Serotonin Activates Bacterial Quorum Sensing and Enhances the Virulence of Pseudomonas aeruginosa in the Host
Bacteria in humans play an important role in health and disease. Considerable emphasis has been placed in understanding the role of bacteria in host-microbiome interkingdom communication. Here we show that serotonin, responsible for mood in the brain ...
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4. Discussion
This study establishes serotonin's role as a bacterial quorum sensing molecule. Our cellular system data show that serotonin can activate the LasR QS pathway at μM concentrations (Fig. 3b). This finding is of great interest as physiological levels of serotonin in the digestive tract of healthy and diseased individuals is 10 μM (Erspamer, 1966) and ~ 100 μM (Miwa et al., 2001) respectively. The raised levels of serotonin in patients with Inflammatory Bowel Disease (IBD), combined with the insight that serotonin can activate QS pathways, may allow for better patient care and therapeutics. Additionally, the additive effects of serotonin combined with QSM (Fig. 3c) are relevant as bacteria exist naturally in the gut, thus it is highly likely that serotonin will co-exist with bacterial QSMs.Previous reports have shown that virulence factor production, such as biofilm formation and protease production, are quorum-dependent (Waters et al., 2008, Valiente et al., 2007), which supports the increase in these factors seen when serotonin was administered in both our in vitro (Fig. 4) and in vivo (Fig. 2, Fig. 5) experiments as serotonin activating the LasR QS system. In vitro, our results show that serotonin mimics the effects of exogenous QSM addition in JP2 cells' biofilm formation (Fig. 4C–D), indicating that serotonin acts as a bacterial signaling molecule that is capable of activating QS-regulated phenotypes. Our developed Pseudomonas infection model demonstrated that serotonin was able to activate Pseudomonas virulence in vivo, within the intestines of the mice (Fig. 2, Fig. 5). Within these experiments, there are two points of particular interest: in the absence of serotonin the JP2 mutant that cannot generate its own QSMs was not able to establish an infection, and the effects of PAO1 infection alone were similar to the effects of JP2 with serotonin. When administered the same CFU of JP2 cells compared to PAO1 cells, the JP2 mutant Pseudomonas did not establish any detectable colonies (Fig. S3B). This lack of infection is supported by the SEM images (Fig. 5A) and histology data (Fig. 5B–C). As the JP2 mutant is only lacking in QSM synthesis, this inability to establish an infection both supports the current literature that Pseudomonas virulence is QS dependent and allows for the assessment of restoration of function, specifically the ability to establish an infection. Thus when we restore the infectivity of JP2 by administering serotonin (Fig. S3B), it indicates that serotonin is fulfilling the role of a QSM. The infection with PAO1 alone and JP2 with serotonin resulted in similar levels of CFUs harvested from the intestines (Fig. S1D, S3B), which further supports serotonin's role as a QSM. This similarity between PAO1 alone and JP2 with serotonin is also noted in the histology data (Figs. 2B–C, 5B–C), with both exhibiting intestinal epithelial damage and villi destruction. These data further demonstrate serotonin's ability to restore function to the JP2 mutant lacking QSMs, providing greater evidence for its role as a QSM.
Our experiments demonstrate that serotonin activates the las quorum sensing pathway, which leads to greater infectivity of P. aeruginosa, however continued experimentation with Pseudomonas infection and serotonin can help to fully elucidate this complex interplay. Our mouse model was limited due to its focus on three day old mouse pups based upon their natural susceptibility to Pseudomonas infection, however, it may be possible to exploit serotonin's enhancement of Pseudomonas virulence to investigate Pseudomonas infections in adult mice. Additionally, there are limitations to current biochemical assays for QSMs, such as a lack of a full crystal structure for LasR. Advancements in this field will allow for binding studies of serotonin with LasR, which would further enhance our findings.
While it is uncertain why Pseudomonas utilizes another organism's molecules to enhance their QS networks, it is not a novel phenomenon. Social cheating by bacteria, where the bacteria exploit sensing molecules produced by other organisms in their vicinity to activate their QS systems, has been observed in many circumstances. Thus serotonin may be a way the opportunistic bacteria P. aeruginosa participates in social cheating to enhance its virulence. While our knowledge of the human microbiome is in its nascent stage, the observed Janus-type behavior of serotonin, acting as both mammalian neurotransmitter and bacterial communication molecule, undoubtedly will contribute to further our knowledge of the complex relationship between the host and its' microbiome. Specifically, these findings could lead to a better understanding of the host regulation of the microbiome, especially during infection, potentially leading to a paradigm shift in the management of intestinal bacterial-related illnesses or disorders of the digestive system that are triggered by bacteria.