Question

During the development of an activatable anti-infective coating based on poly-L-lactic acid and silver nitrate, concerns arose regarding the potential inhibition of the silver's effect by physiological fluids. The literature indicates that silver ions can bind to thiol groups, which can shift the minimum inhibitory concentration (MIC) upwards [1]. Therefore, the glutathione in serum may play a role in the effectiveness of the coating in clinical applications and in vivo studies. To evaluate the potential inhibition or shift of the MIC of the activated coating in situ and in various in vivo or in vitro models, growth curves were applied using the average measured value of silver release from a 6% silver nitrate coating (57.8 mg/L).

Methods

Test solutions were prepared using a 1280 mg/L stock solution of silver nitrate and either human, fetal bovine (FBS), mouse, and rabbit serum, or tryptic soy broth (TSB). The concentrations used were 50, 100, 150, and 200 mg/L for the sera and 50 and 100 mg/L for the TSB. The solutions (100 µL) were incubated with a Staphylococcus epidermidis RP62A culture in TSB (100 µL) containing a bacterial load of 105 CFU/mL. The incubation was carried out in technical duplicates and biological triplicates for 24 hours at 37°C. The OD was measured every 30 minutes at 578 nm.

Results

Concentrations of 50 mg/L silver nitrate and higher inhibited the growth of S. epidermidis in human serum, FBS, mouse serum, and rabbit serum for 24 hours. No differences in growth were observed between TSB and any of the serum samples.

Conclusions

The MIC of silver nitrate is below 50 mg/L for all sera, indicating that the implant coating's silver ion release is at a therapeutically effective level. It is important to note that the MIC may have increased due to the binding of serum components, but this is not relevant to the coating's effectiveness as long as the previous values of silver ion release can be guaranteed in all models.

References

[1] Mulley G et al. (2014) Inactivation of the antibacterial and cytotoxic properties of silver ions by biologically relevant compounds. PLoS One 9:e94409. https://doi.org/10.1371/journal.pone.0094409