Antibodies to the lipopolysaccharide O antigen of Pseudomonas aeruginosa mediate high-level immunity, but protective epitopes have proven to be poorly immunogenic, while nonprotective or minimally protective O-antigen epitopes often elicit the best immune responses. With the goal of developing a broadly protective P. aeruginosa vaccine, we used a gene replacement system based on the Flp recombinase to construct an unmarked aroA deletion mutant of the P. aeruginosa serogroup O2/O5 strain PAO1. The resultant aroA deletion mutant of PAO1 is designated PAO1 Delta aroA. The aroA deletion was confirmed by both PCR and failure of the mutant to grow on minimal media lacking aromatic amino acids. When evaluated for safety and immunogenicity in mice, PAO1 Delta aroA could be applied either intranasally or intraperitoneally at doses up to 5 x 10(9) CFU per mouse without adverse effects. No dissemination of PAO1 Delta aroA to blood, liver, or spleen was detected after intranasal application, and histological evidence of pneumonia was minimal. Intranasal immunization of mice and rabbits elicited high titers of immunoglobulin G to whole bacterial cells and to heat-stable bacterial antigens of all seven prototypic P. aeruginosa serogroup O2/O5 strains. The mouse antisera mediated potent phagocytic killing of most of the prototypic serogroup O2/O5 strains, while the rabbit antisera mediated phagocytic killing of several serogroup-heterologous strains in addition to killing all O2/O5 strains. This live, attenuated P. aeruginosa strain PAO1 Delta aroA appears to be safe for potential use as an intranasal vaccine and elicits high titers of opsonic antibodies against multiple strains of the P. aeruginosa O2/O5 serogroup.