Friday, March 3, 2017

Culturing explant biofilms - shaken or stirred

Many of our readers are familiar with James Bond's martini preference: shaken but not stirred.

The authors of Sonication versus Vortexing of Implants for Diagnosis of Prosthetic Joint Infection
compared sanitation (shaking) versus vortexing (stirring) with respect to the biofilm removal efficacy of vortexing alone in 135 removed prostheses, 35 were diagnosed with infection and 100 with aseptic failure. They used two different cutoffs for a positive result: 1 colony forming unit (CFU) or 50 CFU.

Using a cutoff of >50 CFU/ml, sonication showed higher sensitivity than vortexing (60% versus 40%) (p = 0.151), while the specificities remained equal (99%). Using the lower cutoff value (>CFU/ml), the sensitivities of vortexing and sonication fluid were similar (69% to 71%); however, the specificities decreased to 92% to 93%. In the table below PJI were cases they defined as infection and AF were thought to be aseptic failure.

It is of note that only two of these cases involved the shoulder and only 5 of the recovered organisms were Propionibacterium.

Comment: These authors provide a nice commentary on their work : "Interestingly, vortexing alone demonstrated acceptable sensitivity and specificity, especially in acute PJI, and may be used for the diagnosis of PJI in laboratories where sonication is not available. In addition, vortexing fluid represents a single clinical sample, with such analyses reaching sensitivity comparable to that of analyses using multiple periprosthetic tissue cultures (∼70%). Furthermore, sonication may kill bacteria, especially Gram-negative bacilli and anaerobes, whereas vortexing has not been shown to be harmful to bacteria. In addition, vortexing is an easy and simple procedure which can be performed in most laboratories without additional costs. Despite vortexing having originally been introduced as a preparatory step before sonication to generate microbubbles, which increase the cavitation effect, it seems to be a powerful removal method. High shear forces generated on the interface between the prosthesis and the vortexing fluid may explain the biofilm removal effect. These shear forces could be increased by a pulsatile change of the fluid movement direction (as used in Stomacher analysis). The removal efficiency may be also increased by addition of detergents (e.g., polysorbate 80) or anticoagulants (e.g., EDTA) to the vortexing fluid. Another possibility is the addition of beads to vortexing fluid, as is used for processing tissue samples in a bead mill. However, the contamination risk, increased workload, and costs need to be considered"

Our laboratory has determined that the potential incremental benefit of sonication does not offset the incremental cost and time.
However, we recognize the importance of dislodging bacteria ensconced in a biofilm if we are to be able to detect their presence in failed shoulder arthroplasty. We await further evidence on the value of sonication in recovering Propionibacterium from shoulder implants.