Shoulder Journal Club July 2015: What is the significance of P. Acnes bacteria in Shoulder Surgery

An very interesting, and possibly concerning article was published in the recent issue of JBJS about the high prevalence of Propionibacterium acnes bacteria in cultures taken during open (non-arthroscopic) shoulder surgery (see article).  

The study looked at 117 consecutive shoulder surgeries where multiple cultures were taken during each procedure to see how many cases were contaminated with bacteria.  

The risk of infection is one of the greatest concerns in all of orthopedics.  Orthopedics relies on hardware (metal or plastic implants) for many procedures.  This hardware is very susceptible to infection because bacteria can cling onto it, form biofilm (think of this as super-glue), and elude our bodies' immune system.  This leads to serious infections, repeat surgery, significant morbidity and even mortality.

Some bacteria are known to be bad actors: MRSA (methacillin-resistant staph aureus) is one of the most feared by orthopedic surgeons because of its resistance to most antibiotics and other efforts to remove the bacteria from the site of surgery.    If MRSA is the worst actor, and MSSA (methacillin-sensitive staph aureus) is a bad, but less bad actor, where does P.acnes fall along the spectrum? P.acnes is a low virulence (non-aggressive) bacteria that populates the sebaceous glands around the shoulder (basically it lives in your armpit's sweat glands and makes you smell bad after a workout).  

The three questions that I take away from the study is 1) Can shoulder cultures be trusted to reflect bacterial loads inside the surgical site; and 2) is P.acnes worth testing for (if a bacteria contaminates a surgical site, but its not harmful, then who cares); and 3) are surgeons doing enough to ensure that the surgical site is sterilized before making an incision? 

A 20% contamination rate is astoundingly high, but the true infection rate after open shoulder surgery is much much lower (around 2%...10x less).  So are these cultures accurate? Or if they are accurate, does it even matter if P.acnes is around?  Do we need to start cleaning the shoulder differently before surgery, or giving different antibiotics before surgery to prevent this bacteria from contaminating the surgical site?

In the follow up commentary by Dr. Hasan (see commentary) he suggests that the high culture (+) result indicates a false results.  There is not actually this much bacteria in the surgical wound, the cultures become (+) somewhere else, and therefore, other tests, like measuring different markers of an infection, are needed to properly diagnose a shoulder infection. 

Roughly 20% shoulder surgeries grew out a bacteria on at least one culture, with over 80% being the P.acnes bacteria.  However, there were some concerns about how representative these cultures were of a true infection.  The control sample (not exposed to the surgical site) was positive 13% of the time, indicating that contamination with bacteria often times occurred somewhere outside of the incision.  This finding highlights the idea that cultures may not actually represent what is going on inside of the surgical site itself, and thus should not be used to determine whether patients should return for a repeat surgery based on culture results alone.  But still, even if 13% are false positives, a 7% bacterial infection rate is high, very high.  What does the mean about the way surgeons prepare the shoulder for it being sterilized properly??

Post a comment, we'd like to hear what you think!