Swarmer cell differentiation is a complex process involving the activity of many gene products. In this report, we characterized the genetic locus of Tn5 insertion in each of 12 mutants defective in swarmer cell elongation. The mutations fell into four categories affecting either flagellar biosynthesis or energetics, lipopolysaccharide and cell wall biosynthesis, cellular division, or proteolysis of peptides.Proteus mirabilis swarmer cell differentiation and swarming motility are the end result of at least four separate processes (1, 6). First, the cells must sense cues from the environment, e.g., inhibition of flagellar rotation (6) and the concentration of glutamine (2), and respond accordingly by initiating the transcription of 25 to 50 genes whose expression ultimately produces the differentiated cell. Second, surface-induced transcription of genes whose products inhibit cell septation must produce an elongated cell. Third, the cell must synthesize vastly increased amounts of flagellin for new surface-induced flagella and must express the appropriate genes whose products will control the rotation of the flagella (7, 10). Fourth, multicellular interactions and signaling must occur between swarmer cells to allow them to move out on the agar surface in groups and to consolidate (dedifferentiate) in unison (6). If any one of these events is defective, the end result will be either complete loss of swarming motility or abnormal swarming behavior.In this report, we concentrate on the second feature of swarmer cell differentiation, the surface-induced elongation of cell length. The process of elongation is believed to be due to an inhibition of the normal septation mechanism and takes place with only a slight increase in cell width, often resulting in nonseptated cells 40 to 80 m in length. The overall goal of this research is to understand the molecular mechanism controlling surface-induced elongation of the P. mirabilis swarmer cell.Transposon mutagenesis and phenotypic screening of mutants defective in swarmer cell elongation. Because cellular elongation is intimately involved in the process of swarming and differentiation, we examined a bank of 212 swarming null mutants (Swr Ϫ ) and crippled mutants (Swr cr ; those strains that produce aberrant swarming behavior) previously produced through Tn5-CM (15) mutagenesis (8, 9) for defects in swarmer cell elongation. Swarmer cell elongation was assessed by inoculating 5 l of an overnight culture in the center of a petri dish containing L agar (8). After 4 to 6 h of incubation at 37ЊC, 100 l of 1ϫ phosphate-buffered saline (PBS; 20 mM sodium phosphate, 100 mM NaCl [pH 7.5]) was used to wash swarmer cells from the edge of the colony. The cells in this suspension were then diluted as required in 1ϫ PBS and viewed by phase-contrast microscopy. Cell length was determined by taking photomicrographs of cells under inducing (growth on agar) and noninducing (liquid culture) conditions. The average length of 50 cells was digitally analyzed, and the mean cell length (Ϯ1 standard...