Introduction: C. elegans as a model organism

organism The nematode Caenorhabditis elegans is a genetically tractable multicellular organism that has been the subject of intense study for more than four decades. It has been successfully used as a model system to address fundamental questions in multiple fields of biology, including development, neurobiology and aging. In recent years, this tiny nematode worm has been applied to the study of microbial pathogenesis and host innate immunity, and used for drug discovery and development. C. elegans is a self-fertilizing hermaphrodite with a rapid generation time. Each adult worm grows to a length of approximately 1 mm and under laboratory conditions can produce 300 genetically identical progeny in a 3-day life cycle. This allows the rapid expansion of strains and the establishment of large homogeneous populations. In the lab, C. elegans is propagated on agar plates or in liquid media with the auxotrophic Escherichia coli mutant strain OP50, and can live up to 3 weeks at room temperature. The entire genome sequence for C. elegans is available and many functional genomic approaches have been developed. Notably, RNA interference (RNAi) can be delivered systemically by feeding worms bacteria that express doublestranded RNA (dsRNA) targeting any gene of interest. Because dsRNA expression libraries covering almost 90% of the 20,000 genes in the C. elegans genome are available, many genome-wide RNAi screens have been performed (for a review, see Lamitina, 2006). Furthermore, transgenic C. elegans strains can be readily created via microinjection of DNA (e.g. plasmids and/or PCR products), and its transparency renders the use of fluorescent reporter genes in vivo straightforward, as well as allowing direct real-time monitoring of infectious processes (Aballay et al., 2000; Labrousse et al., 2000). There is an ever-growing list of Grampositive, Gram-negative and fungal pathogens that are known to infect C. elegans, many of which are of clinical relevance (Darby, 2005; Sifri et al., 2005; Powell and Ausubel, 2008). A prominent example is the human opportunistic pathogen Pseudomonas aeruginosa, which was the first microorganism shown to be able to infect and kill C. elegans. In groundbreaking work, the Ausubel laboratory showed that many of the bacterial genes required for full virulence in the nematode were also important in other model systems (Tan et al., 1999a; Tan et al., 1999b). Coupled with the ease of culture and the possibility of automated handling, this has led to numerous in vivo large-scale screens for bacterial virulence factors, using C. elegans as a host (Kurz and Ewbank, 2007). In many cases, these studies have demonstrated that virulence factors involved in the killing of C. elegans are also required for pathogenesis in mammals. This opens up new avenues for the development of novel therapies that target specific virulence mechanisms. Although C. elegans is unlikely ever to encounter many of the pathogens that are used in the laboratory in its natural environment of rotting fruit, C. elegans is continuously exposed to microorganisms, which can be either just a food source or also pathogenic. When confronted with pathogenic microorganisms, C. elegans activates protective mechanisms. These include an avoidance behaviour, triggered by the detection of specific microbial molecules, such as the cyclic pentadepsipeptide biosurfactant serrawettin W2, produced by some strains of Serratia marcescens (Pradel et al., 2007). When a pathogen cannot be avoided, C. elegans mounts an innate immune response, involving the activation of specific signalling pathways and leading to the production and release of defence molecules (Irazoqui et al., 2010). Among these immune effectors are a variety of antimicrobial peptides (AMPs) and proteins (Fig. 1). In the remainder of this Primer, we summarize what is known about these different families of antimicrobial factors, and discuss how C. elegans can be