This group of authors from 11 different institutions sought two evaluate the accuracy of cultures for Cutibacterium using positive control (PC) and negative control (NC) samples. Each institution was sent 12 blinded samples (10 PC and 2 NC). The 10 PC samples included 2 sets of 5 different dilutions of a Cutibacterium strain isolated from the humeral canal tissue of a failed total shoulder arthroplasty with a periprosthetic infection with five out five positive cultures for Cutibacterium. This particular isolate was hemolytic with a subtype IB by traditional subtyping and H1 by single locus sequence typing (SLST)
Ten serial dilutions were performed such that the lowest dilution contained approximately 0.1 colony forming units per 100 ul of culture solution. A set of internal control specimens were cultured and sequenced to ensure that there was no contamination during the dilution process.
Five dilutions were selected; the middle of which had a bacterial load that matched that in the original clinical sample, referred to as the 1 dilution. Two dilutions were more concentrated (10 and 100) and two dilutions were less concentrated (0.1 and 0.01) completed the set of five PC samples.
Negative control (NC) isolates contained sterile saline solution.
Each culture solution was combined with a 10mm x 10mm piece of gauze to replicate a tissue sample and frozen for transport. At each institution the samples were handled as if they were received from the operating room. The culturing protocols were standardized among the institutions.
100% of specimens at the 4 most concentrated dilutions were positive for Cutibacterium.
The least concentrated dilution was positive in 91% of samples.
The NC samples grew Cutibacterium in 3 of 22 samples (14%) at two of the 11 institutions. The other 9 institutions detected no growth in the negative control (NC) samples.
Cutibacterium grew in PC samples at an average of
4.0 +/- 1.3 days, and all grew within 7 days.
Time to positivity was significantly shorter (mean 4.0 days) in true positive cultures compared to that for false positive cultures (8.3 p<0.001)
Strength of culture positivity was recorded as the number of the four quadrants showing growth in a culture plate streaked using the standard method.
As shown below, the strength of positivity (number of quadrants with growth) was linearly related to the concentration of the dilution.
The 11 different institutions reported highly consistent rates of culture positivity for samples with higher bacterial loads; with lower bacterial loads the results were somewhat less consistent.
False positive cultures were not noted for most of the 11 sites. False positive cultures were characterized by low bacterial loads and longer times to culture positivity (see NC points in the graph above).
Comment: This study shows that a standardized approach to specimens culturing can lead to reproducible results among institutions. This inter-institution consistency is critical to comparing treatment protocols and clinical outcomes among institutions.
The study shows that the rate of false positive cultures among these institutions is low.
Even with the most dilute test solution, 91% of the institutions were able to detect Cutibacterium.
In that
(a) the basic definition of a periprosthetic infection is the demonstration of bacteria doing harm, and
(b) in that Cutibacterium is the most common bacteria causing periprosthetic infections (often with a stealth presentation),
a robust and reproducible approach to detecting this bacterium in shoulder wounds is essential for determining diagnosis and treatment.
This approach most include the harvesting of multiple deep tissue and explant specimens for culture and a standardized approach for culturing the specimens as well as for reporting and understanding culture results as described in this article.
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