[Antimicrobial efficacy of a silver layer on hydrogel lenses] Citation]

The high standard of hygiene required in using contact lenses requires regular sterilization, for which the application of a hydrogen peroxide system or storage in liquids containing antimicrobial agents are regarded as the most effective procedures. An approach to avoiding contamination could be to provide the lenses themselves with antimicrobial properties. For this purpose contact lenses equipped with a silver layer were examined for their effect on bacteria.

The antimicrobial efficacy of a silver layer on Weflex 55 hydrogellenses (watercontent 55%, nonionic) was investigated. The lenses were incubated at 37 degrees C for 1 h in suspensions (10(8) CFU/ml) with Pseudomonas aeruginosa or Staphylococcus aureus and were rinsed afterwards. The detachment of the adherent bacteria took place by shaking the lenses in the presence of glass beads (diameter 0.5 mm). Samples of these suspensions were taken, and microbial counts were performed before and after incubation of the contaminated lenses for 6 h and 24 h in a sterile solution of sodium chloride (0.9%, w/v).

Lenses with a silver layer showed markedly fewer test organisms than did unprepared lenses. However, this effect was detectable only when the contaminated lenses were kept in sodium chloride solution (0.9%) for several hours. After incubation for 67 at 25 degrees C the microbial numbers of P. aeruginosa were diminished by about 4 log steps, and those of S. aureus by about 1 log step. Particularly in the case of P. aeruginosa the antimicrobial property of the silver layer was very effective. The microbial numbers of this species, which can cause serious infections of the eyes, were reduced after 24 h by more than 6 log steps. However, S. aureus showed a reduction of only 1.5 log steps. These results reflect the antimicrobial effect of silver ions which are dissolved from depots in the surfaces of the lenses only in traces because of their low dissolution properties.

These first investigations of preserved contact lenses indicate a new possibility for reducing lens-induced infections of the eyes. Due to the weak efficacy against the gram-positive species S. aureus a silver layer probably cannot replace other sterilization procedures completely, but it may represent an additional protection from contamination. Side effects to the eyes due to the negligible toxicity of oligodynamics silver concentrations, such are known to occur with high silver concentrations, should be not expected, but possible allergic reactions caused by the silver-ions should be investigated.