Swarming colonies of the bacterium Proteus mirabilis are capable of self-recognition and territorial behavior. Swarms of independent P. mirabilis isolates can recognize each other as foreign and establish a visible boundary where they meet; in contrast, genetically identical swarms merge. The ids genes, which encode self-identity proteins, are necessary but not sufficient for this territorial behavior. Here we have identified two new gene clusters: one (idr) encodes rhs-related products, and another (tss) encodes a putative type VI secretion (T6S) apparatus. The Ids and Idr proteins function independently of each other in extracellular transport and in territorial behaviors; however, these self-recognition systems are linked via this type VI secretion system. The T6S system is required for export of select Ids and Idr proteins. Our results provide a mechanistic and physiological basis for the fundamental behaviors of self-recognition and territoriality in a bacterial model system.

We announce the complete genome sequence for Proteus mirabilis strain BB2000, a model system for self recognition. This opportunistic pathogen contains a single, circular chromosome (3,846,754 bp). Comparisons between this genome and that of strain HI4320 reveal genetic variations corresponding to previously unknown physiological and self-recognition differences.

Swarming colonies of independent Proteus mirabilis isolates recognize each other as foreign and do not merge together, whereas apposing swarms of clonal isolates merge with each other. Swarms of mutants with deletions in the ids gene cluster do not merge with their parent. Thus, ids genes are involved in the ability of P. mirabilis to distinguish self from nonself. Here we have characterized expression of the ids genes. We show that idsABCDEF genes are transcribed as an operon, and we define the promoter region upstream of idsA by deletion analysis. Expression of the ids operon increased in late logarithmic and early stationary phases and appeared to be bistable. Approaching swarms of nonself populations led to increased ids expression and increased the abundance of ids-expressing cells in the bimodal population. This information on ids gene expression provides a foundation for further understanding the molecular details of self-nonself discrimination in P. mirabilis.The gammaproteobacterium Proteus mirabilis is a leading cause of recurrent urinary tract infections and displays some extraordinary behaviors (15). P. mirabilis undergoes a morphologically distinct developmental cycle during growth on surfaces (reviewed in references 8, 15, and 18). Upon contact with a surface, flagellated rod-shaped swimmer cells (1 to 2 m in length) differentiate into hyperflagellated, polynucleoid, elongated cells (10 to 80 m in length). Colonies can swarm across surfaces rapidly, and the elongated cells, called swarmer cells, do not divide during migration. Expression of many virulence genes appears to be linked to swarmer cell differentiation, and there is a distinction between genes expressed during liquid growth and those expressed during surface-associated growth (1,2,5,6,16,18). The migrating front of a swarm will periodically arrest movement; cells will dedifferentiate to short cells and consolidate through cell division. After another round of differentiation, swarmer cells will migrate forward once again. This behavior results in colonies with a bull's-eyelike pattern (reviewed in references 8 and 18).Of particular interest to us, a boundary will form between approaching swarms of different strains but not between two swarms of a single strain (Fig. 1A). The boundary formation demonstrates that P. mirabilis populations have an ability to distinguish self from nonself. We recently identified a gene cluster, idsABCDEF, involved in P. mirabilis self versus nonself recognition (10). Swarms of Ids mutants form a boundary with their parent. Two of the genes, idsD and idsE, encode functions for determining strain-specific identity. Of the remaining genes, idsB, idsC, and idsF are required for self versus nonself recognition but are not identity determinants. The idsA gene is not required for recognition, but it has been termed an ids gene because idsA is possibly cotranscribed with idsBCDEF. There is no available information on ids gene expression.Bacterial colonies can be considered either as populations of cells or as populat...