Figure 2. S. aureus forms traditional abscesses in bone marrow, but also grow directly on and invade into, living and dead bone fragments.

Murine femurs were extracted, fixed in neutral buffered formalin, and dehydrated in 70% ethanol. Following decalcification in 20% EDTA pH 7.4, femurs were processed and embedded in paraffin. Femurs infected with S. aureus (A) or mock-infected with PBS (B) were sectioned and stained with a modified hematoxylin and eosin (H&E) stain prior to imaging at 1× magnification. Different abscess morphologies, including a traditional abscess (white box “C”) in the bone marrow (C), and sequestra (white box “D”) along cortical bone fragments (D) were observed in the S. aureus infected femurs upon imaging at 10× magnification. Arrowheads in C denote the boundaries of the abscess’ neutrophilic infiltrate. * denotes the staphylococcal abscess community surrounded by an eosinophilic pseudocapsule in the center of the abscess. # in D denotes a nonviable piece of cortical bone (sequestrum) with tightly adherent clusters of staphylococci (arrows) both on the surface of and within the sequestrum. (E–F) A second murine osteomyelitis sample was stained with both modified H&E (E) and tartrate-resistant acid phosphatase (mature osteoclast marker) (F) to demonstrate that S. aureus can also adhere to segments of living cortical bone (denoted by #), as osteoclasts (arrows) are visualized remodeling the same fragment of cortical bone. * denotes a large cluster of staphylococci directly adherent to the bone segment.