Nanoscale chemical imaging of phagocytosis: A battle for metals between host and microbe

The human body employs nutritional immunity to restrict essential micronutrients, such as zinc, from invading pathogens, impeding their growth and replication. Here, we applied an advanced nanochemical imaging technique, synchrotron radiation-based X-ray fluorescence (SR-XRF), on vitrified polymorphonuclear neutrophils (PMNs) during the occurrence of phagocytosis of Saccharomyces cerevisiae. Nanoscopic SR-XRF provided trace elemental distributions at 50 nm spatial resolution, revealing the metal interplay between PMNs and S. cerevisiae. Our results were complemented with X-ray holographic nanotomography (XNH), confirming phagocytosis and providing complementary intracellular morphological information. A systematic decrease in zinc was observed between free and phagocytosed S. cerevisiae within the same XRF maps, suggesting active zinc depletion by PMNs. Other elements, such as sulfur, show an increase in the phagosome, likely indicative of the increase in proteins in the vicinity of phagocytic events. Through 2D/3D nanoimaging and time-lapse microscopy, we confirmed the reduction of zinc within phagocytosed yeast. Hence, our findings challenge the currently accepted hypothetical model that PMNs intoxicate engulfed microbes with an overwhelming influx of zinc ions into the phagosome. Furthermore, antimicrobial assays demonstrated that S. cerevisiae can cope well with sudden zinc spikes. Even high zinc concentrations imposed on S. cerevisiae grown under zinc-limiting conditions did not have adverse effects on viability. Contrarily, S. cerevisiae was more resistant to phagocytic killing by PMNs when grown under high zinc concentrations before infection. Our findings further consolidate zinc deprivation as an effective antimicrobial strategy. A better understanding of the metal deprivation mechanisms could inspire new exploitable targets for antimicrobial therapies.