The diagnosis of prosthetic joint infections with low-virulence organisms, such as P. acnes, is challenging because of the absence of classical clinical evidence of infection, as well as the challenges associated with culturing these organisms. We have analyzed the culture results for a large series of patients having upper extremity revision surgery for painful or loose prosthetic arthroplasty. Using a 28-day culture incubation period for both aerobic and anaerobic culture media, we retrospectively compared the characteristics of culture-positive specimens between P. acnes PJI and nondiagnostic events. In this regard, we have shown that (i) incubation beyond 13 days is associated with increasing recovery of nondiagnostic isolates, (ii) both aerobic and anaerobic culture media should be incubated for a minimum of 13 days to maximize recovery of P. acnes from PJI specimens, (iii) negative histologic findings were frequently observed in P. acnes-infected events, and (iv) PJI specimens were 6.3 times more likely to have ≥2 culture media positive for growth with P. acnes than specimens from nondiagnostic events.
Prevalence of infection subsequent to shoulder arthroplasty has previously been estimated to range from 0.4 to 15.4% (4
). At 21%, the infection rate observed in this study is higher than these published estimates, although it is lower than that in a recently published study (22
). While it is possible that this higher infection rate could reflect increased recovery of contaminants that met the criteria for infection due to the extended culture incubation period employed, i.e., false positives, we do not believe this to be the case for the following reasons: first, published infection rates are based on a collection of studies where various culture incubation periods, frequently of 1 week or less in duration, were employed. Thus, the prevalence of infection subsequent to shoulder arthroplasty in many previous reports may have been underestimated. Second, a high prevalence of infection is not unexpected given the status of our institution as a reference center for revision arthroplasty. Had a 1-week culture incubation period been employed in this study, the infection rate would have been 15.6%, a figure in line with previously published rates. These data therefore demonstrate the importance of utilizing optimized culture incubation conditions to maximize recovery of P. acnes
from PJI specimens.
The optimum culture incubation period for the recovery of P. acnes from PJI specimens has not been determined previously. Consistent with previous findings, we retrospectively determined that a 13-day culture incubation is necessary for the recovery of P. acnes from PJI specimens, although incubation beyond this period was associated with increasing recovery of P. acnes isolates that were nondiagnostic. Furthermore, we have demonstrated that a diagnosis of PJI would have been missed in 29.4% of infected patient events had extended culture incubation only been applied to anaerobic culture media. Although it is possible that additional culture incubation may be required, determination of the significance of recovery of P. acnes from specimens after 13 days of incubation will ultimately require large outcome studies with many patients. In addition, even if such data were available, incubation beyond 13 days would be impractical for most laboratories due to increased labor costs and the costs associated with increased recovery of nondiagnostic isolates.
One limitation of this study, as well as of others, is the absence of an accepted universal definition for the diagnosis of PJI. This is particularly problematic in cases of PJI due to more indolent organisms, such as P. acnes
, where overt clinical and laboratory signs of infection are often absent. A variety of criteria for the diagnosis of PJI can therefore be found in the literature, ranging from a single culture-positive periprosthetic specimen in the absence of histologic findings (11
) to the requirement for a minimum of three culture-positive specimens (1
). One of the criteria used in this study was the requirement for morphologically indistinguishable organisms to be recovered from two or more specimens. This criterion was previously shown to correlate well with the presence of inflammation on histologic analysis, although this association was observed for a collective of different bacterial species and not for P. acnes
). Using optimized culture incubation conditions, we observed that only 40% of patient events with ≥2 specimens positive for growth with P. acnes
had evidence of acute inflammation, consistent with previously published findings (31
). Previous studies suggest that the presence of other findings on histology may also be indicative of P. acnes
infection, e.g., chronic inflammation and macrophages (29
). Interestingly, all patients diagnosed preoperatively with P. acnes
PJI in a previous study (i.e., patients with classic signs of infection) had evidence of acute inflammation, while neutrophils were observed in only 50% of those patients diagnosed postsurgically with P. acnes
). Our data provide additional support for the assertion that acute inflammation is often absent in patients with delayed PJI due to P. acnes.
Like many previously published studies, this study considers a single culture-positive specimen in the absence of histologic findings to be nondiagnostic and most likely represents contamination. In spite of this, a number of instances have been described in the literature where this assumption has been inaccurate (8
). For example, Kelly and colleagues noted a patient with only one of four periprosthetic specimens positive for growth of P. acnes
who subsequently developed recurrent P. acnes
). While recovery of P. acnes
from a single periprosthetic specimen most likely represents contamination from either the patient's skin, the skin of the medical staff, or laboratory processing, it could also represent low-level colonization of the prosthetic joint. Such colonization may in turn lead to the development of subsequent infection. While this scenario may not represent clinically significant infection at the time of specimen harvesting, outcomes data are required to determine the clinical significance of such findings, as well as the most appropriate and clinically efficacious action to be taken by clinicians. These data also suggest that the generation of a universal definition for the diagnosis of PJI caused by more indolent organisms may need to also incorporate criteria for the diagnosis of joint colonization. Such definitions, as well as the criteria contained therein, will ultimately require a large study set with detailed outcome data, a task that is beyond the scope of this analysis.
Because the definition of infection used in this study was solely microbiological, it was not possible to determine the sensitivity of culture. However, the sensitivity of periprosthetic culture for the diagnosis of PJI in previous studies has often been determined using the presence of overt signs of infection as a comparator, e.g., presence of a sinus tract communicating with the prosthesis and the presence of acute inflammation. As mentioned previously, such a comparison may not be entirely appropriate for PJI due to more indolent agents, such as P. acnes
, which can present either acutely or indolently. Recent data have demonstrated that the sensitivity of culture for the diagnosis of PJI can be further improved by subjecting the removed explant to sonication, followed by culture of the sonicate fluid (27
). With respect to the diagnosis of shoulder PJI, explant sonication was shown to be 12% more sensitive than traditional periprosthetic culture (66.7% versus 54.5%) (21
). Similar findings have also been observed for the diagnosis of PJI of additional joints such as prosthetic knees and elbows (27
). The improved sensitivity observed is believed to result from the propensity of agents of PJI, including P. acnes
, to form biofilms on the surface of the prosthesis (7
). However, in the case of shoulder PJI, the incubation period employed differed between periprosthetic tissue (5 to 7 days) and sonicate cultures (14 days; anaerobic culture media). It is therefore unknown to what extent the sensitivity of sonicate culture truly surpasses that of periprosthetic culture for the recovery of P. acnes
from patients with PJI. Future studies will be required to determine whether the sensitivity of periprosthetic culture using extended incubation of both aerobic and anaerobic culture media is equivalent to that of sonicate cultures. Nonetheless, one would predict that the sensitivity of sonicate culture will be further improved by employing extended culture incubation of both aerobic and anaerobic culture media for this culture type.
A further aim of this study was to determine whether other parameters that are easily obtained within the microbiology laboratory, but generally not reported to clinicians, would correlate with the presence of infection. To our knowledge, this is the first study to specifically examine the culture characteristics of P. acnes-positive specimens, other than culture time to positivity. Although the latter characteristic did not appear to be useful in the differentiation of infected events from nondiagnostic events, the data observed raise important questions for future research. For example, it is unknown whether recovery of P. acnes from a single specimen is more likely to represent colonization than contamination if the organism is recovered earlier in the culture incubation process. Furthermore, it is also unknown whether differences in patient presentation (i.e., acute versus indolent infection) may correlate with culture time to positivity. However, it is reasonable to hypothesize that acute infections could be more likely to become culture positive earlier than indolent ones due to the possibility of increased numbers of microorganisms in the acutely, overtly infected prosthetic joint. Finally, it is unknown whether culture time to positivity could play a role in predicting patient outcome. Future outcome studies will be required to address these questions.
In this study, we also observed that specimens from P. acnes
-infected events were more commonly associated with the presence of ≥2 media positive for growth. In particular, recovery of P. acnes
from sheep blood agar was exclusively associated with the presence of infection. Because not all strains of P. acnes
are aerotolerant, it is possible that aerotolerance may be associated with increased virulence. However, we do not believe this to be the explanation for our observation. Rather, we believe that this finding most likely represents a higher organism burden in patients with PJI than in those without, as P. acnes
was also recovered from the anaerobic culture media for each of these patient events. In addition, aerotolerance was observed for P. acnes
strains isolated from both infected and nondiagnostic events when this characteristic was examined (data not shown). Because no specific anaerobic device was used for the transport of periprosthetic specimens to the laboratory in this study, it is also possible that there could have been a selection bias in favor of the recovery of aerotolerant P. acnes
strains. However, we do not believe this to be the case, as the majority of tissue specimens received were over 1 cm3
in size, a size known not to require strict anaerobic transport conditions (26
). Furthermore, all specimens were processed within 1 h of collection. Clinical outcomes studies will ultimately be required to determine whether patient events with a single periprosthetic specimen positive for P. acnes
where 2 culture media are positive for growth are more likely to represent a clinically significant or insignificant finding.
Other laboratory methods for pathogen detection, most notably, direct 16S rRNA gene PCR, were previously demonstrated to be of limited utility in the diagnosis of PJI due to P. acnes. Thus, bacterial cultures of periprosthetic and sonicate specimens currently remain the mainstay for identification of etiological agents of infection. As a result, it is critically important that clinical microbiology laboratories utilize optimized culture conditions for the recovery of P. acnes from prosthetic joint specimens. While the implementation of 13-day culture incubation for both aerobic and anaerobic culture media is clearly associated with increased expense to the laboratory, the improvement in recovery of P. acnes is clear. Laboratories should also consider implementation of an extended culture incubation period where P. acnes is a common agent of infection e.g., cerebrospinal fluid specimens obtained through shunt devices, heart valve specimens, etc. Finally, the recovery of P. acnes from multiple culture media where noted should also be reported to clinicians. The use of optimized culture conditions by laboratories, coupled with the reporting of additional available microbiology data, will likely facilitate the future generation of a universal definition of prosthetic joint infection and colonization.