High definition DIC imaging uncovers transient stages of pathogen infection cycles on the surface of human adult stem cell-derived intestinal epithelium
Authors: van Rijn Jorik M. , Eriksson Jens , Grüttner Jana , Sundbom Magnus , Webb Dominic-Luc , Hellström Per M. , Svärd Staffan G. , Sellin Mikael E. 
Published: August 2021

Interactions between individual pathogenic microbes and host tissues involve fast and dynamic processes that ultimately impact the outcome of infection. Using live-cell microscopy, these dynamics can be visualized to study e.g. microbe motility, binding and invasion of host cells, and intra-host-cell survival. Such methodology typically employs confocal imaging of fluorescent tags in tumor-derived cell line infections on glass. This allows high-definition imaging, but poorly reflects the host tissue’s physiological architecture and may result in artifacts. We developed a method for live-cell imaging of microbial infection dynamics on human adult stem cell-derived intestinal epithelial cell (IEC) layers. These IEC monolayers are grown in alumina membrane chambers, optimized for physiological cell arrangement and fast, but gentle, differential interference contrast (DIC) imaging. This allows sub-second visualization of both microbial and epithelial surface ultrastructure at high resolution without using fluorescent reporters. We employed this technology to probe the behavior of two model pathogens, Salmonella enterica Typhimurium (Salmonella) and Giardia intestinalis (Giardia), at the intestinal epithelial surface. Our results reveal pathogen-specific swimming patterns on the epithelium, showing that Salmonella adheres to the IEC surface for prolonged periods before host-cell invasion, while Giardia uses circular swimming with intermittent attachments to scout for stable adhesion sites. This method even permits tracking of individual Giardia flagella, demonstrating that active flagellar beating and attachment to the IEC surface are not mutually exclusive. Thereby, this work describes a powerful, generalizable, and relatively inexpensive approach to study dynamic pathogen interactions with IEC surfaces at high resolution and under near-native conditions.

Importance

Knowledge of dynamic niche-specific interactions between single microbes and host cells is essential to understand infectious disease progression. However, advances in this field have been hampered by the inherent conflict between the technical requirements for high resolution live-cell imaging on one hand, and conditions that best mimic physiological infection niche parameters on the other. Towards bridging this divide, we present methodology for differential interference contrast (DIC) imaging of pathogen interactions at the apical surface of enteroid-derived intestinal epithelia, providing both high spatial and temporal resolution. This alleviates the need for fluorescent reporters in live-cell imaging and provides dynamic information about microbe interactions with a non-transformed, confluent, polarized and microvilliated human gut epithelium. Using this methodology, we uncover previously unrecognized stages of Salmonella and Giardia infection cycles at the epithelial surface.