Necrotizing Soft Tissue Infections Staphylococcus aureus - but not Streptococcus pyogenes-isolates display high rate of internalization and cytotoxicity toward human myoblasts

Abstract

Necrotizing Soft Tissue Infections (NSTIs), often reaching the deep fascia and muscle, are mainly caused by group A Streptococcus (GAS) and to a lesser extent by Staphylococcus aureus (SA). Conversely SA is a leading etiologic agent of pyomyositis suggesting that SA could have a specific tropism for the muscle. To assess the pathogenicity of these two bacterial species for muscles cells in comparison to keratinocytes, adhesion and invasion of NSTI-GAS and NSTI-SA were assessed on these cells. Bloodstream infections (BSI) SA isolates and non-invasive coagulase negative Staphylococci (CNS) isolates were used as controls.SA isolates from NSTI and from BSI exhibited stronger internalization into human keratinocytes and myoblasts than CNS or NSTI-GAS. While the median level of SA internalization culminated at 2% in human keratinocytes, it reached over 30% in human myoblasts due to a higher percentage of infected myoblasts (>11%) as compared to keratinocytes (<3%) assessed by transmission electron microscopy. Higher cytotoxicity for myoblasts of NSTI-SA as compared to BSI-SA, was attributed to higher levels of psmα and RNAIII transcripts in NSTI group as compared to hematogenous group. However, the two groups were not discriminated at the genomic level. The cellular basis of high internalization rate in myoblasts was attributed to higher expression of α5β1 integrin in myoblasts as compared to keratinocytes. Major contribution of FnbpAB-integrin α5β1 pathway to internalization was confirmed by isogenic mutants.Our findings suggest the contribution of NSTI-SA severity by its unique propensity to invade and kill myoblasts, a property not shared by NSTI-GAS.ImportanceNecrotizing Soft Tissue Infection (NSTI) is a severe infection caused mainly by group A Streptococcus (GAS) and occasionally by S. aureus (SA); the latter being more often associated with pyomyositis. NSTIs frequently involve the deep fascia and may provoke muscle necrosis. The goal of this study was to determine the tropism and pathogenicity of these two bacterial species for muscle cells. The results revealed a high tropism of SA for myoblasts and myotubes followed by cytotoxicity as opposed to GAS that did not invade these cells. This study uncover a novel mechanism of SA contribution to NSTI with a direct muscle involvement, while in GAS NSTI this is likely indirect, for instance, secondary to vascular occlusion.

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“…This interaction not only tightly anchors S. aureus to its eukaryotic host cells but also promotes the internalization of the microbe by human epithelial and endothelial cell and mouse fibroblasts (Dziewanowska et al, 1999;Sinha et al, 1999;Fowler et al, 2000;Jett Bradley and Gilmore Michael, 2002) (Figure 4). In addition, an in vitro study found that one study found that necrotizing soft tissue infections with S. aureus isolates showed high rates of internalization and cytotoxicity to human myocytes, and the cellular basis of the high internalization rate in myocytes was attributed to the higher expression of a5b1 integrins in myocytes (Baude et al, 2019). The ability of S. aureus to be internalized by and survive in host cells, such as keratinocytes, may contribute to developing persistent or chronic infections, eventually leading to deeper tissue infection or dissemination.…”