Importance of the Global Regulators Agr and SaeRS in the Pathogenesis of CA-MRSA USA300 Infection
Daum RS (2010) Importance of the Global Regulators Agr and SaeRS in the Pathogenesis of CA-MRSA USA300
Infection. PLoS ONE 5(12): e15177. doi:10.1371/journal.pone.0015177
Importance of the Global Regulators Agr and SaeRS in the Pathogenesis of CA-MRSA USA300 Infection
Christopher P. Montgomery 0
Susan Boyle-Vavra 0
Robert S. Daum 0
Frank R. DeLeo, National Institute of Allergy and Infectious Diseases, National Institutes of Health, United States of America
0 1 Section of Critical Care, Department of Pediatrics, University of Chicago , Chicago , Illinois, United States of America, 2 Section of Infectious Diseases, Department of Pediatrics, University of Chicago , Chicago, Illinois , United States of America
CA-MRSA infection, driven by the emergence of the USA300 genetic background, has become epidemic in the United States. USA300 isolates are hypervirulent, compared with other CA- and HA-MRSA strains, in experimental models of necrotizing pneumonia and skin infection. Interestingly, USA300 isolates also have increased expression of core genomic global regulatory and virulence factor genes, including agr and saeRS. To test the hypothesis that agr and saeRS promote the observed hypervirulent phenotype of USA300, isogenic deletion mutants of each were constructed in USA300. The effects of gene deletion on expression and protein abundance of selected downstream virulence genes were assessed by semiquantitative real-time reverse-transcriptase PCR (qRT-PCR) and western blot, respectively. The effects of gene deletion were also assessed in mouse models of necrotizing pneumonia and skin infection. Deletion of saeRS, and, to a lesser extent, agr, resulted in attenuated expression of the genes encoding a-hemolysin (hla) and the Panton-Valentine leukocidin (lukSFPV). Despite the differences in hla transcription, the toxin was undetectable in culture supernatants of either of the deletion mutants. Deletion of agr, but not saeRS, markedly increased the expression of the gene encoding protein A (spa), which correlated with increased protein abundance. Each deletion mutant demonstrated significant attenuation of virulence, compared with wild-type USA300, in mouse models of necrotizing pneumonia and skin infection. We conclude that agr and saeRS each independently contribute to the remarkable virulence of USA300, likely by means of their effects on expression of secreted toxins.
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Funding: This work was supported by the Pediatric Critical Care Scientist Development Program and the National Institutes of Health (NIH) Grant AI076596-01A1
to C.P.M., and the Grant Healthcare Foundation and NIH Grant AI040481-08A1 to R.S.D. and S.B-V. The funders had no role in study design, data collection and
analysis, decision to publish, or preparation of the manuscript.
Competing Interests: The authors have declared that no competing interests exist.
Infections caused by methicillin-resistant Staphylococcus aureus
(MRSA) are common and frequently severe [1,2]. In the last
decade, an increasing percentage of these infections have occurred
among previously healthy individuals without traditional risk
factors, including children [3]. Among these
community-associated MRSA (CA-MRSA) infections, uncomplicated skin and soft
tissue infections predominate; however, serious disease, including
complicated skin and soft tissue infections and necrotizing
pneumonia requiring hospitalization, also occur frequently [1].
The first reports of severe CA-MRSA infections in the United
States implicated the genetic background USA400, as defined by
pulsed-field gel electrophoresis, as the predominant cause [4,5].
However, USA400 has essentially disappeared from the U.S. and
has been replaced by USA300 [1,6]. The reasons for the
dominance of USA300 are not known, but some have interpreted
the success as evidence for a fitness advantage conferred by the
USA300 background. In support of this, comparative studies in
several animal models of S. aureus disease have demonstrated that
USA300 isolates are hypervirulent, when compared with USA400
[7] or selected health-care associated MRSA strains [8]. Although
the relationship between fitness and virulence is a subject of
ongoing discussion, understanding the molecular mechanisms of
the extraordinary virulence of USA300 may provide insight into
the pathophysiology of this remarkable genetic background.
In vitro and in vivo studies have uncovered unique molecular
features of USA300 that may contribute to virulence. Genome
sequencing has identified multiple mobile elements containing
putative virulence genes, including enterotoxins (designated sek2
and seq2), the arginine catabolic mobile element (ACME), and the
Panton-Valentine leukocidin (PVL) [9,10]. It is tempting to
speculate that the CA-MRSA epidemic, and by extension the
success of USA300, has been driven by acquisition of one or more
of these novel virulence determinants. However, although
ACME may enhance fitness [11], it does not enhance virulence
in ro (...truncated)