Aerotaxis governs floating biofilm formation in Shewanella oneidensis

Floating biofilm, or pellicle, is a biofilm found at the air-liquid interface. Here, we show that pellicle development of the aquatic bacterium Shewanella oneidensis is under the control of the chemotaxis system (Che3), a regulatory system known to pilot planktonic cell motion according to environmental cues. Deletion of the histidine kinase cheA3 or the response regulator cheY3 gene led to a heterogeneous pellicle or to the absence of pellicle respectively. In addition, a non-phosphorylatable CheY3-D56A mutant was unable to promote pellicle formation. Kinetic analysis revealed that pellicle formation occurs in three steps: rapid formation of a thin pellicle evolving into a heterogeneous biofilm and finally into a thick homogeneous biofilm. Depletion of oxygen not only abolished initiation of pellicle formation but also blocked pellicle maturation. This study thus demonstrates an essential role of aerotaxis (chemotaxis towards oxygen) in floating biofilm development in S. oneidensis, and it also reveals that pellicle formation is a step-by-step process.