows, disruption of cytoplasmic contents; asterisks, cell debris; oval, disruption and breakage of bacterial cell wall. Bars: 500 μm.

FIG. 4.

Blue light prophylaxis and treatment of community-acquired methicillin-resistant Staphylococcus aureus (CA-MRSA) infections in mouse skin abrasions. (A) Successive bacterial luminescence images of a representative mouse skin abrasion infected with 3×10⁶ colony-forming units (CFU) of USA300 LAC::lux and exposed to 41.4 J/cm² blue light at 30 min (day 0) after bacterial inoculation (top row). Corresponding images of a MRSA infected representative mouse skin abrasion kept in the dark for the amount of time necessary to deliver blue light to the treated mice. (B) Dose response curves showing mean fraction of luminescence remaining from both blue light treated mice (n=4) and dark control mice (n=4). Bars are SD. *p<0.5; ***p<0.001 versus dark controls.

FIG. 5.

(A) Successive bacterial luminescence images of a representative mouse skin abrasion infected with 3×10⁵ CFU of USA300 LAC::lux and exposed to 108 J/cm² blue light at 24 h (day 1) after bacterial inoculation (top row). Corresponding images from a dark control mouse at day 1 kept in the dark for the equivalent period of time. (B) Dose response curves showing mean fraction of luminescence remaining from both blue light treated mice (n=4) and dark control mice (n=4). Bars are SD. *p<0.5; ***p<0.001 versus dark controls.