Other genes involved in the response to bacterial infection that have been identified in RNAi screens are required for host cells to clear ingested bacteria. Again, these screens defined a set of generally required genes that limit bacterial survival or replication, such as genes encoding endosomal sorting complex required for transport (ESCRT) proteins (Philips et al., 2008) , as well as genes preventing the growth of specific pathogens, such as lysosomal hexosaminidase, which restricts growth of Mycobacterium marinum but not Listeria monocytogenes or Salmonella typhimurium (Koo et al., 2008) . In other standard genetic studies, intracellular microorganisms such as Wolbachia were found to also engage in mutualistic symbiotic relationships with the host, such as protecting the host against viral infection (Hedges et al., 2008; Teixeira et al., 2008) and nutritional supplementation (Brownlie et al., 2009) , which presumably arose during co-evolution of the endosymbiont and host.