Minimum Contact Time Determination for Silverlab’s ionic⁺ Colloidal Silver Antibacterial Activity 


IONIC SILVERᐩ was submitted in 18 ppm and 42ppm variants for this study.

The aim of the study was to determine the minimum contact time required to produce at least a 5 log reduction in viable bacteria.

The test was conducted in a suspension test loosely based on the method used for determination of bactericidal efficacy of chemical disinfectants EN1276. Certain deviations from the standard method had to be made to eliminate the presence of any traces of Salt (NaCl) which binds and inactivates the silver ions in the suspension.

Bacterial suspensions were prepared from overnight cultures on nutrient agar and standardised to 10cfu/ml in sterile distilled water. Suspensions were standardised by adjusting the optical density to between 0.08 and 0.1 at 625nM using a spectrophotometer.

The test suspension was prepared with a volume of 8ml of the Ionic silver product, 1ml 3g/L bovine serum albumin to give a concentration of 0.3g/L in the final test mixture, and 1 ml of the bacterial suspension.

The aim was to determine the minimum contact time to eliminate >5log of viable bacterial cells in the test suspension, therefor the test was carried out on a mixed bacterial suspension containing equal volumes of Escherichia coli ATCC10536 ; Staphylococcus aureus ATCC6538 ; Pseudomonas aeruginosa ATCC15442 and Enterococcus hirae ATCC10541

A 1ml portion of the test mixture was removed at time intervals of 30 Seconds, 60 seconds, 3 minutes, 5 minutes, 10 minutes and 30 minutes. The test portion was placed in a petri dish and immediately overlaid with Plate count agar containing 5g/L NaCl to bind and neutralise the Ionic Silver.

A control sample containing only distilled water instead of the Ionic silver was included as a negative control sample.

Plates were incubated at 37°C for 48 hours before interpretation.