Differentiation of Serratia marcescens 274 into swimmer and swarmer cells

We describe a new sensory response in the enteric bacterium Serratia marcescens. When grown in liquid media, the bacteria were short rods with one to two flagella and displayed classical swimming behavior. Upon transfer to a solid surface (0.7 to 0.8T% agar medium), the bacteria underwent a dramatic change of form. They ceased septation, elongated, and expressed numerous (10 to 100) flagella that covered the lateral sides of the cells. The bacteria now displayed a different form of locomotion--swarming--which allowed them to rapidly move over the top of the solid surface. The differentiation to either swimmer or swarmer cells could be reversed by growth on solid or liquid medium, respectively. To identify conditions that influence this differentiation, the growth environment of S. marcescens was manipulated extensively. The swarming response was monitored by visual and microscopic observation of cell movement on solid surfaces, by immunofluorescent labeling followed by microscopic observation for the presence of elongated, profusely flagellated cells, as well as by estimation of induction of flagellin protein, using Western immunoblot analysis. Conditions that imposed a physical constraint on bacterial movement, such as solid or viscous media, were the most efficient at inducing the swarming response. No chemical constituent of the medium that might contribute to the response could be identified, although the existence of such a component cannot be ruled out. Both swimmer and swarmer cells had flagellin proteins of identical molecular weight, which produced similar proteolysis patterns upon digestion with trypsin.