Detection and avoidance of a natural product from the pathogenic bacterium Serratia marcescens by Caenorhabditis elegans

The nematode Caenorhabditis elegans is present in soils and composts, where it can encounter a variety of microorganisms. Some bacteria in these rich environments are innocuous food sources for C elegans, whereas others are pathogens. Under laboratory conditions, C elegans will avoid certain pathogens, such as Serratia marcescens, by exiting a bacterial lawn a few hours after entering it. By combining bacterial genetics and nematode genetics, we show that C elegans specifically avoids certain strains of Serratia based on their production of the cyclic lipodepsipentapeptide serrawettin W2. Lawn-avoidance behavior is chiefly mediated by the two AWB chemosensory neurons, probably through G protein-coupled chemoreceptors, and also involves the nematode Toll-like receptor gene tol-1. Purified serrawettin W2, added to an Escherichia coli lawn, can directly elicit lawn avoidance in an AWB-dependent fashion, as can another chemical detected by AWB. These findings represent an insight into chemical recognition between these two soil organisms and reveal sensory mechanisms for pathogen recognition in C elegans.