Discussion
Osteomyelitis leads to bone infection and destruction, mainly bacterial infection, the most common is Staphylococcus aureus, which is a major problem in the treatment of orthopaedic diseases[22, 23]. Osteomyelitis often occurs in more serious open fractures, where the bone fracture ends are exposed, which is easy to be invaded by bacteria, leading to bone tissue infection or local residual bacterial lesions[24]. Due to the destruction of skin soft tissue and blood vessels around the bone fracture end, it is very easy to lead to bacterial growth and bacterial biofilm adhesion, resulting in the repeated development of clinical Osteomyelitis, which is difficult to cure[25, 26]. At present, the clinical treatment for chronic Osteomyelitis is mainly debridement and drainage, and systemic sensitive antibiotics[23, 27, 28]. However, due to the poor local blood supply of the focus and the formation of dead bones to varying degrees, the antibiotic treatment has poor effect and large side effects[29].
As a member of MSCRAMM family, SDRC protein is mainly used to promote the biofilm formation of Staphylococcus aureus. In Osteomyelitis infected by Staphylococcus aureus, SDRC protein plays a huge role in the progression and prognosis of Osteomyelitis[30]. In our study, we constructed a model of Staphylococcus aureus induced Osteomyelitis by knocking out the SDRC protein sequence of Staphylococcus aureus and infecting rat, founding that compared with wild type Staphylococcus aureus infected Osteomyelitismouse , its IL-6, TNF- α The inflammatory indexes such as ALP, OST and RunX2 were significantly reduced, the infection of bone tissue was significantly lighter, and the osteogenic indexes such as ALP, OST and RunX2 were also significantly increased, suggesting that SDRC protein played a huge role in the occurrence and progress of Osteomyelitis infected by Staphylococcus aureus. Previous studies[19, 31]have shown that the most important biological function of SDRC protein is its ability to promote the formation of bacterial biofilms, which is particularly important in the process of bacterial infection of the host[32, 33]. Therefore, understanding the molecular mechanism mediated by SDRC protein is of great significance for the treatment of Staphylococcus aureus infectious diseases. Our research results showed that after the SDRC protein is knocked out, the biofilm forming ability of Staphylococcus aureus is significantly weakened, and it has an invasive virulence pathogenic factor α- HL (P=0.1868) and PSM (P=0.0361) were significantly reduced, which confirmed that SDRC protein played an important role in the virulence of Staphylococcus aureus, and provided a new direction for the treatment of Staphylococcus aureus infected Osteomyelitis.
As a member of the CWA protein family, the SDRC protein mainly functions to promote the formation of biofilms in Staphylococcus aureus through its own dimerization, helping bacteria to proliferate on the host surface. Derived from human neuronal cells β Axonal protein( β- Neurexin )can interact with SDRC protein, and this interaction can inhibit SDRC protein mediated biofilm formation, slowing down the infection of Staphylococcus aureus to the host. In current in vivo and in vitro experimental studies, it has been found that Staphylococcus aureus can invade osteoblasts and cause their death. Staphylococcus aureus, which invades osteoblasts, can also live and proliferate within it and is believed to be related to the occurrence of chronic osteomyelitis and recurrent osteomyelitis. In terms of bone formation, bacterial biofilms and inflammatory cytokines inhibit the osteogenic differentiation ability of bone marrow mesenchymal stem cells, while Staphylococcus aureus inhibits the proliferation of osteoblasts and induces their apoptosis.
In order to further explore the mechanism of SDRC protein promoting the occurrence and development of Staphylococcus aureus infected Osteomyelitis, we found that Myh7 was significantly increased in the SDRC protein knockout group through Transcriptome sequencing, and by constructing a stable cell line with MYH7 knockdown and functional experiments, we found that after infection with SDRC knockout staphylococcus, MYH7 knockdown significantly reduced its osteogenic indicators and Ossification, compared with shNC, suggesting that MYH7 may inhibit the biological function of Staphylococcus aureus SDRC protein. The MYH7 gene is mainly located in the long arm of chromosome 14 and contains a total of 40 exons (E1-E40)[34, 35]. It was first reported by Geisterfer Lowrance et al. in 1990. The MYH7 gene, as the first pathogenic gene found to be associated with hypertrophic cardiomyopathy, has been extensively reported in studies such as myocardial injury[36]. In China, the MYH7 gene mutation range is relatively widespread in patients with hypertrophic cardiomyopathy, except for E6, E7, E10, E17, E24, E25, E29 Pathogenic mutations are present in all 27 exons except for E32 and E33, with 7 mutation sites located in E22[36, 37]. At the same time, it has been found in Western populations that the most common malignant mutations in patients with myocardial hypertrophy are associated with R403Q, R453C, R719W, and R723G mutations in the MYH7 gene[38, 39]. In our study, we found for the first time that MYH7 was significantly reduced in the knockout of SDRC protein in Staphylococcus aureus osteomyelitis, indicating that MYH7 plays a potential biological role in the occurrence and development of Staphylococcus aureus osteomyelitis, and may have a significant correlation with the SDRC protein of Staphylococcus aureus. This provides a solid theoretical basis for us to further explore the deep mechanism of SDRC protein in Staphylococcus aureus osteomyelitis in the future.

Acknowledgements

Funding
This study was funded by National Natural Science Foundation of
China (Grant No. 82072392);
The Grants from Yunnan Orthopedics and Sports Rehabilitation
Clinical Medicine Research Center (Grant No. 202102AA310068);
and Yunnan Prvincial Clinical Orthopaedic Trauma Medical Center
(Grant No. ZX20191001);
Clinical Key Subject Construction Project of PLA;
Medical key subject of Joint Logistic Support Force of PLA.

Author contributions

BQ, MG ,and LY conceived and designed the experiments. BQ, CM, HS, TC, HL,
and LS performed the experiments. CL and YX analyzed the data. CL, LY, YX
and TC contributed reagents, materials, and analysis tools. BQ and MG wrote
the paper.

Data availability

The data of this study can be obtained from the corresponding author
upon reasonable request.

Competing interests

The authors declare no competing interests.

Ethics approval and consent to participate

The animal experiment program was approved by the Medical Ethics Committee of
the 920th Hospital of the PLA Joint Logistics Support Force (Approval No: Lun
shen 2019-010 (Section) -01), and carried out in accordance with the Guidelines
for the Care and Use of Experimental Animals.

Footnotes

These authors contributed equally: Baochuang Qi, Minzheng Guo, LiLi Yang

Contributor Information

Yongqing Xu: Email:xuyongqingkm@163.net
Chuan Li:Email:lichuankaka@163.com
Tao chen: Email:henpao@126.com