Real-time monitoring of pharmacokinetics of antibiotics in biofilms with Raman-tagged hyperspectral stimulated Raman scattering microscopy

Abstract

Antibiotics resistance developed by biofilms has posed a clinical challenge in the effective treatment of bacterial infections. However, the resistance mechanisms have not been well understood due to a lack of suitable tools for dynamic observation of the interplay between antibiotics and biofilm. In this work, with the use of rapid hyperspectral stimulated Raman scattering microscopy associated with an aryl-alkyne-based Raman tag synthesized, we investigate dynamic interactions between vancomycin and Staphylococcus aureus (S. aureus) biofilm to gain new insights into the resistance mechanisms of the biofilm. Methods: We utilize spectral focusing hyperspectral stimulated Raman scattering microscopy ensued with multivariate curve resolution analysis to spectrally decompose S. aureus biofilm into its major components (i.e., bacteria and extracellular polymeric substances). Concurrently, vancomycin is conjugated with aryl-alkyne Raman tag (Raman peak at 2218 cm-1) for in vivo tracking of its uptake into biofilm without tissue interference. Results: We find that vancomycin penetration is a non-uniform diffusion process with penetration depths limited by the preferential affinity to the cell clusters. Semi-quantitative analysis shows that the majority of vancomycin binds to the bacteria, achieving intracellular concentrations of up to 4- to 10- fold higher than the administered dosage. The diffusion constant of ~3.16 µm2/min based on the diffusion and antibiotic binding equations is obtained that well accounts for the antibiotic penetration into the biofilm. SRS longitudinal monitoring of antibiotic effect on the growth of biofilms shows that the antibiotics can eradicate the upper layer of the biofilm exposed to sufficient dosages, while the lower layer of the biofilm at a sub-inhibitory dose remains viable, eventually re-growing to significant bio-volume. Conclusion: The Raman-tagged hyperspectral SRS microscopy developed is a powerful imaging tool for dynamic monitoring of inhibitory effects of antibiotics on the growing biofilm in vivo, which would facilitate the formulation of new antibiotics for more effective treatments of bacterial infections in near future.