Why is MRSA so virulent?

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        Why is MRSA so virulent?
        By Javad Parvizi, MD, FRCS; Nitin Goyal, MD

        Staphylococcus aureus is a ubiquitous pathogen that is the leading cause of bacterial infections worldwide, according to data published by Diekema and colleagues in 2001. Methicillin-resistant Staphylococcus aureus (MRSA) emerged in the early 1960s and was originally regarded as a nosocomial pathogen. This notion has evolved dramatically over the last 2 decades and community-associated (CA-MRSA) has now become a chief public health concern in the United States.

        According to recent estimates, published by Klevins and colleagues in 2007, deaths due to MRSA infections in the United States exceed those due to HIV/AIDS. In the United States the epidemic of CA-MRSA is almost completely due to the exceptionally infectious USA300 strain. Infections due to the USA300 strain may be limited to the skin and soft tissue, but may also easily result in severe deep, disseminated infection that spreads readily among humans. In contrast with health care-associated MRSA (HA-MRSA), CA-MRSA strains can cause infections in healthy individuals with no significant predisposing risk factors. It is vital that clinicians recognize the virulence of CA-MRSA is unique not only in the resistance to α-lactam antibiotics, but with the facility with which it may proliferate, evade host immune cells, and cause tissue necrosis through an assembly of virulence factors.


        Discussed in detail in the last Infection Watch column (Biofilm infections: Newer understanding of old problem, September, page 28), CA-MRSA produces a biofilm which is more problematic in the setting of implants. The CA-MRSA adheres to the implant and secretes a glycocalyx layer which allows it to protect itself from access by antibiotics, phagocytosis, and opsonization. The minimum inhibitory concentration (MIC) in biofilm-forming bacteria is significantly increased, demonstrating a considerable resistance to antibiotics.

        Antibiotic resistance

        Methicillin-resistance in S aureus is conferred by the mecA gene. The mecA gene produces penicillin binding protein-2a (PBP-2a). Involved in the cell wall production, PBP-2a has a very low affinity for ß-lactam antibiotics. This impairs the antibiotic ability to disrupt the bacterial cell wall. The mecA gene is carried by a mobile genetic element designated staphylococcal cassette chromosome (SCCmec). The SCCmec typing is strong evidence of the independent evolution of HA-MRSA and CA-MRSA clones; SCCmec types I, II, and III are predominantly observed in HA-MRSA strains, while SCCmec types IV and V are primarily found in the CA-MRSA strains. However, it cannot be overemphasized that it is much more than simply the resistance to methicillin that defines the virulence of CA-MRSA.


        S. aureus is an exceptionally successful pathogen that has been capable of adapting and evolving dynamically its resistance and virulence. The ability of any bacteria to cause disease is largely related to the bacteria’s aptitude for evading the innate immunity of the host. This usually includes resistance to human polymorphonuclear leukocytes (PMNs) and other white blood cells of the innate human immune system. CA-MRSA strains can cause particularly severe infections in otherwise healthy people. CA-MRSA produces a wide gamut of molecules that aid in evading the host immune response and contribute to its pathogenesis. The comparison of CA-MRSA with other S. aureus strains has allowed for the identification of these genes and toxins that are active in CA-MRSA but not in other strains. These may represent excellent candidates for the factors that underlie the success of CA-MRSA as a virulent, endemic bacterium.

        Panton-Valentine Leukocidin (PVL)

        Panton-Valentine Leukocidin (PVL) was one of the very first of these toxins that was identified to be present in CA-MRSA and absent in other, less virulent forms, including HA-MRSA. Initially, PVL was held to be largely responsible for the success of CA-MRSA. This was an attractive hypothesis as PVL was identified to be a biocomponent toxin produced by the lukPV operon that causes pore formation in leukocytes. Therefore, it had a direct impact on white blood cells, causing lysis, which could easily explain the increased virulence in CA-MRSA.

        Since its discovery, the importance of PVL in the virulence of CA-MRSA has been thoroughly investigated. Recent results from multiple animal studies using deletions of PVL-encoding genes in a variety of CA-MRSA strains demonstrated that PVL has minimal role in the pathogenesis and virulence of CA-MRSA. Additionally, the absence of PVL did not have any bearing on the ability of the bacteria to lyse human white blood cells. Hence, the role of PVL is likely less significant than initially theorized.

        α-Hemolysin (Hla)

        Hla, also known as α-toxin, has long been known to be an important virulence factor in S. aureus. It is a well-characterized hemolytic toxin that lyses many types of host cells, including most leukocytes — not PMNs — and has significant pro-inflammatory effects. In recent experiments in murine models, Hla-negative CA-MRSA strains were essentially avirulent, and immunization against Hla protected against sepsis. Although this toxin was uncovered some time ago, this indicates a remarkable role in the CA-MRSA pathogenicity and provides support that Hla may be one of the primary determinants of virulence. Hla has a high rate of expression in USA300 as compared with other S. aureus strains, likely contributing to its high virulence. Additionally, Hla is encoded within the core genome of CA-MRSA, in contrast with PVL genes which are encoded by a mobile genetic element.

        α-Type phenol soluble modulins

        Phenol-soluble modulins (PSMα)are short (~20 amino acids), amphipathic, and á-helical peptides that recruit and lyse neutrophils. PSMá have a primary role in the mechanism to circumvent eradication by neutrophils and other phagocytes in the early immune response. Discovered in an effort to identify molecules that are the underlying cause of the enhanced virulence, the PSMá peptides were found to have a marked cytolytic effect on PMNs in vivo and in vitro, according to Wang and colleagues in 2007. Importantly, CA-MRSA strains, especially US300, generate PSMá peptides at significantly higher levels than the major HA-MRSA strains. Also, in murine models of infection and sepsis, deletion of PSMá in a variety of CA-MRSA strains radically reduced virulence. Similar to Hla and in contrast with PVL genes, PSMá is encoded in the core genome of CA-MRSA. The increased expression of PSMα peptides in the strain USA300 may be in part related to higher activity of the accessory gene regulator in this strain.

        Type I ACME

        Genomic sequencing of the epidemic strain of CA-MRSA, USA300, has uncovered the acquisition of the type I arginine catabolic mobile element (ACME) from S. epidermidis. This genetic element contains genes that aid in the bacterium survival on human skin, which may contribute to the success of USA300. The presence of type I ACME is exclusive to the USA300 strain of CA-MRSA and appears to be physically linked to SCCmec by its utilization of the same ccr recombinases for transfer and mobilization. Therefore, the dominance of the USA300 strain of CA-MRSA may be related to the increased colonization facility and transmissibility conferred by type 1 ACME as well as the antibiotic resistance provided by SCCmec. Further animal model research is necessary to delineate the specific role of the type I ACME in greater detail.


        The rapid emergence and dissemination of CA-MRSA strains worldwide and the epidemic within the United States highlight the difficulty encountered when treating a drug-resistant bacterium combined with several virulence factors. Future drug targeting must utilize novel approaches to control virulence and attenuate disease severity. Active areas of research include studies aimed at the passive immunization against distinct toxins (Hla and PSMα) as well as attention to new intravenous agents to directly target MRSA.


        Diekema DJ, Pfaller MA, Schmitz FJ, et al: Survey of infections due to staphylococcus species: Frequency of occurrence and antimicrobial susceptibility of isolates collected in the united states, canada, latin america, europe, and the western pacific region for the SENTRY antimicrobial surveillance program, 1997-1999. Clin Infect Dis 2001;32:S114-132.
        Klevens RM. Morrison MA. Nadle J. et al. . Active Bacterial Core surveillance (ABCs) MRSA Investigators: Invasive methicillin-resistant staphylococcus aureus infections in the united states. JAMA 2007;298:1763-1771.
        Bubeck Wardenburg J, Palazzolo-Ballance AM, Otto M, Schneewind O, DeLeo FR: Panton-valentine leukocidin is not a virulence determinant in murine models of community-associated methicillin-resistant staphylococcus aureus disease. J Infect Dis 2008;198:1166-1170.
        Bubeck Wardenburg J, Bae T, Otto M, et al. Poring over pores: Alpha-hemolysin and panton-valentine leukocidin in staphylococcus aureus pneumonia. Nat Med 2007;13:1405-1406.
        Diep BA, Palazzolo-Ballance AM, Tattevin P, et al. Contribution of panton-valentine leukocidin in community-associated methicillin-resistant staphylococcus aureus pathogenesis. PLoS ONE 2008;3:e3198.
        Voyich JM, Otto M, Mathema B, et al. Is panton-valentine leukocidin the major virulence determinant in community-associated methicillin-resistant staphylococcus aureus disease? J Infect Dis 2006;194:1761-1770.
        Wang R, Braughton KR, Kretschmer D, et al. Identification of novel cytolytic peptides as key virulence determinants for community-associated MRSA. Nat Med 2007;13:1510-1514.
        Diep BA, Stone GG, Basuino L, et al. The arginine catabolic mobile element and staphylococcal chromosomal cassette mec linkage: Convergence of virulence and resistance in the USA300 clone of methicillin-resistant staphylococcus aureus. J Infect Dis 2008;197:1523-1530.
        Diep BA, Gill SR, Chang RF, et al. Complete genome sequence of USA300, an epidemic clone of community-acquired meticillin-resistant staphylococcus aureus. Lancet 2006;367:731-739.

        Note: Also see the topic and video interview with Dr. Michael Otto, Pathogen Molecular Genetics Section NIAID (Video & Excerpt), which deals with MRSA and related topics:


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