The value of sonication on orthopaedic implants in an everyday clinical setting - an exploratory study.

BACKGROUND: Sonication of removed orthopaedic implants in suspected implant-associated infections (IAI) is widely applied internationally. However, evaluation of the utility of sonication on all implants removed in everyday standard practice is scarce. This exploratory study was performed to evaluate the application of sonication fluid (SF) culture on removed orthopaedic implants, irrespective of the reason for removal. METHODS: Out of 100 removed orthopaedic implants collected between August 2019 and September 2020, 77 implants with availability of concurrent tissue culture samples were included in the study. Removed implants were categorized into a confirmed or suspected IAI group and a presumed aseptic group based on pre-operative diagnosis by the responsible surgeon. Implants were sonicated and SF culture performed under both aerobic and anaerobic conditions. The significance of all bacterial isolates was evaluated based on the CFU/mL cut-offs of the EBJIS guidelines, except for C. acnes where additional investigations were performed. RESULTS: The results of SF culture in the two groups were compared with their corresponding tissue cultures. Out of the 12 cases in the confirmed/suspected IAI group, SF culture was positive in 11 cases and had increased diagnostic yield in two (17%) cases compared to tissue culture. Increased diagnostic yield of SF compared to tissue culture was seen in seven (11%) of the 65 implants in the presumed aseptic group. If growth of Cutibacterium species isolates were interpreted based on EBJIS cut-off for SF culture instead of the study-specific criteria, then two isolates considered to represent infection might have been missed while three other isolates considered contaminants would have fallen under the 'infection confirmed' category in the EBJIS guidelines. CONCLUSION: Sonication with SF culture has increased diagnostic yield compared to tissue cultures in all implants irrespective of reason for removal. However, positive SF cultures with Cutibacterium species should always be interpreted with extreme care.

Shotgun sequencing of sonication fluid for the diagnosis of orthopaedic implant-associated infections with Cutibacterium acnes as suspected causative agent.

Orthopaedic implant-associated infections (OIAIs) due to Cutibacterium acnes can be difficult to diagnose. The aim of this pilot study was to determine if metagenomic next-generation sequencing (mNGS) can provide additional information to improve the diagnosis of C. acnes OIAIs. mNGS was performed on sonication fluid (SF) specimens derived from 24 implants. These were divided into three groups, based on culture results: group I, culture-negative (n = 4); group II, culture-positive for C. acnes (n = 10); and group III, culture-positive for other bacteria (n = 10). In group I, sequence reads from C. acnes were detected in only one SF sample, originating from a suspected case of OIAIs, which was SF and tissue culture-negative. In group II, C. acnes sequences were detected in 7/10 samples. In group III, C. acnes sequence reads were found in 5/10 samples, in addition to sequence reads that matched the bacterial species identified by culture. These samples could represent polymicrobial infections that were missed by culture. Taken together, mNGS was able to detect C. acnes DNA in more samples compared to culture and could be used to identify cases of suspected C. acnes OIAIs, in particular regarding possible polymicrobial infections, where the growth of C. acnes might be compromised due to a fast-growing bacterial species. However, since SF specimens are usually low-biomass samples, mNGS is prone to DNA contamination, possibly introduced during DNA extraction or sequencing procedures. Thus, it is advisable to set a sequence read count threshold, taking into account project- and NGS-specific criteria.

Amplicon-Based Next-Generation Sequencing as a Diagnostic Tool for the Detection of Phylotypes of Cutibacterium acnes in Orthopedic Implant-Associated Infections.

The diagnosis of orthopedic implant-associated infections (OIAIs) caused by the slow-growing anaerobic bacterium Cutibacterium acnes is challenging. The mild clinical presentations of this low-virulent bacterium along with its ubiquitous presence on human skin and human-dominated environments often make it difficult to differentiate true infection from contamination. Previous studies have applied C. acnes phylotyping as a potential avenue to distinguish contamination from infection; several studies reported a prevalence of phylotypes IB [corresponding to type H in the single-locus sequence typing (SLST) scheme] and II (SLST type K) in OIAIs, while a few others found phylotype IA1 (more specifically SLST type A) to be abundant. However, phylotype determination has mainly been done in a culture-dependent manner on randomly selected C. acnes isolates. Here, we used a culture-independent amplicon-based next-generation sequencing (aNGS) approach to determine the presence and relative abundances of C. acnes phylotypes in clinical OIAI specimens. As amplicon, the SLST target was used, a genomic fragment that is present in all C. acnes strains known to date. The aNGS approach was applied to 30 sonication fluid (SF) samples obtained from implants removed during revision surgeries, including 17 C. acnes culture-positive and 13 culture-negative SF specimens. In 53% of the culture-positive samples, SLST types were identified: relative abundances were highest for K-type C. acnes, followed by H- and D-type C. acnes. Other types, including A- and C-type C. acnes that are more prevalent on human skin, had low relative abundances. The aNGS results were compared with, and confirmed by a culture-dependent approach, which included the isolation, whole genome sequencing (WGS) and phylotyping of 36 strains of C. acnes obtained from these SF samples. Besides serving as a powerful adjunct to identify C. acnes phylotypes, the aNGS approach could also distinguish mono- from heterotypic infections, i.e., infections caused by more than one phylotype of C. acnes: in eight out of nine culture-positive SF samples multiple C. acnes types were detected. We propose that the aNGS approach, along with the patient's clinical information, tissue and SF cultures and WGS, could help differentiate C. acnes contamination from true infection.

Diagnosis of orthopaedic-implant-associated infections caused by slow-growing Gram-positive anaerobic bacteria - a clinical perspective.

Slow-growing Gram-positive anaerobic bacteria (SGAB) such as Cutibacterium acnes are increasingly recognized as causative agents of implant-associated infections (IAIs) in orthopaedic surgeries. SGAB IAIs are difficult to diagnose because of their non-specific clinical and laboratory findings as well as the fastidious growth conditions required by these bacteria. A high degree of clinical suspicion and awareness of the various available diagnostic methods is therefore important. This review gives an overview of the current knowledge regarding SGAB IAI, providing details about clinical features and available diagnostic methodologies. In recent years, new methods for the diagnosis of IAI were developed, but there is limited knowledge about their usefulness in SGAB IAI. Further studies are required to determine the ideal diagnostic methodology to identify these infections so that they are not overlooked and mistakenly classified as aseptic failure.