114 A Comparison of in vitro Disc Diffusion and Time‐Kill Kinetic Assays for the Evaluation of Antimicrobial Activity of Silver Containing Wound Dressings

Abstract

There is a plethora of new silver containing dressings on the market today. Various manufacturers attempt to show that their dressings are the most efficacious and therefore should be preferentially employed by health care workers based on the results of their in vitro tests. However, there have been no studies to date that clearly identify which in vitro tests are appropriate for comparison purposes. The available literature suggests that there are problems with disc sensitivity assays in terms of assessing the antimicrobial efficacy of silver. Spadaro (1985) has shown that the diameters of zones of inhibition were not proportional to the concentration of anodic silver in complex test media. Further, Richards et al (2001) have shown that zones of inhibition did not appear to correlate to log reduction assay data for silver containing dressings. Other authors favour log reduction or time kill assays as they better compare to clinical data. The purpose of this study was to determine which in vivo test is most appropriate for evaluating the antimicrobial efficacy of silver‐containing dressings. This was done by testing 8 different silver containing dressings and 2 non‐silver (mafenide acetate or hexamethylene biguanide) containing topical agents against 17 clinically relevant microorganisms using both zone of inhibition assays and time‐kill kinetic assays in complex media. The results for the two assays were then correlated to determine if the methods generated similar results. It was determined that the two methods do not correlate at all. This is most likely a result of the silver interacting with the media in the zone of inhibition test, thus invalidating the results of this test. We therefore conclude that zone of inhibition data generated for silver‐containing dressings is of little value when assessing antimicrobial efficacy, and that time‐kill assays are of greater use.