Given that the dearth of new antibiotic development loads an existential burden on successful infectious disease therapy ¹ , health organizations are calling for alternative approaches to combat methicillin-resistant Staphylococcus aureus (MRSA) infections. Here, we report a drug-free photonic approach to eliminate MRSA through photobleaching of staphyloxanthin, an indispensable membrane-bound antioxidant of S. aureus ^2-5 . The photobleaching process, uncovered through a transient absorption imaging study and quantitated by absorption spectroscopy and mass spectrometry, decomposes staphyloxanthin and sensitizes MRSA to reactive oxygen species attack. Consequently, staphyloxanthin bleaching by low-level blue light eradicates MRSA synergistically with external or internal reactive oxygen species. The effectiveness of this synergistic therapy is validated in MRSA culture, MRSA-infected macrophage cells, S. aureus biofilms, and a mouse wound infection model. Collectively, these findings highlight broad applications of staphyloxanthin photobleaching for treatment of MRSA infections.

Confronted with the rapid evolution and dissemination of antibiotic resistance, there is an urgent need to develop alternative treatment strategies for drug-resistant pathogens. Here, we present an unconventional approach to restore the susceptibility of methicillin-resistant S. aureus (MRSA) to a broad spectrum of conventional antibiotics via photo-disassembly of functional membrane microdomains. The photo-disassembly of microdomains is based on effective photolysis of staphyloxanthin, the golden carotenoid pigment that gives its name. Upon pulsed laser treatment, cell membranes are found severely disorganized and malfunctioned to defense antibiotics, as unveiled by membrane permeabilization, membrane fluidification, and detachment of membrane protein, PBP2a. Consequently, our photolysis approach increases susceptibility and inhibits development of resistance to a broad spectrum of antibiotics including penicillins, quinolones, tetracyclines, aminoglycosides, lipopeptides, and oxazolidinones. One Sentence Summary: Using photons to crash S. aureus cell membrane and its formidable defense against a broad spectrum of antibiotics.

Streptococcus agalactiae , also known as Group B Streptococcus (GBS), is increasingly recognized as a major cause of soft tissue and invasive diseases in the elderly and diabetic populations. Antibiotics like penicillin are used with great frequency to treat these infections, although antimicrobial resistance is increasing among GBS strains and underlines a need for alternative methods not reliant on traditional antibiotics. GBS hemolysin/cytolysin and granadaene pigment are two major linked virulence factors that contribute to GBS pathogenicity. Here we show that photobleaching of the antioxidant granadaene renders the pathogen more susceptible to killing by mouse macrophages and to hydrogen peroxide killing. Photo-treatment also leads to loss of activity of the linked hemolysin/cytolysin although photobleaching disproportionally affected the activity of the two factors. Treatment with light also affected GBS membrane permeability and contribute to increased susceptibility to the cell membrane active antibiotic daptomycin and to penicillin. Overall our study demonstrates a dual effect of photobleaching on the virulence and antimicrobial susceptibility of GBS and suggests a novel approach for the treatment of GBS infection. Our findings further provide new insight on the relationship between GBS hemolysin and the granadaene pigment. Importance: For elderly individuals or those with chronic underlying conditions (such as diabetes), skin infections caused by Streptococcus agalactiae represent a significant risk for the development of invasive disease. S. agalactiae strains are becoming increasingly resistant to antibiotics. By utilizing blue light to neutralize the granadaene pigment present in S. agalactiae , this paper presents a non-invasive and non-antibiotic reliant process capable of reducing GBS virulence while increasing the antimicrobial susceptibility of the bacterium. The differential effect of blue light on the linked GBS hemolysin/cytolysin and granadene pigment further provides new insight on the relationship between the two virulence factors. Overall photo-treatment represents a novel strategy for the treatment of S. agalactiae infections.