Diamond Relaxometry as a Tool to Investigate the Free Radical Dialogue between Macrophages and Bacteria

Abstract

Although free radicals, which are generated by macrophages play a key role in antimicrobial activities, macrophages sometimes fail to kill <i>Staphylococcus aureus</i> (<i>S. aureus</i>) as bacteria have evolved mechanisms to withstand oxidative stress. In the past decades, several ROS-related staphylococcal proteins and enzymes were characterized to explain the microorganism's antioxidative defense system. Yet, time-resolved and site-specific free radical/ROS detection in bacterial infection were full of challenges. In this work, we utilize diamond-based quantum sensing for studying alterations of the free radical response near <i>S. aureus</i> in macrophages. To achieve this goal we used <i>S. aureus</i>-fluorescent nanodiamond conjugates and measured the spin-lattice relaxation (T1) of NV defects embedded in nanodiamonds. We observed an increase of intracellular free radical generation when macrophages were challenged with <i>S. aureus</i>. However, under a high intracellular oxidative stress environment elicited by lipopolysaccharides, a lower radical load was recorded on the bacteria surfaces. Moreover, by performing T1 measurements on the same particles at different times postinfection, we found that radicals were dominantly scavenged by <i>S. aureus</i> from 80 min postinfection under a high intracellular oxidative stress environment.