Effect of increasing silver content in phosphate-based glasses on biofilms of Streptococcus sanguis

Silver is a powerful antibacterial ion that may be useful for dealing with localized infections, such as periodontitis. However, the use of silver in this role could be significantly improved by the development of an effective means of delivery. Phosphate-based glasses may provide a means of delivering the ions in a controlled manner. In our study, we have examined the effect of increasing silver content in phosphate-based glasses on biofilms of Streptococcus sanguis. Glasses of nominally the same dissolution rate were doped with silver at 1, 5,10, and 15 mol % and the number of colony-forming units (CFUs) determined after 6 and 24 h in a constant depth film fermenter (CDFF). The 1 mol % silver showed little change between 6 and 24 h. However, only 0.5 log CFUs were present on the glass containing 5 mol % at 6 h, and this reduced to virtually zero at 24 h. Few viable bacteria were found on the 10 mol % glass at both 6 and 24 h. The 15 mol % glass was investigated in this experiment, but no viable counts were detected. In a second set of experiments, glasses with 10 and 15 mol % silver were tested in the CDFF for up to 192 h. For the 10 mol % silver glass, there was approximately 0.8 log CFUs on this glass, which dropped to almost zero at 50 h. This was approximately 1.5-2.0 log reduction in CFUs compared to controls, and this difference was maintained for the first 50 h. After 50 h, there was a slow increase in the CFUs on all samples. However, CFUs on the 10 mol % silver glass were still suppressed up to 192 h compared to the controls. However, both controls also exhibited a decrease in viable counts at 50 h; this may have been due to carryover of silver into the control sample holders. However, this was minimized by the specimen layout in the CDFF and by having gaps between specimen sets. For the 15 mol % silver glass, counts for both this glass and the controls decreased to virtually zero between 24 and 48 h, but the numbers slowly increased up to 170 h, but the number of CFUs was suppressed compared to the 10 mol % glass at the same time point. The decrease seen is clearly the effect of the silver ions; however, the slow increase in CFUs may be accounted for by the biofilms forming thick layers on top of the glass discs inhibiting the release of ions from the glass by forming a "sacrificial layer" through which further ions have to diffuse.