Sub-Inhibitory Concentrations of Mupirocin Strongly Inhibit Alpha-Toxin Production in High-Level Mupirocin-Resistant MRSA by Down-Regulating agr, saeRS, and sarA.

Ye Jin,Jingjing Duan,Li Liu,Meilan Li,Jingye Pan,Fangyou Yu,Yang Wu,Chun Chen,Xiaofei Shen,Zhihao Hao,Yongpeng Shang,Zhihui Lv

doi:10.3389/fmicb.2018.00993

Abstract

Mupirocin, a topical antibiotic, has been utilized for decades to treat Staphylococcus aureus skin infections, as well as to decolonize patients at risk of methicillin-resistant S. aureus (MRSA) infection. The aims of this study were to investigate the expression of α-toxin (encoded by the hla gene) in ten clinical MRSA strains (MIC = 1024 μg/ml) in response to a sub-inhibitory concentration of mupirocin (1/32 minimum inhibitory concentration [MIC]) (32 μg/ml) by using α-toxin activity determination and enzyme-linked immune sorbent assay (ELISA). Subsequently, real-time polymerase chain reaction (RT-PCR) was used to examine the expression of saeR, agrA, RNAIII, and sarA genes under sub-inhibitory concentration of mupirocin in order to investigate the mechanism of action of this treatment regarding its strong inhibition of α-toxin expression. For all the strains tested, mupirocin dramatically reduced mRNA levels of α-toxin. The results indicated that α-toxin activity in mupirocin-treated groups was significantly lower than that in untreated groups. The results show that the levels of agrA, RNAIII, saeR, and sarA expression significantly decrease by 11.82- to 2.23-fold (P < 0.01). Moreover, we speculate that mupirocin-induced inhibition of α-toxin expression may be related to the inhibition of regulatory loci, such as agr, sarA and saeRS. More specifically, we found that the mechanism involves inhibiting the expression of agrA and RNAIII. In conclusion, sub-inhibitory concentrations of mupirocin strongly inhibit alpha-toxin production in high-level mupirocin-resistant MRSA by down-regulating agr, saeRS and sarA, which could potentially be developed as a supplemental treatment to control high-level mupirocin-resistant MRSA infection and reduce the risk of infection and colonization.

Highlights

Methicillin-resistant Staphylococcus aureus (MRSA) is one of the major causes of hospital- and community-acquired infections, including those associated with burn injury, sepsis, pneumonia, keratitis, and nasosinusitis (David and Daum, 2010)
As it is difficult to achieve the concentrations of mupirocin >256 μg/ml (1/4MIC7) in serum, we explored the effects of mupirocin on α-toxin expression using concentrations of 16–128 μg/ml (1/64 to 1/8 MIC)
We find that sub-inhibitory concentrations of mupirocin reduce hla expression by interfering with at least three key regulatory loci: agr, saeRS, and staphylococcal accessory gene regulator (sarA)

Summary

Introduction

Methicillin-resistant Staphylococcus aureus (MRSA) is one of the major causes of hospital- and community-acquired infections, including those associated with burn injury, sepsis, pneumonia, keratitis, and nasosinusitis (David and Daum, 2010). Α-toxin is a 33.2-kDa extracellularly secreted protein encoded by the hla gene (Bhakdi and TranumJensen, 1991). It is a pore-forming toxin that causes damage and death of cells (Bhakdi and Tranum-Jensen, 1991). This protein has been documented as a virulence factor in many severe infections, including keratitis, mastitis, nasosinusitis, peritonitis, and skin and soft tissue infections (SSITs) (Bayer et al, 1997). The sae locus includes a two-component signal-transduction system encoded by saeS and saeR that positively regulates the expression of hla at the transcriptional level. Sae activation is affected by agr, as well as by some stress environment

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