Data have been presented indicating thatStaphylococcus aureus produces several cell surface proteins which bind specifically to different host extracellular matrix proteins and plasma proteins (12,13,32). For many of the cell surface proteins a role in colonization and virulence has been demonstrated in animal models of infection (17,23,27,33). Two highly homologous fibronectin-binding proteins (FnBPA and FnBPB), encoded by fnbA and fnbB, have been characterized (14, 21, 25, 41) and shown to be involved in adherence to damaged heart valves (23) and to promote internalization of S. aureus by epithelial cells (9). Although S. aureus is primarily considered to be an extracellular pathogen, the intracellular niche could promote long-term colonization and maintenance of chronic infections.Protein A (Spa), which binds immunoglobulin G (IgG) by the Fc segment, is a major surface protein present in virtually all strains of S. aureus (10, 11). Strains of S. aureus with a high content of Spa are more resistant to phagocytosis by human neutrophils in vitro than strains with less Spa (34). Reduced virulence of a spa mutant compared to that of the corresponding wild type was demonstrated in a mouse intraperitoneal infection (31).We have recently shown that transcription of the fnbA and fnbB genes is negatively regulated by agr and by an agr-independent mechanism that restricts fnb mRNA synthesis to the early exponential phase of growth (38). A similar temporal control of fnb transcription was also found in another strain of S. aureus (Newman) (43). However, only fnbA appeared to be regulated by agr in this strain. It was also found that fnbA, but not fnbB, was positively regulated by sarA. As for fnbA and fnbB, transcription of spa is negatively regulated by agr (20). However, unlike for fnbA, transcription of spa is negatively controlled by sarA (3,42).Data from recent studies indicate that both FnBPs and protein A may be degraded by extracellular proteases (3, 26, 42). Four major extracellular proteases are produced by S. aureus (1): staphylococcal serine protease (V8 protease) (SspA), a metalloprotease named aureolysin (Aur), a cysteine protease (Scp) named staphopain (18), and a second cysteine protease (SspB) encoded within the same operon as SspA (2, 36).

The production of cell surface proteins in Staphylococcus aureus is generally down-regulated in the postexponential growth phase by the global regulator agr. The effector of this regulation is the RNAIII molecule, which is encoded within the agr locus. RNAIII seems to regulate most target genes at the level of transcription, but it also has an effect on the translation of some genes. To study the role of agr on the expression of fibronectin binding proteins (FnBPs), we investigated the transcription and translation of fnb genes in agr mutant strain WA250 and its parent strain, 8325-4. The results show that fnb genes are negatively regulated by agr and also by an agr-independent mechanism that restricts fnb mRNA synthesis to the early exponential phase of growth. Transcription and Western blot analysis of cell-associated FnBPs demonstrated that synthesis of both FnBPA and FnBPB in the wild-type and agr mutant strains took place preferentially during the first hour of growth and rapidly decreased after the second hour. We also confirmed previous results showing that the agr mutant strain has an increased capacity to bind fibronectin compared to its parent agr ؉ strain. However, while the concentrations of fnb mRNAs and proteins differed by a factor of 16 between the strains, the difference in fibronectin binding was only twofold, indicating that the binding of fibronectin to the bacteria is not proportional to the amount of FnBPs on their surface.

BackgroundViperid snakebite envenoming is characterized by prominent local tissue damage, including muscle necrosis. A frequent outcome of such local pathology is deficient skeletal muscle regeneration, which causes muscle dysfunction, muscle loss and fibrosis, thus provoking permanent sequelae that greatly affect the quality of life of patients. The causes of such poor regenerative outcome of skeletal muscle after viperid snakebites are not fully understood.Methodology/Principal FindingsA murine model of muscle necrosis and regeneration was adapted to study the effects of the venom and isolated toxins of Bothrops asper, the medically most important snake in Central America. Gastrocnemius muscle was injected with either B. asper venom, a myotoxic phospholipase A2 (Mtx), a hemorrhagic metalloproteinase (SVMP), or saline solution. At various time intervals, during one month, tissue samples were collected and analyzed by histology, and by immunocytochemical and immunohistochemical techniques aimed at detecting muscle fibers, collagen, endothelial cells, myoblasts, myotubes, macrophages, TUNEL-positive nuclei, and axons. A successful regenerative response was observed in muscle injected with Mtx, which induces myonecrosis but does not affect the microvasculature. In contrast, poor regeneration, with fibrosis and atrophic fibers, occurred when muscle was injected with venom or SVMP, both of which provoke necrosis, microvascular damage leading to hemorrhage, and poor axonal regeneration.Conclusions/SignificanceThe deficient skeletal muscle regeneration after injection of B. asper venom is likely to depend on the widespread damage to the microvasculature, which affects the removal of necrotic debris by phagocytes, and the provision of nutrients and oxygen required for regeneration. In addition, deficient axonal regeneration is likely to contribute to the poor regenerative outcome in this model.

Skeletal muscle regeneration after myonecrosis involves the activation, proliferation and fusion of myogenic cells, and a coordinated inflammatory response encompassing phagocytosis of necrotic cell debris, and the concerted synthesis of cytokines and growth factors. Myonecrosis often occurs in snakebite envenomings. In the case of venoms that cause myotoxicity without affecting the vasculature, such as those of many elapid snakes, regeneration proceeds successfully. In contrast, in envenomings by most viperid snakes, which affect the vasculature and extracellular matrix in addition to muscle fibers, regeneration is largely impaired and, therefore, the muscle mass is reduced and replaced by fibro-adipose tissue. This review discusses possible causes for such poor regenerative outcome including: (a) damage to muscle microvasculature, which causes tissue hypoxia and affects the inflammatory response and the timely removal of necrotic tissue; (b) damage to intramuscular nerves, which results in atrophy of regenerating fibers; (c) degradation of muscle cell basement membrane, compromising the spatial niche for proliferating myoblasts; (d) widespread degradation of the extracellular matrix; and (e) persistence of venom components in the damaged tissue, which may affect myogenic cells at critical points in the regenerative process. Understanding the causes of poor muscle regeneration may pave the way for the development of novel therapeutic interventions aimed at fostering the regenerative process in envenomed patients.

No abstract

Soft-tissue infection is commonly found in patients bitten by Latin American Bothrops snakes. Staphylococcus aureus, which is not present in the mouth of the snake, is frequently isolated from these infections. The effects of B. asper venom on infection with S. aureus were analyzed in a model of infection in envenomated mouse gastrocnemius muscle. Inoculation of 50 colony-forming units (cfu) of S. aureus was enough to cause infection in envenomated muscle, compared with >5x104 cfu without venom. This effect was also achieved by injection of venom myotoxin III (an A(2) phospholipase). A sarA mutant strain in which production of extracellular toxins and enzymes is up-regulated and binding of fibronectin, fibrinogen, and other host proteins is down-regulated was much less virulent than the corresponding parental strain, indicating that the ability of S. aureus to mask itself with host molecules might be more important than the effects of secreted toxins and enzymes in this kind of infection.

Background: Viperid snakebite envenoming is characterized by prominent local tissue damage, including muscle necrosis. A frequent outcome of such local pathology is deficient skeletal muscle regeneration, which causes muscle dysfunction, muscle loss and fibrosis, thus provoking permanent sequelae that greatly affect the quality of life of patients. The causes of such poor regenerative outcome of skeletal muscle after viperid snakebites are not fully understood.Methodology/Principal Findings: A murine model of muscle necrosis and regeneration was adapted to study the effects of the venom and isolated toxins of Bothrops asper, the medically most important snake in Central America. Gastrocnemius muscle was injected with either B. asper venom, a myotoxic phospholipase A 2 (Mtx), a hemorrhagic metalloproteinase (SVMP), or saline solution. At various time intervals, during one month, tissue samples were collected and analyzed by histology, and by immunocytochemical and immunohistochemical techniques aimed at detecting muscle fibers, collagen, endothelial cells, myoblasts, myotubes, macrophages, TUNEL-positive nuclei, and axons. A successful regenerative response was observed in muscle injected with Mtx, which induces myonecrosis but does not affect the microvasculature. In contrast, poor regeneration, with fibrosis and atrophic fibers, occurred when muscle was injected with venom or SVMP, both of which provoke necrosis, microvascular damage leading to hemorrhage, and poor axonal regeneration.Conclusions/Significance: The deficient skeletal muscle regeneration after injection of B. asper venom is likely to depend on the widespread damage to the microvasculature, which affects the removal of necrotic debris by phagocytes, and the provision of nutrients and oxygen required for regeneration. In addition, deficient axonal regeneration is likely to contribute to the poor regenerative outcome in this model.

Our observations suggest that traces of venom in muscle tissue might inhibit myotube formation and preclude a successful regenerative response.