Using NanoSIMS coupled with microfluidics to visualize the early stages of coral infection by Vibrio coralliilyticus

Global warming has triggered an increase in the prevalence and severity of coral disease, yet little is known about coral/pathogen interactions in the early stages of infection. The point of entry of the pathogen and the route that they take once inside the polyp is currently unknown, as is the coral's capacity to respond to infection. To address these questions, we developed a novel method that combines stable isotope labelling and microfluidics with transmission electron microscopy (TEM) and nanoscale secondary ion mass spectrometry (NanoSIMS), to monitor the infection process between Pocillopora damicornis and Vibrio coralliilyticus under elevated temperature.

Three coral fragments were inoculated with ¹⁵N-labeled V. coralliilyticus and then fixed at 2.5, 6 and 22 h post-inoculation (hpi) according to the virulence of the infection. Correlative TEM/NanoSIMS imaging was subsequently used to visualize the penetration and dispersal of V. coralliilyticus and their degradation or secretion products. Most of the V. coralliilyticus cells we observed were located in the oral epidermis of the fragment that experienced the most virulent infection (2.5 hpi). In some cases, these bacteria were enclosed within electron dense host-derived intracellular vesicles. ¹⁵N-enriched pathogen-derived breakdown products were visible in all tissue layers of the coral polyp (oral epidermis, oral gastrodermis, aboral gastrodermis), at all time points, although the relative ¹⁵N-enrichment depended on the time at which the corals were fixed. Tissues in the mesentery filaments had the highest density of ¹⁵N-enriched hotspots, suggesting these tissues act as a "collection and digestion" site for pathogenic bacteria. Closer examination of the sub-cellular structures associated with these ¹⁵N-hotspots revealed these to be host phagosomal and secretory cells/vesicles.

This study provides a novel method for tracking bacterial infection dynamics at the levels of the tissue and single cell and takes the first steps towards understanding the complexities of infection at the microscale, which is a crucial step towards understanding how corals will fare under global warming.