The prevalence of phenotypic silver resistance in clinical isolates

Research has shown silver to be an effective antimicrobial agent against bacteria, virus, yeast, and fungi. Due to the increased use of silver-based wound products during the past decade, concerns of widespread silver resistance have been raised.1 The purpose of this study was to assess the prevalence of phenotypic silver resistance in clinical isolates.

A total of 130 different microorganism strains were collected from patients admitted to a tertiary care hospital. To determine phenotypic silver resistance, a corrected zone of inhibition (CZOI) test was used. The isolate (0.1 mL) was streaked on nutrient agar in 3 directions to form a confluent lawn. A silver dressing and a control gauze dressing were placed on the lawn and incubated for 24 hours. The CZOI was determined by averaging the zone of clearance in both directions across the dressing and then subtracting the dressing size. Corrected zone of inhibition tests were conducted in duplicate. To confirm the silver dressing killed the microorganism and did not simply hinder bacterial growth, a culture was taken from underneath each silver dressing and plated separately.

All of the isolates showed no growth when tested against the silver-based dressing. The CZOI values ranged between 0.0 mm and 7.25 mm. No growth was observed in the secondary culture from underneath the dressing, indicating the silver dressing was bactericidal for all 130 isolates tested and not simply bacteriostatic. The control gauze dressing did not show any antimicrobial properties.

The threat of widespread silver resistance in clinical isolates remains low. However, continued monitoring for silver resistance should be maintained.