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.

Caenorhabditis elegans microRNAs of the let-7 family act in innate immune response circuits and confer robust developmental timing against pathogen stress

“…7). OMVs are secreted by P. aeruginosa and known to function as a virulence factor and toxin delivery platform (39). These results suggest that the prolonged survival of let-7(mg279) animals on P. aeruginosa reflects an enhanced countervirulence activity in let-7(mg279) animals compared with WT animals.…”

“…WT (N2) and let-7(mg279) animals were synchronized and grown on NGM plates seeded with E. coli HB101. L4-stage animals were transferred onto P. aeruginosa slow-killing plates and incubated at 25°C for 8, 24, and 48 h. Worms were harvested and prepared for transmission EM as described (39).…”

Caenorhabditis elegans responses to bacteria from its natural habitats

“…elegans Strains and Maintenance. N2-Bristol, Phsp-4::GFP, Phsp-6::GFP, Pgst-4:: GFP (acquired from the Caenorhabditis Genetics Center); PF35E2.5, Pclec-60::GFP, Pirg-1::GFP (gifts from F. Ausubel, Massachusetts General Hospital, Boston) (39)(40)(41). All strains were maintained on NGM plates (no antibiotics) spotted with E. coli OP50 at 20°C-22°C using standard protocols (42) unless otherwise noted.…”

Selective Sorting of Cargo Proteins into Bacterial Membrane Vesicles

“…OMV production is increased upon exposure of bacterial cells to harsh conditions, such as the addition of chemical stressors to medium or the host environment during infection (5,6). This has been shown in the Caenorhabditis elegans model system, where an abundant amount of OMV accumulated during infection with Pseudomonas aeruginosa (7). An increase in OMV production under stressful conditions is correlated with bacterial survival (5,6).…”