Identification of Odor Blend Used by Caenorhabditis elegans for Pathogen Recognition

Abstract

Animals have evolved specialized pathways to detect appropriate food sources and avoid harmful ones. Caenorhabditis elegans can distinguish among the odors of various species of bacteria, its major food source, but little is known about what specific chemical cue or combination of chemical cues C. elegans uses to detect and recognize different microbes. Here, we examine the strong innate attraction of C. elegans for the odor of the pathogenic bacterium, Serratia marcescens. This initial attraction likely facilitates ingestion and infection of the C. elegans host. Using solid-phase microextraction and gas chromatography coupled with mass spectrometry, we identify 5 volatile odors released by S. marcescens and identify those that are attractive to C. elegans. We use genetic methods to show that the amphid chemosensory neuron, AWCON, senses both S. marcescens-released 2-butanone and acetone and drives attraction to S. marcescens. In C. elegans, pairing a single odorant with food deprivation results in a reduced attractive response for that specific odor. We find that pairing the natural odor of S. marcescens with food deprivation results in a reduced attraction for the natural odor of S. marcescens and a similar reduced attraction for the synthetic blend of acetone and 2-butanone. This result indicates that only 2 odorants represent the more complex odor bouquet of S. marcescens. Although bacterial-released volatiles have long been known to be attractive to C. elegans, this study defines for the first time specific volatile cues that represent a particular bacterium to C. elegans.

Efficiency of Gastrointestinal Cancer Detection by Nematode-NOSE (N-NOSE)

“…Furthermore, N-NOSE is less costly and non-invasive. The method used in N-NOSE is based on the C. elegans chemotaxis assay, which has been established in previous C. elegans studies (11,(17)(18)(19)(20); the assay can be performed using basic laboratory instruments and requires less than 100 μl of urine samples. In addition, it is non-invasive compared to the sampling of blood serum and endoscopic examination.…”

“…Measurement method of N-NOSE. The behavior of the nematode C. elegans towards odors has been analyzed by a number of researchers, and the method to quantify the olfactory behavior has been established (11,12,(17)(18)(19)(20). In the present study, the culture of animals and chemotaxis assay were performed according to previous C. elegans studies.…”

“…elegans was cultured on Nematode Growth Media (NGM) agar plate seeded with Escherichia coli as a food source, according to standard methods (11,12,(17)(18)(19)(20). The method for measuring N-NOSE, a biological cancer test using C. elegans, was according to previous C. elegans olfaction studies (11,(17)(18)(19)(20) and as described in Hirotsu et al (13). Briefly, 1 μl of diluted urine sample was spotted at one side of the agar plate (9 cm).…”

“…The value (-1~0) means repulsion to urine sample; the value (0~1) means attraction to urine sample. Using this value of chemotaxis index, past C. elegans studies quantified the olfactory behavior to various odor samples and concentrations (11,(17)(18)(19)(20). The average of chemotaxis indices of more than 10 assay plates were used for ROC analysis.…”

Identification of attractive odorants released by preferred bacterial food found in the natural habitats of C. elegans

“…elegans is attracted to many volatile compounds that are released by bacteria, including alcohols, ketones, aldehydes, esters and amines, and can discriminate among mixtures of volatile compounds released by different kinds of bacteria [ 3 – 6 ]. Recently, specific attractive volatiles important for attraction to particular bacteria have been identified [ 7 – 10 ]. However, many of these bacteria have been pathogens which likely attract the C .…”