Toxin-antitoxin (TA) systems are small genetic elements composed of a toxin gene and its cognate antitoxin that are important for plasmid stabilization (plasmid-encoded) and bacterial virulence (chromosome-encoded). These systems are also related to biofilm and persister cell formations.Pseudomonas aeruginosais an antibiotic-resistant human pathogen that produces virulence factors modulated by quorum sensing (QS) and can form biofilms. The type II PumAB TA system of pUM505, isolated from a clinical strain ofP. aeruginosa, confers plasmid stability. Additionally, the PumA toxin increasesP. aeruginosavirulence and is neutralized by the PumB antitoxin. In this study, we determined whether virulence conferred by PumA toxin is regulated by QS. ThepumA gene was transferred toP. aeruginosa lasI/rhlI, a mutant strain in the LasI and RhlI QS systems, to analyze the effect on virulence of the transformants.pumA transfer did not increase bacterial virulence in lettuce andCaenorhabditis elegans, suggesting that the virulence conferred by PumA requires QS modulation.pumA mRNA levels drastically decreased in theP. aeruginosa lasI/rhlI (pUC_pumA) strain, suggesting positive regulation ofpumA gene expression by QS. Supplementation of the growth medium ofP. aeruginosa lasI/rhlI (pUC_pumA) with C4-AHL and 3-oxo-C12-AHL autoinducers increasedpumA mRNA levels and restored bacterial virulence, suggesting that both autoinducers complemented the mutations and positively regulated the toxic effects of PumA. This strengthened the hypothesis that QS regulates bacterial virulence conferred by the PumA toxin. Thus, this report establishes an important function of QS in the virulence conferred by plasmid-encoded TA systems in bacterial pathogens.