JCM Accepted Manuscript Posted Online 12 October 2016
J. Clin. Microbiol. doi:10.1128/JCM.01435-16
These authors retrospectively studied whether reducing cultivation time to 7 days allowed accurate diagnosis without losing sensitivity. They identified patients with at least one positive P. acnes sample between 2005 and 2015 and grouped into ‘infection’ and ‘no infection’. They defined an ‘infection’ when at least two samples from the same case were positive. Clinical and microbiological data including time to positivity for different cultivation methods were recorded.
They identified 70 cases meeting their definition of a P. acnes ‘infection’, in which 262 out of 379 samples were positive (69.1%). 47 cases did not fulfill our criteria for a proven ‘infection’ with only one positive sample (47 out of 215 samples, 21.9%). The most common sample site was shoulder (total n = 77), followed by hip (n=26). In the ‘infection’ group, they diagnosed PJI in 35 (50%) cases, and implant-associated infections with large (plates, intramedullary nail) or small implants (screws, anchors) in 10 (14.3%) and 17 (24.3%), respectively. Five cases presented with septic arthritis (arthroscopy-associated), and three cases with osteomyelitis (two in the shoulder after infiltration or trauma and one in the clavicula associated with b-cell lymphoma). In the ‘no infection’ group there were pain due to mechanical reasons in 22 (46.8%), aseptic loosening of an implant in 9 (19.2%), and
other reasons in 16 (34%) cases diagnosed at revision surgery.
The cases of P. acnes "infection" had a significant faster median time to positivity of 6 days (range 2- 11 days) compared to 9 days in 47 cases with P. acnes identified as a contamination (p<0.0001).
Their yield with sonication was lower than expected and they suggest the possibility that P. acnes might be inhibited or killed using ultrasound baths.
They conclude that a prolonged cultivation time is necessary for P. acnes identification. They would have missed 21.4% of the P. acnes infections if cultivation time had been reduced to 7 days. Thioglycolate broth as an enrichment method for tissue samples showed a high sensitivity. The best positive predictive value was seen with direct incubation on anaerobic agar plates. Time to positivity of P. acnes growth did not seem to be affected by a prolonged transportation time, which showed that P. acnes in the biofilm of musculoskeletal infections can survive for hours even in a fastidious environment.
Comment: This study effectively points out that longer incubation times result in a higher rate of culture positivity for Propionibacterium and the importance of anaerobic and broth cultures.
The definition of 'infection' as two or more positive cultures is, however, arbitrary due to the fact that Propionibacterium are not evenly distributed throughout the wound (see this link); whether or not a given specimen is culture positive depends on the 'luck of the draw'.
Rather than applying an arbitrary definition of 'infection' (and there are many), it would be clearer to report the number of specimens submitted from (a) tissue, (b) explant, and (c) fluid and the percent of each that was culture positive.
The bacterial load matters. Higher total numbers of bacteria in the wound will increase the percent of specimens that are culture positive, but a low percent of positive cultures does not mean the wound is aseptic.
We could take a number of samples of this lawn and not find two that show a mushroom, but that doesn't mean mushrooms are not really there.
Higher numbers of bacteria in a specimen will result in a shorter time to the recognition of a culture being positive, but a long time to positivity does not mean that the result is a 'contaminant'.
Consider the example below in which 1000 bacteria need to be present in order for the culture is recognized as positive. The red specimen contained 4 bacteria while the blue specimen contained one. The red specimen was 'culture positive' at nine days, while the blue specimen was interpreted as showing 'no growth' at 10 days.
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