Naftifine enhances photodynamic therapy against Staphylococcus aureus by inhibiting staphyloxanthin expression

Highlights

• STX significantly improved the survival rate of S. aureus treated with PACT.

• STX is a limiting factor of PACT using either Type I or Type II photosensitizer.

• Naftifine inhibits STX expression and increases membrane permeability of S. aureus.

• PACT combined with STX biosynthesis inhibitor is an effective way to treat S. aureus infection.

Abstract

Staphyloxanthin (STX) is a virulence factor produced by Staphylococcus aureus (S. aureus) including methicillin-resistant S. aureus (MRSA), a widely distributed Gram-positive bacterium related to several clinical human infections. We herein reported that Naftifine, a broad-spectrum antifungal agent, plays an important role on photodynamic antimicrobial therapy (PACT) for S. aureus. STX attenuated the PACT effect on S. aureus in a dose-dependent manner, which probably due to its biological function of scavenging reactive oxygen species (ROS). On the contrary, the sensitivity of S. aureus to PACT was significantly improved after an incubation with Naftifine (10 μM) compared with that of S. aureus untreated with Naftifine. The reason for this phenomenon is that Naftifine inhibits STX biosynthesis and improves membrane permeability. Taken together, we identified STX as a limiting factor of PACT in S. aureus. Meanwhile, we found that the combination of PACT and Naftifine, an inhibitor of STX biosynthesis, has synergistic antibacterial effect on S. aureus, and provide a new way for the rapid and efficient treatment of S. aureus infection without drug resistance.

Introduction

Staphylococcus aureus (S. aureus), a Gram-positive pathogen, has been an important human pathogen throughout history and is currently a leading cause of bacterial infections worldwide [1]. It exists in the skin, oral cavity, mucosa membrane and other parts of the human body [2], with unique local infection ability and leading to fatal diseases such as osteomyelitis and sepsis [3,4]. Moreover, the emergence of highly virulent drug-resistant strains such as methicillin-resistant S. aureus (MRSA), known as super-bacteria, is a major therapeutic concern [5,6]. These situations pose an appalling challenge to developing new ways to treat S. aureus infections.

Photodynamic antimicrobial chemotherapy (PACT) is a non-traditional antimicrobial method which has attracted much attention in recent years [7]. In PACT, the nontoxic photosensitizer gathered on the surface of bacteria can be converted into high-energy triplet state under the light of specific wavelength, and then form cytotoxic radicals, such as superoxide anion (O₂⁻) and hydroxyl radical (HO·) through hydrogen extraction or electron transfer reaction (type I reaction), or singlet oxygen (¹O₂) through transferring energy directly to molecular oxygen (type II reaction) [[8], [9], [10]]. These ROS are potent oxidants that are toxic to nearby microorganisms [11]. Compared with traditional antibacterial therapy, PACT is a non-invasive therapy with broad-spectrum antibacterial activity [12]. It can attack multiple sites of bacteria at the same time and act quickly, so as to effectively inactivate multi-resistance microorganisms and avoid new resistance [13]. Meanwhile, PACT can inhibit the activity of a series of protein virulence factors of S. aureus, such as α-hemolysin, V8 protease and sphingosine myelinase [14], which play an important role in reducing the infectivity of S. aureus to the host [[14], [15], [16]].

Staphyloxanthin (STX), identified as β-d-glucopyranosyl 1-O-(4,4-diaponeurosporen-4-oate)-6-O-(12-methyltetradecanoate) [17], is one of the most important virulence factors of S. aureus [18]. STX is a membrane carotenoid, which can enhance the survival of bacteria in harsh environment, especially in the process of infection [19,20]. It was found that STX not only increased the resistance of bacteria to dehydration and linoleic acid, but also protected S. aureus from being killed by the host immune system [[20], [21], [22]]. Considering that STX has antioxidant activity to the ROS produced by host neutrophils [23,24], we evaluated the effect of STX on the PACT of S. aureus and explored the possible mechanism of the synergistic effect of Naftifine, an STX biosynthetic inhibitor, and PACT in this study. Our results showed the protective effect of STX on S. aureus, and the synergistic antibacterial effect of PACT and STX biosynthetic inhibitor. This work suggests a new prospect for the treatment of S. aureus infection.