Osteomyelitis is a progressive inflammatory condition of the skeletal system, caused by microbial infection and subsequent destructive invasion of the bone matrix. It includes osteomyelitis caused by local spread from adjacent lesions (such as trauma or joint arthroplasty), and secondary to vascular insufficiency or neuropathy (such as diabetic foot), and hematogenous osteomyelitis (Urish and Cassat, 2020). Even after treatment with antibiotics and surgical debridement, about 20 % of acute osteomyelitis cases eventually develop into chronic osteomyelitis (Gimza and Cassat, 2021, Kalinka et al., 2014a, Krauss et al., 2019, Wang et al., 2023). Chronic osteomyelitis is associated with high rates of bone deformation and destruction, leading to disability, and even death (Rao et al., 2011), imposing a significant burden on patients and society.

Staphylococcus aureus (S. aureus) is the most common pathogen responsible for osteomyelitis, accounting for approximately 75 % of cases (Wang et al., 2023). The difficulty in treating S. aureus osteomyelitis lies in its multiple survival strategies during co-existence and co-evolution with the host, including antibiotic resistance (Chen et al., 2025, Zelmer et al., 2024), biofilm formation, abscess formation, intracellular infection, and osteocyte-lacunar-canalicular network (OLCN) colonization (Muthukrishnan et al., 2019). Notably, methicillin-resistant S. aureus (MRSA) has not only demonstrated resistance to β-lactam antibiotics and multiple other antimicrobial agents, but has also evolved additional sophisticated antibiotic resistance mechanisms (Gardete and Tomasz, 2014, Peacock and Paterson, 2015, Tasneem et al., 2022, Zhan and Zhu, 2018).

The bone cortex provides a unique environment where immune cells are difficult to access, allowing MRSA to survive within cortical bone lacunae and intracellularly infect osteocytes, this may represent one of the underlying pathogenic mechanisms contributing to the refractory nature of osteomyelitis (Masters et al., 2019, Yang et al., 2018). Researches have shown that within cortical bone lacunae or osteocytes, S. aureus can transform into low-metabolism state, called small colony variants (SCVs), which cause a lower release of host inflammatory factors compared to the wild type, thereby facilitating the intraosseous survival of S. aureus (Cai et al., 2022, Loss et al., 2019, Zhou et al., 2022). Recent studies suggested that S. aureus SCVs, frequently isolated from clinical chronic osteomyelitis cases, play a key role in cortical bone colonization and antibiotic resistance (Freiberg et al., 2024, Guo et al., 2022, Tuchscherr et al., 2016). However, the mechanisms underlying SCV-associated bone colonization and antibiotic resistance in cortical bone MRSA infections remain unclear.

Pyroptosis is a type of programmed cell death that is mediated by inflammasomes (Du et al., 2021), such as the NOD-like receptor protein 3 (NLRP3). During pyroptosis, Caspase-1 is activated, which cleaves the gasdermin D (GSDMD) protein, resulting in the formation of pores in the cell membrane. Simultaneously, Caspase-1 catalyzes the maturation of inflammatory cytokines IL-1β and IL-18, which are then released through the pores to intensify the inflammatory response (Man et al., 2017). This mechanism also facilitates the release of intracellular bacteria and triggers immune responses to help control the infection (Miao et al., 2010). However, the activation of the pyroptosis pathway during bacterial osteomyelitis can also trigger osteoclast activation and contribute to bone destruction (Zhu et al., 2019). Different pathogens have evolved various mechanisms to suppress the pyroptosis response, promoting their survival and spread (Deng et al., 2022). Similarly, S. aureus can reduce the sensitivity of inflammasomes to avoid the activation of immune cell pyroptosis pathways, thereby enhancing its survival within host cells (Pushkaran et al., 2015, Shimada et al., 2010). It remains unclear whether MRSA, after invading cortical bone during osteomyelitis, regulates the pyroptosis process of osteocytes to influence immune inflammation. Investigating these mechanisms could deepen our understanding of osteomyelitis and provide a theoretical foundation for developing new treatment strategies.

In this study, we revealed that MRSA undergoes adaptive changes in cortical bone, characterized by active downregulation of the virulence factor, Panton-Valentine leukocidin (PVL), and concurrent suppression of osteocyte pyroptosis. This inhibition of osteocyte pyroptosis exerts a dual effect: (1) compromised host immune clearance of bacteria, and (2) stimulation of pathological bone formation. The resulting aberrant osteogenesis, coupled with prolonged osteocyte survival, facilitates MRSA OLCN and osteocytes colonization, as well as the formation of antibiotic resistance by creating physical barriers to antibiotic penetration. Importantly, we demonstrated that pharmacological induction of osteocyte pyroptosis reverses this phenotype, enhancing antibiotic penetration through cortical bone, thereby improving antimicrobial efficacy in osteomyelitis (Graphic abstract).