Branching dendrite patterns originated from the point of inoculation. The dendrites thickened and further branching from the original dendrite arms was observed through time. A full swarming pattern was usually observed

3–4 weeks after inoculation. The swarm front is preceded by a clear slimy layer (Fig. 1, inset), which appeared to be devoid of bacteria as observed under phase-contrast microscopy (data not shown). Differentiation into swarmer cells usually involves remarkable changes in cell morphology, such as hyperflagellation and cell elongation (Fraser & Hughes, 1999). To determine whether SCH772984 R. leguminosarum swarmer cells exhibit these morphological changes, transmission electron microscopy

was used to examine vegetative and swarmer cells (Fig. 3). Cells at the edge of the swarming colony of VF39SM are hyperflagellated (Fig. 3c). The number of flagella in swarmer cells increased three to five times when compared with the vegetative cells. VF39SM vegetative cells exhibited four to seven flagella per cell, whereas the swarmer cells exhibited around 21 flagella per cell (Fig. 3a and c). Rhizobium leguminosarum 3841 vegetative cells had an average of two subpolar flagella, while the majority of the swarmer BMS-907351 cells had three flagella per cell (Fig. 3d and e). A t-test on the number of flagellar filaments indicates that the differences observed

between 3841 vegetative and swarmer cells are statistically very significant at P<0.0001 (Student’s t-test). Notably, VF39SM swarmer cells have substantially more flagella compared with 3841 swarmer cells, and the additional flagellation may contribute to the difference in the swarming pattern of the two rhizobial strains described above (Fig. 2b and f). The hyperflagellated cells are not elongated and the cells appear to be of the same size as the vegetative cells. Cells obtained at the center of the swarming colony (at the point of inoculation) demonstrated the same number of flagella (Fig. 3b) and the same cell length as the vegetative cells. It has also been observed that the swarmer cells are arranged in rafts, with the adjacent cells connected together along their long axis (Fig. 3f). The expression of the motility-related genes flaA, rem, and visN in VF39SM swarmer cells was compared with gene expression in nonswarming cells. The expression of flaA increased sixfold under swarming conditions compared with broth cultures, while visN increased expression threefold (Fig. 4). Gene expression by swarmer cells was also higher when compared with the expression of cells grown on solid medium. The flagellar regulatory gene visN showed an increase in expression to as much as 14-fold and the flaA transcript showed a 21-fold increase.