Distinct Pathogenesis and Host Responses during Infection of C. elegans by P. aeruginosa and S. aureus

Abstract

The genetically tractable model host Caenorhabditis elegans provides a valuable tool to dissect host-microbe interactions in vivo. Pseudomonas aeruginosa and Staphylococcus aureus utilize virulence factors involved in human disease to infect and kill C. elegans. Despite much progress, virtually nothing is known regarding the cytopathology of infection and the proximate causes of nematode death. Using light and electron microscopy, we found that P. aeruginosa infection entails intestinal distention, accumulation of an unidentified extracellular matrix and P. aeruginosa-synthesized outer membrane vesicles in the gut lumen and on the apical surface of intestinal cells, the appearance of abnormal autophagosomes inside intestinal cells, and P. aeruginosa intracellular invasion of C. elegans. Importantly, heat-killed P. aeruginosa fails to elicit a significant host response, suggesting that the C. elegans response to P. aeruginosa is activated either by heat-labile signals or pathogen-induced damage. In contrast, S. aureus infection causes enterocyte effacement, intestinal epithelium destruction, and complete degradation of internal organs. S. aureus activates a strong transcriptional response in C. elegans intestinal epithelial cells, which aids host survival during infection and shares elements with human innate responses. The C. elegans genes induced in response to S. aureus are mostly distinct from those induced by P. aeruginosa. In contrast to P. aeruginosa, heat-killed S. aureus activates a similar response as live S. aureus, which appears to be independent of the single C. elegans Toll-Like Receptor (TLR) protein. These data suggest that the host response to S. aureus is possibly mediated by pathogen-associated molecular patterns (PAMPs). Because our data suggest that neither the P. aeruginosa nor the S. aureus–triggered response requires canonical TLR signaling, they imply the existence of unidentified mechanisms for pathogen detection in C. elegans, with potentially conserved roles also in mammals.

Author Summary

Pseudomonas aeruginosa and Staphylococcus aureus are bacteria that can form part of the human microbiota, but can also cause severe disease. Despite their great clinical importance, little is known about their interactions with the human host before disease onset, particularly regarding the molecules that host cells use to prevent and combat infection. The invertebrate Caenorhabditis elegans is a powerful model host to study host-pathogen interactions and, because of its simplicity and highly-developed experimental methods, can illuminate fundamental mechanisms of bacterial pathogenesis. We used C. elegans to understand the cellular events that occur during early stages of P. aeruginosa and S. aureus infection. We found that P. aeruginosa slowly colonizes the intestine, producing virulence-related membrane vesicles, and causing accumulation of electron-dense biofilm-like material on intestinal cells and abnormalities in them. In contrast, S. aureus colonizes rapidly, disrupting microvilli and lysing host cells. We found that these different strategies result in different host gene expression responses. Focusing on S. aureus infection, the C. elegans response is mainly microbicidal and detoxifying, aiding host survival and bearing similarities with the human response. Our study provides new insights into mechanisms that P. aeruginosa and S. aureus use to cause disease, and into C. elegans defenses, with potential implications for human immunity and disease.