Synergistic effect between silver nanoparticles and antifungal agents on Candida albicans revealed by dynamic surface-enhanced Raman spectroscopy

Identifying the mechanisms of action of new potential antibiotics is a necessary but time-consuming and costly process. We have developed an ultra-rapid, highly sensitive, and reproducible dynamic surface-enhanced Raman spectroscopy (D-SERS) method to discriminate and evaluate the sensitivity of Candida albicans to antifungal agents with different mechanisms by using silver nanoparticles (Ag NPs). Although Ag NPs have been used conventionally for the enhancement of Raman signals, the accompanying influence of Ag NPs on the microbes has not been investigated. Herein, surface charge and concentration of Ag NPs are likely to be the main influencing factors. Then different concentrations of Ag NPs with the same surface charge as C. albicans were prepared to find the optimal conditions for enhancement of Raman signals while minimally affecting tested fungi. Spectral variations were observed with increasing concentrations of Ag NPs, as well as those of antifungal agents, including echinocandin and azole drugs. The results indicated that the combination of sub-lethal Ag NPs and echinocandin drugs revealed potent synergistic effects against fungi. This could be explained by the metabolism of fungi, the result of which has also been verified by transmission electron microscopy (TEM). Lastly, the combination of sub-lethal Ag NPs and echinocandin drugs was used for a mammalian cell toxicity assay to demonstrate whether the optimal combination could cause lower cytotoxicity to mammalian cells. This work opens a window not only for the evaluation of antifungal agents with different mechanisms, but also for the clinical treatment of fungal infections or even new drug development.