Bacteria Responsible for Incurable Bone Infection Hide in Bone Micro-Channels

Bacteria that cause life-threatening and incurable bone infections may elude immune or antibiotic attack by hiding in tiny channels within bone, according to researchers at the University of Rochester Medical Center (URMC). Researchers in the Center for Musculoskeletal Research (CMSR) conducted the first systematic study to define where and how Staphylococcus aureus hides in bones, yielding the first demonstration that the bacteria can change shape and “move” to colonize tiny channels in mouse bone.

S. aureus is a common bacteria that can cause painful skin infections or life-threatening blood or bone infections. Because it can form bacterial communities deep within bone where it can survive for long periods of time, S. aureus bone infections are extremely difficult and costly to treat. Patients with S. aureus bone infections are often treated with antibiotics and undergo surgery to remove infected tissue, but infection recurs in 40 percent of patients and amputation of infected limbs is sometimes necessary.

“The challenge with bone infections is that they tend to be incurable,” said Edward Schwarz, Ph.D., Richard and Margaret Burton Distinguished Professor in Orthopaedics and director of the CMSR. “Surgeons take extra margins around infected bone, reconstruct, and the infection comes back. They don't understand why this infection keeps coming back and neither did we.”

To investigate, lead author, Karen de Mesy Bentley, M.S., director of the Electron Microscopy Shared Laboratory and faculty associate in the Department of Pathology and Laboratory Medicine at URMC, performed systematic transmission electron microscopy (TEM) of S. aureus infected mouse femurs and tibias. Mice were infected with S. aureus directly or via contaminated implants 14 days prior to imaging.

Bentley discovered the bacteria within the tiny channels, called canaliculi, during her many hours of TEM examination of the tissue. It appeared that S. aureus must squeeze from round to rod-shaped, which has never before been documented, to fit into canaliculi that are many times smaller in diameter than the bacteria. Bentley’s images also suggest that S. aureus can “move”, though not in the traditional sense.  Since it lacks the typical appendages of motile bacteria, S. aureus must move by dividing asymmetrically and pushing daughter cells in a desired direction, such as into canaliculi.

Schwarz and Bentley originally hypothesized that the bacteria were only getting into the canaliculi when enormous pressure from the dividing bacteria forced them through. However, several images showed lone bacteria attaching to canaliculi openings in the absence of any crowding pressure. They also appeared to be dividing abnormally - aligning the plane of division near the opening rather than splitting down the middle, so their daughter cell will be extruded into the channel.

Three parallel canaliculi with varying stages of S.aureus infection.

Image by Karen de Mesy Bentley, M.S.