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Journal of Clinical Microbiology, February 2008, p. 488-492, Vol. 46, No. 2
0095-1137/08/$08.00+0 doi:10.1128/JCM.01762-07
Copyright © 2008, American Society for Microbiology. All Rights Reserved.

Departments of Clinical Microbiology,1 Orthopaedic Surgery, Fundación Jiménez Díaz-UTE, Universidad Autónoma de Madrid, Madrid, Spain,2 Department of Orthopaedic Surgery, Hospital la Princesa, Universidad Autónoma de Madrid, Madrid, Spain3
Received 5 September 2007/ Returned for modification 16 October 2007/ Accepted 28 November 2007
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Septic implant analysis is especially interesting due to the fact that infected materials act as bacterium reservoirs, impairing implant function and propagating infection into the bone. Moreover, both the diagnosis and treatment of prosthetic osteoarticular infections are further complicated by the development of a bacterial biofilm, where the bacteria have changed their phenotypes to an extremely resistant sessile form of life (3, 5, 7). The surge of multiresistant microorganisms that easily adhere to inert surgical materials stresses the value of adequate diagnosis leading to proper therapy for these patients. Although periprosthetic tissue culture remains the standard microbiological diagnostic method, organisms adhered to the prosthesis are occasionally impossible to detect by common bacterial cultures. Different approaches to obtain data from the potentially infected prosthetic material (11, 14, 18, 21, 25) include sampling from the surface of the implant (direct swab), surrounding fluids, and the implant after sonication. False-negative and false-positive results are found with these methods, compared with the clinical diagnosis of infection. False-negative cultures frequently occur due to empirical antibiotic treatments, low numbers of circulating bacteria in the implant-surrounding tissues, or biofilm formation. On the other hand, false-positive cultures frequently come from contamination, as orthopedic infections are often caused by the same bacteria responsible for the contamination of cultures (4, 18), such as coagulase-negative Staphylococcus (CNS) or Propionibacterium spp. Highly sensitive techniques can lead to unacceptably high numbers of false-positive determinations. Another potential source of error is the contamination of the removed implant during transportation to the diagnostic unit, especially because of leakages in plastic transport bags (19). Besides, the basic media commonly used for bacterial isolation do not adequately allow the isolation of uncommon organisms such as mycobacteria or fungi, while the determination of bacterial DNA in the synovial fluid around the implant by means of PCR may increase the risk of false-positive determinations (15).
The ideal diagnostic approach would require high sensitivity and specificity to confirm orthopedic implant infection. We have designed a prospective study to evaluate the diagnostic value of quantitative cultures performed after orthopedic implant sonication, associated with the inoculation of a broad range of media, to discriminate between contamination and true infection according to the number of CFU detected in the cultures.
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Clinical diagnosis of prosthetic infection was based on the standard preoperative and intraoperative signs associated with orthopedic implant infection: fistula, purulent discharge from the wound, intraoperative identification of periprosthetic purulence and/or a sinus tract communicating with the prosthesis, laboratory findings (maintained elevation of C-reactive protein and erythrocyte sedimentation rate), radiological signs, and/or gamma scan findings (2, 9, 17).
Removal of the prosthetic device was performed under aseptic conditions as a regular orthopedic surgery procedure. Multiple (three to five) periprosthetic tissue samples were sent to microbiology laboratories for routine cultures. Prosthetic devices were sent to the reference laboratory (Fundación Jiménez Díaz microbiology department) in sterile closed containers for specimen processing. Samples were stored at 4°C until processing (maximum delay of 24 h).
Sample processing. In the reference laboratory, samples were aseptically introduced and hermetically closed in 20- by 40-cm sterile plastic bags with 50 ml of sterile phosphate buffer (pH 6.8) (bioMérieux, Marcy-L'Étoile, France). Bags were previously steam sterilized and sealed until use. The samples were sonicated with an Ultrasons-H 3000840 low-power bath sonicator (J. P. Selecta, Abrera, Spain) during 5 min. The bags were visually inspected before and after sonication to detect leaks in the bag.
The sonicate was removed under an aseptical manipulation protocol and placed into 50-ml Falcon tubes. Samples were then centrifuged at 3,000 x g during 20 min, and the supernatant was discharged. The sediment was resuspended in 5 ml of sterile phosphate buffer, and 10 µl was inoculated onto the following culture media: tryptic soy-5% sheep blood agar, chocolate agar, Schaedler-5% sheep blood agar, MacConkey agar, Sabouraud-chloramphenicol agar, and Middlebrook 7H10 agar. Ten microliters was also prepared for Gram staining. Samples were streaked onto each medium for quantitative culture. The media were then incubated under different conditions: at 37°C in a 5% CO2 atmosphere during 7 days (tryptic soy-5% sheep blood agar and chocolate agar) or 15 days (Middlebrook 7H10 agar), at 37°C under a normal atmosphere during 1 day (MacConkey agar), at 37°C under an anaerobic atmosphere during 7 days (Schaedler-5% sheep blood agar), and at room temperature and atmosphere during 30 days (Sabouraud-chloramphenicol agar). All media were inspected daily for microbial growth. Isolated organisms were identified according to commonly used commercial biochemical tests (API20NE, API Strep, and Rapid ID32A; bioMérieux, Marcy-L'Étoile, France) or commonly accepted protocols (6). Susceptibility testing of the organisms was performed using the Kirby-Bauer disc plate assay, and the results were interpreted according to Clinical and Laboratory Standards Institute (formerly NCCLS) standards (12).
When two phenotypically identical strains were isolated from different patients, the strains were analyzed by randomly amplified polymorphic DNA (RAPD) analysis using previously described protocols (24) with primers Akopyanz (CCG CAG CCA A), p3 (AGA CGT CCA C), and p15 (AAT GGC GCA G).
Statistical analysis. Fisher's exact test was used for evaluations of the presence of confluent bacteria related to clinical diagnosis. All calculations were performed with EPI-INFO 3.4.1 software (Centers for Disease Control and Prevention).
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TABLE 1. Results of conventional and sonicate cultures
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Among the patients with clinical diagnosis of infection, all but one (with clinical diagnosis of total knee prosthetic infection) had positive results for the sonicate (sensitivity of 94.1%), and two had negative results by conventional culture methods (sensitivity of 88.2%). Specificity, however, was lower for the sonication (42.8%) than for the conventional culture (specificity of 100% for nine patients with conventional cultures). The positive predictive value was 66.7% for sonication and 100% for conventional cultures, and the negative predictive value was 85.7% for sonication and 81.8% for conventional cultures. One patient with total hip prosthetic infection had S. aureus isolated from the sonicate culture, but the conventional cultures were negative.
Of the cases with no clinical diagnosis of infection, but where conventional cultures were performed (n = 9), four had positive results for the sonicate. After reviewing the clinical chart, one of these cultures was considered to be a true-positive result (Pseudomonas aeruginosa), while the other three were considered to be without clinical significance (one Stenotrophomonas maltophilia and two Burkholderia sp. isolates). Of the cases where no conventional cultures were performed (n = 5), one was considered to be clinically significant after reevaluation (Staphylococcus aureus), and the true significance of another one was doubtful (one isolation of Mycobacterium fortuitum). The rest of these cases were considered to be of no clinical relevance (one isolate of Burkholderia cepacia and one isolate of Sphingomonas paucimobilis). The S. maltophilia isolate was considered to be contamination caused by visible leakage found in the plastic sonication bag. No other cases of bag leakage were detected.
After these late considerations, if we include all the cases with a final diagnosis of infection, the sensitivity of sonication remained at 94.7%, while conventional cultures showed a sensitivity of 84.2%. However, specificity remained lower for sonication (50%) than for conventional cultures (100% of eight cases). The positive predictive value rose to 75% for sonication (conventional cultures remained at 100%), but the negative predictive value was not affected.
In relation to the isolated organisms, all the samples from four patients grew two different organisms (S. aureus and Streptococcus agalactiae, S. aureus and P. aeruginosa, S. aureus and CNS, and CNS and Aspergillus terreus). All the other positive cultures grew only one organism: gram-positive cocci were the most common of them (S. aureus in nine cases and CNS in six cases); P. aeruginosa, S. maltophilia, and Burkholderia spp., were isolated in two cases each, and one isolate was detected for other organisms (Table 2). No identical strains were detected in two different patients according to RAPD results.
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TABLE 2. Characteristics of the two groups of patients
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Clinical evaluation, both perioperatively and intraoperatively, has many pitfalls, and several reports have shown that patients with clinically aseptic loosening may in fact have oligosymptomatic infection that cannot be diagnosed until microbiological evaluation is performed (13, 23).
The pathogenesis of these infections may explain the diagnostic problems. Biofilm is an extremely important form of bacterial life that seems to be a crucial part of the development of biomaterial-related infection (3, 5, 7, 8). Other potential factors related to infections are intracellular survival of bacteria and the ability of low-pathogenic-potential organisms to develop infection when foreign bodies are present through long time periods (16). All these factors lead to difficulties in diagnoses, as bacterial isolates that are part of a biofilm or are inside cells grow poorly in culture media. Moreover, there could be fastidious organisms involved, such as mycobacteria or fungi, that cannot be isolated unless special culture media or prolonged incubation times are used (10).
Sonication of the implanted material seems to be a valid approach for the diagnosis of device-related infections, as has been stated previously (19, 20, 22, 25). This technique dislodges the adhered organisms and allows their detection through conventional microbiological cultures. Reports describing approaches similar to the one used in the present work (20, 22) have shown better sensitivity results for sonication than for conventional cultures, as confirmed in our work. However, the specificity in our results was lower than that that reported previously by Trampuz et al. (20), while no specificity was reported previously by Tunney et al. (22). Several factors could explain this difference. First, the number of conventional cultures taken among patients without infection was very low in our report, so the specificity of conventional cultures could be lower than what was detected. Second, it is possible that sonication in bags could increase the number of positive cultures due to contamination, as previously stated (19), so the specificity reported previously by Trampuz et al. was lower than that of conventional cultures (88.5% versus 90.9%) (20); however, in those studies, the sonication bags were perforated, allowing the entrance of water from the sonicator with subsequent contamination of the sample; we detected only one case of a damaged bag in our work where S. maltophilia was subsequently isolated, while in all the other cases, no rupture could be detected despite careful inspection of the bags before and after sonication. Moreover, we changed the water of the sonicator after each sonication, and the sonicator remained empty so as to avoid bacterial overgrowth in the water and subsequent contaminations. Third, the number of culture media employed by us is higher than those used by Trampuz et al. (20), so there is a possibility that some isolates may not be detected if only conventional media are used; in fact, Burkholderia sp. isolates grew only on Middlebrook 7H10 agar plates, and A. terreus isolates grew both in Sabouraud-chloramphenicol agar tubes and on Middlebrook 7H10 agar plates; if we had used the culture media described previously (20), we would not have detected these isolates, and the specificity would have increased to 66.7%.
The fact that two cases of true infection caused by pathogenic organisms were detected without initial suspicion of infection raises the question of clinical evaluation as a gold standard (23).
On the opposite side, and even more interesting, is the fact that organisms isolated in our study as being false positives share a common characteristic: all of them are nonfermenter gram-negative rods or uncommon isolates (environmental mycobacteria and fungi) with a low pathogenic potential for humans. However, their true significance in these samples is questionable because no clinical symptoms were detected in the patients. These organisms could have been attached to the implant surface and may not have led to clinical infection but may have contributed to the loosening of the prosthesis in the long-term. The high number of colonies detected and the absence of these organisms in control cultures from the sonicator make this hypothesis reasonable. As the use of multiple culture media in our study was done to increase the detection possibilities of fastidious or uncommon organisms, the significance of these organisms as cause of contamination or a true pathogen remains doubtful. Although no clinical disease could be found, the patients with Burkholderia sp. isolates had symptoms leading to prosthesis removal (due to pseudoarthrosis and aseptic loosening), as was also the case for the patient with M. fortuitum (with motion problems). In the case of A. terreus, the fungus was isolated together with CNS, a more common pathogen, in a patient with clinical diagnosis of infection, so its role in the disease is difficult to establish.
In conclusion, the use of sonication, together with a broad spectrum of culture media, increases the possibilities for the diagnosis of device-related orthopedic infections. The significance of some isolates that appeared with high colony counts but without clinical symptoms or signs needs further evaluation to classify them properly as contaminants or pathogens.
There are no conflicts of interest for any author.
Published ahead of print on 12 December 2007. ![]()
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