Over the past three years, my colleagues and I have studied Staphylococcus aureus (S. aureus) bacteria and their ability to infect and reside inside human living osteocytes. This phenomenon may explain why periprosthetic joint infections (PJIs) are difficult to treat and clear, as well as why they can recur months or years after infection treatment or primary surgery. Our research, which was recently published, was presented during the 2018 Musculoskeletal Infection Society Annual Scientific Meeting, where it was awarded the Jeanette Wilkins Award for Best Basic Science Paper.

PJI is a complication of joint replacement surgery. Osteocytes are the most numerous and long-lived cell type in mineralized bone, forming an interconnected, multifunctional syncytium. The focus of our investigation was whether these cells have a role in PJI (Fig. 1). Although many pathogens have been shown to cause PJI, Staphylococci are predominant, and S. aureus is the species most commonly identified. We examined the ability of human osteocytes to become infected by S. aureus and the reciprocal responses of both the host cell and pathogen in this scenario. We also sought evidence of osteocyte infection in patients with PJI.

Exposure of human primary osteocyte-like cells to S. aureus at defined multiplicities of infection resulted in intracellular infection. The infected osteocytes showed no signs of increased cell death compared to uninfected cultures, suggesting that, unless cleared, osteocyte infection could persist for long periods of time. Bacteria recovered from the cultures displayed an increasing switch to the small colony variant (SCV) phenotype over a five-day period. SCVs are slow-growing, low-virulence forms associated with persistence in biofilm and intracellular infection. SCVs also tend to be antibiotic-resistant and difficult to eradicate. They can switch to a planktonic form spontaneously or in response to environmental cues to cause reinfection (Fig. 2).

Using specific immunostaining, we also found infection of viable osteocytes in bone samples from several patients with S. aureus–infected PJIs, confirming that infection of osteocytes is a previously unknown feature of human PJI. We also have been able to culture S. aureus from the bone of patients with PJI.

In our experimentally infected osteocyte-like cells, stringent bioinformatics analysis of gene microarrays revealed 1,526 genes with a fold-change of more than two and a false discovery rate set at P < 0.01, differentially expressed between infected and uninfected controls. Similar patterns of host gene expression were observed between the experimentally infected osteocyte-like cultures and the experimentally infected human bone sourced from patients undergoing primary total hip arthroplasty for osteoarthritis. The increased expression of certain genes also was observed in bone samples taken from the acetabulum of patients being treated for hip PJI by staged revision surgery, being compared with biopsies taken from the iliac wing, which we considered to be a noninfected control site, and with similar samples obtained from patients being revised for aseptic loosening.

Our work sheds new light on the etiology of PJI; human osteocytes can become infected by S. aureus. The bony location of infected osteocytes may render them refractory to clearance by immune cells, and osteocytes may, therefore, be an immune-privileged cell type. The phenotypic switch of S. aureus to the persistent SCV form, capable of reverting to an active infectious form, suggests that osteocytes may behave as a disease nidus for future infection. Given the naturally long lifespan of these cells, the survival of infected osteocytes implies that osteocyte intracellular infections may persist indefinitely unless the infected area is effectively resected or the patient is treated with effective antimicrobials.

These findings may help explain the high prevalence of infections that become apparent only after long periods of time or recur following surgical/medical management. Our findings also serve as a biologic rationale for the efficacy of aggressive bone débridement during surgical treatment of PJI; however, it remains to be seen how distal from the contaminated prosthesis bacteria can infect osteocytes.

Dr. Atkins’ study coauthors are Dongqing Yang, PhD; Asiri R. Wijenayaka, PhD; Lucian B. Solomon, MD, PhD; Stephen Pederson, PhD; David M. Findlay, PhD; and Stephen P. Kidd, PhD.

Gerald J. Atkins, PhD, is professor of biomedical orthopaedic research and a National Health and Medical Research Council of Australia senior research fellow. He is a scientific director of the University of Adelaide Centre for Orthopaedic & Trauma Research in Australia.

References:

1. Yang D, Wijenayaka AR, Solomon LB, et al: Novel insights into Staphylococcus aureus deep bone infections: the involvement of osteocytes. MBio 2018;9:e00415-18.