Analysis of Congo red-induced changes in the cell surface and macrocolony structure of the bacterium Azospirillum brasilense

Adsorption of the vital dye Congo red suppresses swarming of Azospirillum brasilense in a semiliquid medium, and the bacteria become able to spread with the formation of microcolonies. By using direct and stereoscopic light microscopy, the patterns of the front of Azospirillum spreading in a semiliquid medium containing the dye were analyzed. It was found that in a medium with Congo red, small motile colonies were formed among the individual cells, and once formed, they left the boundaries of the swarming front. The microcolonies produced by azospirilla in the presence of the dye were ordered bacterial structures, rather than random cell aggregates. Transmission electron microscopy revealed that the cells grown without the dye had polar flagella, whereas the cells from the medium with Congo red had no flagella and were covered with a layer of fibrillike material. Immunochemical data for the cell surface changes resulting from interaction with the dye make it possible to consider Azospirillum lipopolysaccharide as a probable Congo red receptor.