The refractory nature of Pseudomonas aeruginosa infections is due in part to the presence of specialized cells, termed persisters, within the population. To identify genes involved in P. aeruginosa persister formation, a PAO1 transposon (Tn) library was challenged en masse with 1,000 μg/ml of carbenicillin and was enriched for mutants that were able to survive in the presence of this antibiotic. For one mutant that was further characterized, the carbenicillin MIC was equal to that of PAO1, but persister formation exhibited a 20-fold increase after exposure to the antibiotic. Sequence analysis revealed that the Tn had inserted into PA4115, a gene encoding a putative lysine decarboxylase. A PA4115 mutant that produced 48-fold and 20-fold more survivors than PAO1 after 10-h exposures to carbenicillin and ticarcillin, respectively, was generated by allelic exchange. Furthermore, the rate of carboxypenicillin-induced lysis was reduced in the PA4115 mutant. Under certain pH conditions, lysine decarboxylase converts lysine to cadaverine. By measuring cadaverine production, we discovered that the PA4115 mutant had significantly reduced lysine decarboxylase activity. To determine if reduced cadaverine levels are responsible for the increase in carbenicillin and ticarcillin persistence, viability and lysis assays were performed in the presence of exogenous cadaverine. Cadaverine increased the rate of killing and lysis of the PA4115 mutant in the presence of both antibiotics. These findings suggest that cadaverine may be able to enhance the effectiveness of carboxypenicillins against P. aeruginosa by reducing persister formation.