Know Your Microbial Enemy

Commentary “Know your enemy.” This far-seeing counsel by Sun Tzu in The Art of War1 remains relevant even 2,000 years after the words were written. Although periprosthetic joint infections (PJIs) are arguably the most devastating complications of arthroplasty, a PJI cannot occur without a causative organism, the enemy. The war against PJI pathogens cannot be won without first knowing them; we need to know what they are and how they behave. Tarabichi et al. report the culture results and the time to positivity of intraoperative samples obtained in 536 PJIs. Although the most common organisms were methicillin-sensitive Staphylococcus aureus and Staphylococcus epidermidis, many other pathogens were identified, including gram-negative rods and Candida species. As such, it is difficult to predict in each individual case “who” will be the enemy. Nevertheless, the study did provide important information on when the pathogens are likely to be identified in culture, with methicillin-resistant S. aureus, gram-negative rods, and methicillin-sensitive S. aureus having the shortest times to positivity (median, <2 days) and Cutibacterium acnes having the longest time to positivity (median, 7 days). These data represent a small but meaningful step forward to understanding the enemy and how it behaves. The findings of the study are helpful in several ways. First, the data highlight the wide range of pathogens that cause PJIs. Others have also reported that causative organisms in PJI are not limited to the Staphylococcus species2, and, therefore, wider-spectrum prophylactic antibiotics may be indicated in patients at risk for more biologically complex infections. The challenge remains to identify these patients prior to the surgical procedure. Second, if the clinical presentation indicates that a highly virulent pathogen is likely, cultures can be assessed early, at 24 to 28 hours, and antibiotics can be tailored to the specific organisms isolated. Third, the findings highlight the importance of maintaining cultures to 14 days, as some pathogens can be identified as late as this date. Fourth, extrapolation from the sepsis and urosepsis literature, as comprehensively referenced by Tarabichi et al., suggests that patients with rapidly identified PJI pathogens may be at increased risk for poor clinical outcomes and therefore should be treated aggressively. Finally, although only 2 clinical sites participated in the study, the data are international; given that and the large sample size, the results of this study can be considered somewhat generalizable. There is still much more that we need to know to defeat the enemy in joint reconstruction. Many PJIs are the result of bacterial colonization of biofilm that is formed on the implant. These sessile organisms are less likely to be detected in intraoperative samples as they are not planktonic and therefore sonication of the implant would be required to release them into fluid or tissue culture specimens. Sonication was not performed in the current study and, therefore, cultures may have missed biofilm-associated bacteria. In addition, Tarabichi et al. did not correlate culture results with clinical outcome. These outcomes were confounded by nonstandardized treatment protocols, and, therefore, knowing the behavior of the bacteria in vitro may not give clinicians enough information, at least at this point, to direct the treatment approach and clarify the prognosis. Overuse of antibiotics in limb reconstruction has been shown to increase the risk of serious antibiotic complications and may lead to resistant organisms3. As such, subsequent research could aim to clarify the microbiologic criteria for PJI prognostication and, therefore, provide guidance for de-escalation of antimicrobial treatment when appropriate. Overall, Tarabichi et al. provide useful information as a starting point to understand the in vitro behavior of PJI pathogens, the enemy. The more we understand these pathogens, the closer we will be to defeating them. In the meantime, clinicians now have a greater understanding of culture dynamics and the timeline for assessing and acting upon culture results. This knowledge can curb overtreatment and eventually lead to more patient-specific antibiotic prophylaxis and PJI management. Furthermore, future research that provides direct clinical extrapolation will take us closer to knowing the enemy and understanding the art of the war against it.