Accessing real-time interaction between antimicrobial liposomes and live Staphylococcus epidermidis using surface plasmon resonance microscopy.

Staphylococcus epidermidis (SE) is a Gram-positive bacterium that is a major cause of healthcare-associated infections (HAIs), largely due to its ability to form biofilms on medical devices and its resistance to many antibiotics. Cholesteryl linoleate (CL) is a lipid secreted by epithelial cells that has been shown to have antimicrobial effects against SE, but how it interacts with the bacteria remains poorly understood. In this study, we developed a surface plasmon resonance microscopy (SPRm) protocol to observe the interaction between CL formulated in phospholipid (CL-PL) liposomes and live stationary phase SE cells in real time. Buffer control experiments showed that the bacteria remained viable throughout the SPRm procedure. Our results provide evidence that CL directly binds to surface of SE and showed that CL-PL binding to SE is concentration-dependent and much more pronounced than binding of PL only. The binding was not uniform but instead varied from one SE cell region to another, with some cell regions showing strong binding and others showing much less indicative of a heterogenous bacterial population as expected for stationary phase bacteria. This work supports further exploration of CL and similar lipids to treat antibiotic-resistant infections caused by SE and other pathogens associated with HAIs, and presents a novel approach of studying antimicrobial activities capable to discern heterogeneity within live bacterial populations.