Although Staphylococcus aureus osteomyelitis is considered to be incurable, the major bacterial reservoir in live cortical bone has remained unknown. In addition to biofilm bacteria on necrotic tissue and implants, studies have implicated intracellular infection of osteoblasts and osteocytes as a mechanism of chronic osteomyelitis. Thus, we performed the first systematic transmission electron microscopy (TEM) studies to formally define major reservoirs of S. aureus in chronically infected mouse (Balb/c J) long bone tissue. Although rare, evidence of colonized osteoblasts was found. In contrast, we readily observed S. aureus within canaliculi of live cortical bone, which existed as chains of individual cocci and submicron rod‐shaped bacteria leading to biofilm formation in osteocyte lacunae. As these observations do not conform to the expectations of S. aureus as non‐motile cocci 1.0 to 1.5 μm in diameter, we also performed immunoelectron microscopy (IEM) following in vivo BrdU labeling to assess the role of bacterial proliferation in canalicular invasion. The results suggest that the deformed bacteria: (1) enter canaliculi via asymmetric binary fission; and (2) migrate toward osteocyte lacunae via proliferation at the leading edge. Additional in vitro studies confirmed S. aureus migration through a 0.5‐μm porous membrane. Collectively, these findings define a novel mechanism of bone infection, and provide possible new insight as to why S. aureus implant‐related infections of bone tissue are so challenging to treat. © 2016 American Society for Bone and Mineral Research.

Multi-cell migration is important for tissue development and repair. An experimentally accessible example of multi-cell migration is provided by the classic scratch-wound assay. In this assay, a confluent monolayer is `injured' by forcibly removing a strip of cells, and the remaining monolayer `heals' through some combination of cell migration, spreading and proliferation. The scratch wound has been used for decades as a model of wound healing and an assay of cell migration, however the mechanisms that underlie the coherent expansion of cells in the surviving monolayer are still debated. Here we develop an agent-based computational model that predicts the most robust characteristics of healing in scratch wounds. The cells in our model are simple mechanical agents that respond to cell contact by redirecting migration and slowing division. We imbued model cells with crawling and growth dynamics and measured for individual L1 fibroblasts and found that simulated recovery occurs in a steady, sheet-like and division-independent fashion to mimic healing by L1s. The lack of cohesion and biochemical cell-cell communication in the model suggests that these factors are not strictly necessary for cells to migrate as a group. Instead, our analysis suggests that steady sheet migration can be explained by cell spreading in the monolayer.