**6. How helminths, particularly** *C. elegans***, are killed by pathogens**

To answer this question, several research groups have developed nematode bacteria experimental systems. Their results can be grouped into five different mechanisms: (1) Colonization: The worm is killed slowly through an infection-like process, which

correlates with the accumulation of bacteria within the worm's intestine [15]. (2) Infection persistence: In this mechanism, contact between the worm and live bacterial cells is necessary as they accumulate in the intestinal tract of the animal host. Additionally, the proliferation of bacterial cells inside the worm intestine is also needed to establish a persistent infection. This mechanism suggests that some bacterial species may adhere to the intestinal receptors in worms [16]. (3) Invasive: Bacterial cells, such as *S. enterica*, use a type III secretion system to invade *C. elegans* [17]. In this worm, there are two major surfaces that act as nutritious bacterial interfaces: the first is at the chemosensory and olfactory neurons of the amphids, and the second is at the apical surface of intestinal cells. (4) Biofilms: These occur when bacteria form an obstructive matrix over the animal pharyngeal opening, which accumulates over time and prevents normal feeding and nutrition [18]. (5) Toxin-mediated killing: Bacterial pathogens kill worms via the production of diffusible toxins. Under these conditions, no bacterial but rather a soluble toxin is necessary to kill the worm [19]. For the first four mechanisms, direct contact with the pathogen microbes is necessary. The fifth mechanism requires that a soluble toxin reaches the worm. These mechanisms indicate that toxin-mediated killing associated with ROS products is feasible, as all redox reactions are developed under soluble conditions. For example, if the *C. elegans* mutants mev-1(kn1) and rad-8(mn163) are fed with the human opportunistic pathogen *P. aeruginosa* strain PA14, they will be killed. This is because *P. aeruginosa* secretes phenazines, and this organic compound exerts its toxic effect on *C. elegans* mutants undergoing redox cycling for ROS production.
