Table 1.

Examples of effector mechanisms involved in inter-species and inter-kingdom interactions.

Effector molecule(s)	Relevant aspects for microbiota studies	Reference(s)
Secreted phosopholipases (PldA and PldB) of Pseudomonas aeruginosa.	Killing of competing bacteria (PldA, PldB), promoting P. aeruginosa internalization by host cells (PldB).	Russell et al., 2013; Jiang et al., 2014
Autoinducer-2 (AI-2)	Mediates inter-species signaling among the microbiota. Production of AI-2 by Bifidobacterium breve UCC2003 (a probiotic strain) is required for gut colonization and protection against Salmonella infection in C. elegans.	Christiaen et al., 2014
Autoinducer-3 (AI-3), epinephrine and norepinephrine	Autoinducer 3 (AI-3) produced by the intestinal microbiota can cross-signal with host hormones epinephrine and norepinephrine.	Sperandio et al., 2003
	Epinephrine/Norepinephrine can be sensed by enterohemorrhagic E. coli, inducing the expression of the LEE pathogenicity island and the flagella regulon. The AI-3 signaling cascade is conserved across several bacterial species.	Reading and Sperandio, 2006