Necrotizing Soft Tissue Infection Staphylococcus aureus but not S. pyogenes Isolates Display High Rates of Internalization and Cytotoxicity Toward Human Myoblasts.

BACKGROUND: Necrotizing soft tissue infections (NSTIs) caused by group A Streptococcus (GAS) and occasionally by Staphylococcus aureus (SA) frequently involve the deep fascia and often lead to muscle necrosis. METHODS: To assess the pathogenicity of GAS and S. aureus for muscles in comparison to keratinocytes, adhesion and invasion of NSTI-GAS and NSTI-SA isolates were assessed in these cells. Bloodstream infections (BSI-SA) and noninvasive coagulase-negative staphylococci (CNS) isolates were used as controls. RESULTS: NSTI-SA and BSI-SA exhibited stronger internalization into human keratinocytes and myoblasts than NSTI-GAS or CNS. S. aureus internalization reached over 30% in human myoblasts due to a higher percentage of infected myoblasts (>11%) as compared to keratinocytes (<3%). Higher cytotoxicity for myoblasts of NSTI-SA as compared to BSI-SA was attributed to higher levels of psmα and RNAIII transcripts in NSTI-SA. However, the 2 groups were not discriminated at the genomic level. The cellular basis of high internalization rate in myoblasts was attributed to higher expression of α5ß1 integrin in myoblasts. Major contribution of FnbpAB-integrin α5ß1 pathway to internalization was confirmed by isogenic mutants. CONCLUSIONS: Our findings suggest a factor in NSTI-SA severity is the strong invasiveness of S. aureus in muscle cells, a property not shared by NSTI-GAS isolates.

Cytotoxic activity of Flavobacterium psychrophilum in skeletal muscle cells of rainbow trout (Oncorhynchus mykiss).

Flavobacterium psychrophilum is the etiologic agent of bacterial coldwater disease (BCWD) and rainbow trout fry syndrome (RTFS), which cause significant worldwide losses in aquaculture. Juvenile rainbow trout are particularly susceptible to F. psychrophilum infection, the main external clinical signs of which are extensive necrotic myositis and ulcerative lesions. Despite the economic relevance of this pathogen in aquaculture, little is known about the molecular mechanisms underlying F. psychrophilum infection and pathogenesis. In this study, cultured skeletal muscle cells from rainbow trout (Oncorhynchus mykiss) were co-incubated with the virulent strain of F. psychrophilum JIP02/86 (ATCC 49511). Trypan blue exclusion analysis at 48h post-incubation revealed decreased cellular viability. Direct bacteria-myoblast contact was found a key factor in inducing F. psychrophilum cytotoxicity. Apoptosis was characterized by nuclear DNA fragmentation, decreased plasma membrane integrity, increased caspase activity, and the proteolytic cleavage of poly(ADP-ribose)polymerase-1 (PARP-1). Moreover, bacterial infection induced an early inhibition of NF-κB signaling, as well as a differential expression of the pro- and anti-apoptotic genes, bax and bcl-2. These findings suggest that F. psychrophilum induces rainbow trout muscle apoptosis through the modulation of the NF-κB signaling as a mechanism for nutrient acquisition and survival.