Basis of Virulence in a Panton-Valentine Leukocidin-Negative Community-Associated Methicillin-Resistant Staphylococcus aureus Strain

Infections with methicillin-resistant Staphylococcus aureus (MRSA) are the most frequent cause of death by an infectious agent in the United States and a leading source of morbidity and mortality in hospitals worldwide [1, 2]. While traditionally limited to hospitals and predisposed patients, community-associated (CA) MRSA infections emerged in the late 1990s, presenting largely as infections of the skin and soft tissues [3]. These occurred in otherwise healthy patients with no contact to the hospital setting. The strains causing CA-MRSA infections are different from those involved in hospital infections. In the United States, virtually all CA-MRSA cases are caused by a clone called pulsed-field type USA300. USA300 has become the leading cause of skin infections reporting to emergency departments in the United States [4]. Globally, endemic CA-MRSA clones have emerged at different geographical locations throughout the world [3], and many countries currently report a rise of CA-MRSA infections [5–7]. The reason for the extraordinary success of CA-MRSA strains as pathogens has been described as a situation where “resistance and virulence converge” [8]. All CA-MRSA strains harbor an SCCmec element, carrying methicillin resistance determinants, which is significantly smaller than those found in hospital-associated (HA)–MRSA clones. This is believed to cause less of a fitness cost, but experimental results have been inconsistent [9, 10]. Similarly, what underlies the increased virulence potential of CA-MRSA strains has remained controversial [11, 12]. Two hypotheses were developed. Both are based on the key role that cytolysis, in particular lysis of neutrophils, plays as immune evasion strategy of S. aureus during the establishment of infection [13], and the characteristically high cytolytic activity detected in CA-MRSA strains [14]. One hypothesis focuses on the acquisition of a mobile genetic element carrying the genes coding for Panton-Valentine leukocidin [15], a toxin that lyses leukocytes in a receptor-dependent fashion [16]. This hypothesis was mainly based on the finding that most initially found CA-MRSA clones harbor Panton-Valentine leukocidin (PVL)–encoding genes, while HA-MRSA commonly do not [15]. Another hypothesis explains the increased virulence of CA- as opposed to HA-MRSA strains by increased expression of core genome-encoded toxin genes, such as mainly phenol-soluble modulins (PSMs) and α-toxin [17–19]. While the molecular basis of CA-MRSA skin infections remains controversial in particular [20, 21], the considerable research efforts that have been made in this field now present a picture in which both expression of core genome-encoded toxins and acquisition of mobile genetic elements such as the prophage carrying the PVL-encoding genes played a role in the evolution of virulence of CA-MRSA [22]. However, virtually all investigations were performed using strain USA300, and a smaller number in the first isolated CA-MRSA strain USA400, which in the meantime was almost completely replaced by USA300 [3]. Both strains are prevalent mostly in the Americas. The factors contributing to virulence in other global CA-MRSA strains have received much less attention. Notably, this includes a series of CA-MRSA strains that do not have PVL-encoding genes and have emerged as causes of infections similarly severe as those caused by PVL-positive CA-MRSA [11]. For example, the PVL-negative sequence type (ST) 72 predominates among CA-MRSA strains in Korea and can cause severe skin infections [23]. Thus, to gain a more comprehensive understanding of virulence development in CA-MRSA clones, we investigated the basis of virulence in the CA-MRSA strain ST72 from Korea.