Mechanisms that control aging are important yet poorly defined. To discover longevity control genes, we performed a forward genetic screen for delayed reproductive aging in C. elegans. Here, we show that am117 is a nonsense mutation in the phm-2 gene, which encodes a protein homologous to human scaffold attachment factor B. phm-2(lf) mutant worms have an abnormal pharynx grinder, which allows live bacteria to accumulate in the intestine. This defect shortens lifespan on highly pathogenic bacteria but extends lifespan and health span on the standard E. coli diet by activating innate immunity pathways that lead to bacterial avoidance behavior and dietary restriction. eat-2(lf) mutants displayed a similar phenotype, indicating accumulation of live bacteria also triggers extended longevity in this mutant. The analysis of phm-2 elucidates connections between pathogen response and aging by defining a mechanism of longevity extension in C. elegans—bacterial colonization, innate immune activation, and bacterial avoidance behavior. Display omitted •phm-2 encodes a SAFB protein conserved in mammals•phm-2 and eat-2 mutants have a defective pharynx and accumulate bacteria in the gut•Bacterial accumulation activates innate immunity and bacterial avoidance behavior•Bacterial avoidance leads to dietary restriction and delays aging Kumar et al. link immunity and aging in C. elegans. The authors characterize phm-2 and eat-2, mutants that allow live bacteria to accumulate in the intestine, causing delayed reproductive and somatic aging. The mechanism combines molecular immune activation and behavioral food avoidance, leading to dietary restriction and extended lifespan.