Description

Caenorhabditis elegans is fed with the unharmful bacterial food of Escherichia coli strain OP50 in the laboratory but the nematode is also susceptible to infections by ingested Gram-negative bacterial pathogens. Salmonella enterica invades epithelial cells, causes germline cell death, and kills the worm in as short as two days, while E. coli OP50 does not result in any worm death for at least four days (Aballay et al. 2003; Tenor et al. 2004). When the worms are allowed to grow into 1-day-old adults on a lawn of S. enterica, the expression of an innate response gene, PMK-1, is activated, indicating infection by the pathogen and the elicited innate response of the host (Tenor et al. 2004). Here we report that lipopolysaccharide (LPS) extracted from S. enterica stimulated gravid adults to lay more eggs in LPS-containing solution than those in M9 buffer only. Figure 1 shows that the worms that were exposed to 0.1 mg/ml LPS for 1 hour laid an average of 5.57±0.57 eggs (± one standard error; n = 101), which was significantly different (p < 0.0001) than the response of the worms exposed to M9 buffer (1.87±0.25 eggs; n = 112). Serotonin, produced by the hermaphrodite-specific neurons (HSNs), is a known stimulant of the vulval muscle contractions that induce egg laying (Trent et al. 1983). Therefore, we used serotonin in this study as a positive control condition, and the egg-laying level that it induced as a reference to that caused by LPS. Exogenous serotonin at 1 mg/ml in M9 resulted in the laying of 8.40±0.60 eggs in one hour (n = 111; p < 0.0001 compared to the worms in M9). The magnitude of LPS-stimulated egg laying was not comparable to that of the serotonin one (p = 0.0005, indicating significant difference).

Intact LPS on the cell surface of S. enterica is required for C. elegans to initiate its innate immune response through the MAP kinase pathway involving PMK-1 (Aballay et al. 2003), while this study demonstrated that LPS extracted from the same bacterial species stimulated egg laying. Further studies will investigate whether LPS-stimulated egg laying involves activating the MAP kinase pathway and whether the stimulated egg-laying behavior happens simultaneously with the innate immune response in the nematode.

Since serotonin and the neuropeptide NLP-3 are released from the HSNs, to initiate egg-laying behavior (Brewer et al. 2019), further investigation is needed to understand whether the components of serotonin biosynthesis and signaling (Dempsey et al. 2005; Schafer 2006) or those of NLP-3 (which are yet to be identified), mediate LPS-stimulated egg laying. We speculate that LPS, when ingested, may stimulate egg laying by stimulating serotonin or neuropeptide production.