Differential expression of nonagglutinating fimbriae and MR/P pili in swarming colonies of Proteus mirabilis.

The expression of nonagglutinating fimbriae (NAF) and mannose-resistant/Proteus-like (MR/P) pili in swarming colonies of Proteus mirabilis was investigated. Elongated swarmer cells do not express pili, and the relative number of bacteria expressing NAF during swarming and early consolidation phases was very low (<5%). Relative expression of NAF in a terrace increased to approximately 30% at 48 h. We also determined the expression of NAF and MR/P pili in two phenotypically distinguishable regions of each terrace. The expression of both NAF and MR/P pili was always higher in the region closer (proximal) to the middle of the colony than in the distal region of the terrace. The relative numbers of bacteria expressing NAF or MR/P pili in the proximal region were between 39.1 and 63% and between 5.9 and 7.7%, respectively. In the distal region, expression levels were between 20.8 and 27.3% and between 3.7 and 5. 6%, respectively. A time course experiment testing NAF expression in both the proximal and distal regions of a terrace indicated that NAF expression in the proximal regions was always higher than in the distal regions and increased to a plateau 40 to 50 h after the start of the swarming phase for any given terrace. These results indicate that expression of NAF or MR/P pili in swarming colonies of P. mirabilis is highly organized, spatially and temporally. The significance of this controlled differentiation remains to be uncovered.