Pathogen-derived volatiles modulate flight or fight response in Caenorhabditis elegans
In nature, the interaction between two organisms largely relies on their ability to sense and respond to each other, which involves many specific chemicals and signaling pathways. Animals have evolved mechanisms to detect pathogens in their environment and alter their behavior and physiology to avoid infection. In this work, I have studied the nematode Caenorhabditis elegans and its interaction with a bacterial pathogen Pseudomonas aeruginosa PA14 to define the molecular basis of olfaction-driven changes in host behavior and physiology. C. elegans is a bacterivore that forages in decaying organic matter for food. A well-developed chemosensory system including an odor sensory system enables C. elegans to efficiently engage in food search behavior as well as to avoid pathogens. C. elegans exhibits both flight and fight response to P. aeruginosa, a ubiquitous bacterium, and an opportunistic human pathogen. In the first part of my work, we explore the innate ability of C. elegans to respond to P. aeruginosa volatiles. In a systematic study of behavioral response, we find that C. elegans avoid old lawn of P. aeruginosa a flight behavior termed aversion response. Using olfaction defective mutant worms odr-3(n2150) we first show that the volatile cues from the pathogenic lawn majorly drive the avoidance response of the host. Also, this olfaction-mediated aversion response helps the host to increase its chances of survival when exposed to the pathogen. Using solid-phase microextraction (SPME) based gas chromatography-mass spectrometry (GC-MS), we identify six prominent volatiles released by the aversion-inducing old lawn of P. aeruginosa. Dimethyl sulfide, dimethyl disulfide, pyrrole, 1,4-dichlorobenzene, hexanoic acid, 2-ethyl-, methyl ester, and 1-undecene are abundantly present in the P. aeruginosa headspace. By using odor sensory mutants of C. elegans and specific volatiles from P. aeruginosa in chemotaxis assays, we find that C. elegans is repelled by volatile 1-undecene but attracted to volatile pyrrole. Subsequently, we show that 1-undecene is detected by AWB neurons, while pyrrole is sensed by the AWA odor sensory neurons driving repulsion and attraction respectively. We also confirm the role of the odor sensory neurons in sensing 1-undecene and pyrrole by performing odor-evoked in vivo calcium imaging of the respective neurons. In addition, we analyze the behavior of C. elegans in a complex environment when exposed to both the repellent and the attractant and find that the balance of repulsive and attractive odors dictates nematode behavior reflecting its ability to make choices in a complex sensory environment. In the second part of my thesis, I perform a detailed analysis of the pathogen-associated repellent volatile 1-undecene in inducing behavioral and physiological defense responses in the host. We find that 1-undecene induces roaming in C. elegans and also alter the egg-laying pattern. We show that the production of 1-undecene in the older, aversion-inducing lawn is dependent upon non-heme iron oxidase UndA. By performing a screen for regulators in the P. aeruginosa genome, we identify phosphoenolpyruvate -phosphotransferase system, PtsP as a regulator of undA, and olfaction mediated aversion response in C. elegans. Finally, we show that 1-undecene serves as a molecular pattern and induces upregulation of a subset of immune response genes specific to P. aeruginosa in C. elegans, in AWB neuron-dependent manner. Broadly, my thesis work provides a molecular, neuronal, and genetic basis of the flight and fight response of C. elegans upon exposure to the pathogenic bacterium P. aeruginosa.