Two distinctive colony morphologies were noted in a collection of Pseudomonas aeruginosa transposon insertion mutants. One set of mutants formed wrinkled colonies of autoaggregating cells. Suppressor analysis of a subset of these mutants showed that this was due to the action of the regulator WspR and linked this regulator (and the chemosensory pathway to which it belongs) to genes that encode a putative fimbrial adhesin required for biofilm formation. WspR homologs, related in part by a shared GGDEF domain, regulate cell surface factors, including aggregative fimbriae and exopolysaccharides, in diverse bacteria. The second set of distinctive insertion mutants formed colonies that lysed at their center. Strains with the most pronounced lysis overproduced the Pseudomonas quinolone signal (PQS), an extracellular signal that interacts with quorum sensing. Autolysis was suppressed by mutation of genes required for PQS biosynthesis, and in one suppressed mutant, autolysis was restored by addition of synthetic PQS. The mechanism of autolysis may involve activation of the endogenous prophage and phage-related pyocins in the genome of strain PAO1. The fact that PQS levels correlated with autolysis suggests a fine balance in natural populations of P. aeruginosa between survival of the many and persistence of the few.Pseudomonas aeruginosa, although readily defined taxonomically (69), shows no evidence of being constrained to a particular bacterial lifestyle (72). Indeed, this opportunistic pathogen proliferates within hosts as varied as plants, insects, nematodes, and mammals and interacts with amoebae and yeasts as well (13,16,31,44,58). The broad host range of P. aeruginosa can be used both to uncover the role of specific virulence factors and to characterize new regulatory systems controlling virulence (10,12,13,16,21,44,58). One study identified transposon insertion mutants of P. aeruginosa strain PAO1 that were less virulent in the fruit fly model host (16). Most such strains had an insertion in a chemotaxis-like regulatory locus (PA0408 to PA0417) required for surface spreading by twitching motility and controlling unknown factors required for efficient fly killing. Based only on their distinctive compact colony morphology, a large collection of these mutants was assembled for testing in the fly model.In this same study (16), two even more striking colony morphologies were noted, and strains of each type were saved. One set of mutants formed wrinkled colonies and had additional properties described for autoaggregating cells of Pseudomonas fluorescens (59,68) and Salmonella enterica serovar Typhimurium (61,62,83). The second set of mutants formed colonies with visible lysis, a characteristic noted in some of the earliest descriptions of P. aeruginosa isolates (5, 6, 26, 32), but for which the molecular basis has been elusive. Since autoaggregation and autolysis are likely to reflect fundamental aspects of the biology of P. aeruginosa, the genetic basis of the two mutant colony morphologies was explored. Autoaggregat...