Abstract
Community acquired-methicillin resistant Staphylococcus aureus (CA-MRSA) is a socially problematic pathogen that infects healthy individuals, causing severe disease. CA-MRSA is more virulent than hospital associated-MRSA (HA-MRSA). The underlying mechanism for the high virulence of CA-MRSA is not known. The transcription product of the psm-mec gene, located in the mobile genetic element SCCmec of HA-MRSA, but not CA-MRSA, suppresses the expression of phenol-soluble modulin α (PSMα), a cytolytic toxin of S. aureus. Here we report that psm-mec RNA inhibits translation of the agrA gene encoding a positive transcription factor for the PSMα gene via specific binding to agrA mRNA. Furthermore, 25% of 325 clinical MRSA isolates had a mutation in the psm-mec promoter that attenuated transcription, and 9% of the strains had no psm-mec. In most of these psm-mec-mutated or psm-mec-deleted HA-MRSAs, PSMα expression was increased compared with strains carrying intact psm-mec, and some mutated strains produced high amounts of PSMα comparable with that of CA-MRSA. Deletion of psm-mec from HA-MRSA strains carrying intact psm-mec increased the expression of AgrA protein and PSMα, and virulence in mice. Thus, psm-mec RNA suppresses MRSA virulence via inhibition of agrA translation and the absence of psm-mec function in CA-MRSA causes its high virulence property.
Highlights
Community acquired-methicillin resistant Staphylococcus aureus (CA-Methicillin-resistant Staphylococcus aureus (MRSA)), especially the USA300 clone, causes severe infectious diseases in many people in the United States and in European countries
We previously found that psma promoter activity, which is enhanced by AgrA, was decreased in the Newman strain transformed with psm-mec, resulting in a decreased amount of phenol-soluble modulin a (PSMa), but no alteration of the amount of agrA mRNA [12]
We examined whether psm-mec RNA decreases the amount of AgrA in the Community acquired-methicillin resistant Staphylococcus aureus (CA-MRSA) strains MW2 (USA400) and FRP3757 (USA300)
Summary
CA-MRSA, especially the USA300 clone, causes severe infectious diseases in many people in the United States and in European countries. One reason for the high virulence of the CA-MRSA USA300 strains is suggested to be the high amounts of secreted toxins, including PSMa, a-hemolysin, d-hemolysin (Hld), and the Panton-Valentine leukocidin (PVL) [1,2,3]. The USA300 strains show increased expression of the agr locus, which upregulates the production of PSMa, a-hemolysin, and PVL, compared with HA-MRSA strains [1,3,4,5]. The agr locus encodes RNAIII, which is an mRNA of Hld as well as a regulatory RNA that upregulates the expression of various toxins, including a-hemolysin, and downregulates the expression of various cell surface proteins [9]. AgrA activates the transcription of RNAIII and other virulence genes, including the psma operon, by direct binding to the promoter [8,10]. The mechanism that increases the expression of agr in the USA300 strains, is not known
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