Thursday, December 28, 2017

How effective is Vancomycin against Propionibacterium?

In vitro susceptibility of Propionibacterium acnes to simulated intrawound vancomycin concentrations

These authors state that there is convincing evidence supporting the prophylactic use of intrawound Vancomycin powder in spinal fusion surgery and mounting evidence in the arthroplasty literature suggesting that it can reduce surgical site infections. As a result, a number of shoulder arthroplasty surgeons have adopted this practice, despite a paucity of evidence and the presence of a pathogen that is, for the most part, unique to this area of the body—Propionibacterium acnes. 

The purpose of this study was to evaluate the efficacy of vancomycin against planktonic P. acnes in vitro, using time-dependent concentrations one would expect in vivo after intra-articular application based on a prior in vivo study (see this link) in which 2 gm of Vancomycin was applied topically in arthroplasty wounds (although the Vancomycin was not applied as a powder, but rather injected through a drain). Intrawound Vancomycin concentrations were interpolated and extrapolated from these in vivo data.

Planktonic P. acnes was then subjected to a time-kill analysis during 96 hours.

At each time point, the inoculum was centrifuged into pellet form and then reconstituted for serial drop counts onto blood agar plates. After anaerobic incubation, colony-forming units were counted, and log10 colony forming units per milliliter were determined.

Early time points grew to confluence, and thus colony-forming units per milliliter were not calculated. However, at 12 hours of vancomycin treatment, distinct colonies were appreciated. There was a 3 x log10 reduction in colony-forming units per milliliter between 12 and 48 hours, denoting bactericidal activity. P. acnes was completely eradicated after 3 days of treatment.

The authors concluded that when administered in a fashion meant to simulate time-dependent in vivo intrawound concentrations, vancomycin exhibited bactericidal activity against P. acnes. This may lend credence to theprophylactic use of vancomycin in shoulder surgery.

Comment: The vancomycin susceptibility of planktonic forms of P. acnes clinical isolates is fair, with a minimum inhibitory concentration of 0.38 μg/mL. The vancomycin concentration required to eradicate P. acnes biofilms has been estimated to be ≥128 μg/mL.

Our practice is to use Ceftriaxone and Vancomycin intravenously and 2 gm of topical Vancomycin powder, one of which is inserted into the medullary canal immediately prior to insertion of the humeral component.

Only time and careful followup will tell if this is effective.

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