How Relevant Were the Models Used to Measure the Impact of Panton‐Valentine Leukocidin in Human Staphylococcal Infections?

Panton-valentine leucocidin carrying Staphylococcus aureus causing necrotizing pneumonia inactivates the JAK/STAT signaling pathway and increases the expression of inflammatory cytokines

PURPOSE

Methicillin-resistant Staphylococcus aureus (MRSA) carrying Panton-Valentine leukocidin, a pore-forming toxin, is a common cause of necrotizing pneumonia. However, the early pulmonary inflammatory response following PVL(+) MRSA infection is unknown. The purpose of this study was to use a murine model to determine the effect of PVL(+) MRSA on lung tissues and the expression of cytokines and JAK and STAT mRNA and protein.

METHODS

Mice were randomly divided into 3 groups and intra-nasally treated with PBS (control group), recombinant PVL (rPVL group), and PVL(+) MRSA (PVL group). At 24 and 48 h after inoculation, bronchoalveolar lavage fluid (BALF) was tested for cytokine levels, and lung tissues were tested for JAK and STAT mRNA and protein expression, and examined after hematoxylin and eosin staining.

RESULTS

Mice infected with the PVL(+) strain became ill, characterized by impaired mobility, hunched posture, ruffled fur, and labored breathing. Lung tissue exhibited tissue necrosis and hemorrhage. BALF levels of IL-8, TNF-α, IFN-γ, IL-12, sICAM-1, and sVCAM-1 were increased in the rPVL or PVL groups, while levels of IL-10 and IL-4 levels were similar among the groups. JAK1 and STAT1 mRNA expression and protein levels were increased in lung tissue from mice infected with PVL(+) MRSA and rPVL.

CONCLUSIONS

PVL is a significant S. aureus virulence factor, and upregulates the expression of proinflammatory cytokines but does not affect the expression of anti-inflammatory cytokines. The effect of PVL may be due to JAK/STAT pathway activation. Blockade of the JAK/STAT pathway may decrease the severity of PVL(+) MRSA pneumonia.

Methicillin-Resistant Staphylococcus aureus: Molecular Characterization, Evolution, and Epidemiology

Staphylococcus aureus, a major human pathogen, has a collection of virulence factors and the ability to acquire resistance to most antibiotics. This ability is further augmented by constant emergence of new clones, making S. aureus a "superbug." Clinical use of methicillin has led to the appearance of methicillin-resistant S. aureus (MRSA). The past few decades have witnessed the existence of new MRSA clones. Unlike traditional MRSA residing in hospitals, the new clones can invade community settings and infect people without predisposing risk factors. This evolution continues with the buildup of the MRSA reservoir in companion and food animals. This review focuses on imparting a better understanding of MRSA evolution and its molecular characterization and epidemiology. We first describe the origin of MRSA, with emphasis on the diverse nature of staphylococcal cassette chromosome mec (SCCmec). mecA and its new homologues (mecB, mecC, and mecD), SCCmec types (13 SCCmec types have been discovered to date), and their classification criteria are discussed. The review then describes various typing methods applied to study the molecular epidemiology and evolutionary nature of MRSA. Starting with the historical methods and continuing to the advanced whole-genome approaches, typing of collections of MRSA has shed light on the origin, spread, and evolutionary pathways of MRSA clones.

Systemic dissemination and cutaneous damage in a mouse model of staphylococcal skin infections

Serious staphylococcal infections frequently begin in the skin. The present study used a mouse model of such infections to evaluate the ability of Staphylococcus aureus to disseminate from the skin and to determine if cutaneous damage from the infections was required for dissemination. The mice were inoculated with S. aureus onto flank skin prepared by a tape-stripping method that caused minimal disruption of the epidermal keratinocyte layers. After these inoculations the staphylococci were found to disseminate to the spleen and kidneys of almost all animals within 6h. Induction of leucopenia did not affect this process. Cutaneous damage was prominent in these experimental infections and included loss of the epidermis, neutrophil infiltration into the epidermis, and complete necrosis of the dermis. The latter also occurred in cyclophosphamide-treated animals, indicating that the organisms themselves and not the host inflammatory responses were responsible. Dermal necrosis did not develop until 48h after inoculation, a time by which dissemination had already occurred. Therefore, in this mouse model system S. aureus is capable of penetrating the epidermal keratinocyte layers and disseminating rapidly after inoculation; the experimental infections do produce significant dermal damage, but the latter develops after dissemination has already taken place.