Fig. 1 Pulse lavage irrigation is unable to remove biofilm from arthroplasty materials. The biofilm mass of S. aureus transfected with the luciferase gene can be measured using bioluminesence imaging. A strong biofilm signal remained on polymethyl methacrylate (PMMA) after 3 L of direct pulse lavage irrigation.
Courtesy of Kenneth L. Urish, MD, PhD

AAOS Now

Published 2/1/2016
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Kenneth L. Urish, MD, PhD; Nicholas John Giori, MD, PhD; the AAOS Biomedical Engineering Committee

Biofilm a Unique Factor in Orthopaedic Infection

Understanding the resilience of biofilm bacteria is critical
Infection is one of the broadest, most challenging problems in orthopaedic surgery. In total knee arthroplasty (TKA), the infected implant is a leading cause for revision surgery. Avoiding infection in the management of open fractures is one of the core principles of orthopaedic residency training. Chronic osteomyelitis, especially in children, can develop a number of sequelae and was associated with high mortality before the advent of antibiotics. From a historical perspective, infection was a leading cause of death in traumatic extremity battlefield injuries. One reason the treatment of orthopaedic infections can be so demanding lies in the biofilm created by many species of bacteria.

Biofilms are groups of bacteria embedded in a complex extracellular polymeric substance composed of polysaccharides, nucleic acids, and proteins. The extracellular polymeric substance of the biofilm enhances bacterial adhesion, provides a local community in which the bacteria can cooperate and communicate, and serves as a barrier to the host's immune response. Contrary to a popularly reported misconception found in the orthopaedic literature, there is little inhibition of antibiotic diffusion through biofilm. In the course of infection, a biofilm typically becomes established within the first few hours, but takes approximately 2 days to mature.

Biofilms are more or less likely to develop on different materials, most likely based on hydrophobicity. Bacteria in biofilm have a lower metabolism, thereby decreasing the efficacy of antibiotics. Bacteria in a biofilm are not simply the same version of bacteria growing in suspension adhered to a surface. The differences between the same bacteria in suspension (planktonic) and in a biofilm (sessile) are enormous; bacteria in a biofilm are much more difficult to eradicate.

Important steps
Irrigation and débridement (I&D) is often the first surgical step in clearing an infection. Important surgical principles of I&D include complete excision of necrotic tissue and the removal or exchange of implants, when possible. There has been considerable debate in the trauma literature about wound irrigation in open fractures. The multicenter Fluid Lavage of Open Wounds (FLOW) study found no differences in reoperation rates between high- or low-pressure irrigation. Mechanical débridement removes a majority of biofilm from implants, but a sufficient volume of biofilm remains, which can prevent complete eradication of the infection (Fig. 1). Biofilm behaves similarly to a highly viscous fluid. This allows the biofilm to flow over a surface and remain adherent even when high shear force irrigation is applied.

A second possible step in an I&D procedure is the use of antiseptics. Available antiseptics include alcohols, biguanides (like chlorhexidine), halogen-releasing agents (chlorine- and iodine-based solutions), peroxygens (hydrogen peroxide), and phenols. Although their mechanisms of action are incompletely understood, all of these agents interfere in some way with cell membranes and bacterial protein activity. Various antiseptics have been studied in vitro for their ability to eradicate biofilm-forming Staphylococcus aureus (S. aureus) from titanium discs. However, although these agents are able to incrementally reduce the number of bacteria on an infected disc, re-incubation of the discs 6 weeks after treatment has revealed that bacteria remain viable. In addition, all of these agents can cause local host tissue damage, and concentrations need to be monitored.

Fig. 1 Pulse lavage irrigation is unable to remove biofilm from arthroplasty materials. The biofilm mass of S. aureus transfected with the luciferase gene can be measured using bioluminesence imaging. A strong biofilm signal remained on polymethyl methacrylate (PMMA) after 3 L of direct pulse lavage irrigation.
Courtesy of Kenneth L. Urish, MD, PhD
Fig. 2 Biofilm remains on arthroplasty material after treatment with antibiotic. The arrow notes S. aureus at the base of the biofilm. Scale bar represents 1 µm.
Courtesy of Kenneth L. Urish, MD, PhD

The third critical step of an I&D program is treatment with antibiotics. Unfortunately, biofilm has a high tolerance to antibiotics (Fig. 2). Surprisingly, even after 24 hours in supra-therapeutic levels 100-fold higher than typical clinical values, antibiotics were unable to eliminate the biofilm beyond one order of magnitude. Antibiotic selection can be a challenge, as often biofilm remain culture-negative. In periprosthetic joint infection, cultures may be negative in approximately 20 percent of cases. This was the motivation behind the use of sonication—the use of sound energy to agitate particles in a sample—in improving culture results and antibiotic selection.

The role of persisters
These results provide additional evidence for the important role of bacterial persisters in orthopaedic infection. Bacterial persisters comprise a subpopulation of bacteria that become highly tolerant to antibiotics without undergoing genetic change. Persisters are thought to be resilient to antibiotics partly because these bacteria are not undergoing cellular activities that antibiotics can corrupt, resulting in tolerance to the antibiotics (ie, no growth and slow death). In contrast, traditional resistance mechanisms arise from genetic changes that block antibiotic efficacy. When this occurs, resistant bacteria cells can grow even in the presence of antibiotics, whereas persister cells do not grow as they become dormant. Understanding how persister cells form is important if we are to develop strategies for controlling orthopaedic infections.

A strong appreciation for the resilience of biofilm is critical when treating orthopaedic infections. A clinically relevant biofilm can remain despite aggressive mechanical débridement, local treatment with antiseptics, and long-term intravenous antibiotics. Some current research in this area focuses on identifying dispersal agents that can break up the biofilm and on ways to manipulate the quorum sensing behavior that controls the transition of bacteria from the planktonic state to the sessile state. Until such novel treatments become available, however, we must rely on the patient's immune defense to eradicate the infection or at a minimum establish a detente with the biofilm.

Kenneth L. Urish, MD, PhD, and Nicholas John Giori, MD, PhD, are memmbers of the AAOS Biomedical Engineering Committee.

Bottom Line

  • Bacteria in a biofilm are different from bacteria growing in suspension adhered to a surface.
  • Biofilm can flow over a surface and remain adherent even when high shear force irrigation is applied.
  • Contrary to common belief, there is little inhibition of antibiotic diffusion through biofilm.
  • Clinically relevant biofilms often resist aggressive mechanical débridement, local treatment with antiseptics, and long-term intravenous antibiotics.

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