Flagellen-vermittelte Motilität in Shewanella : Mechanismen zur effektiven Fortbewegung in S. putrefaciens CN-32 und S. oneidensis MR-1

Bacteria move efficiently by rotating helical proteinacious filaments called flagella. This elegant type of movement enables microorganisms to migrate towards favorable conditions in a constantly changing environment. Through chemotaxis, bacteria respond to specific stimuli and modulate function and output of their flagellar systems to be able to follow gradients. Bacteria have evolved their flagellar motility repertoire to match requirements of their specific habitats, leading to a high degree of variability of flagellation between organisms. Some bacteria elaborate secondary lateral flagella in addition to their polar flagellar systems, enabling movement through highly viscous environments or across surfaces. Strict regulatory circuits control production and maintenance of the costly lateral flagellar system. Shewanella putrefaciens CN-32 harbors two flagellar systems assembling a Na+-driven polar flagellum and H+-driven lateral flagella. Both systems exhibit significant similarity to dual flagellar systems in other bacterial species. Surprisingly, a subpopulation of S. putrefaciens CN-32 elaborates one or two randomly localizing lateral flagella already during exponential growth in complex liquid media. Phenotypical analyses of defined in frame deletion mutants in concert with advanced fluorescence microscopy demonstrate that, despite the synchronous assembly, structural components are highly specific to their corresponding flagellar system. A single chemotaxis system specifically controls rotational switching of the bidirectional polar flagellum, but it has no influence on unidirectional rotation of lateral flagella. Rotating polar flagella is sufficient to mediate full swimming speed of single cells in liquid environment. However, cells producing additional lateral flagella display a significantly increased directed swimming behavior in liquid or structured habitats, indicating a role of lateral flagella in efficient reorientation of cell bodies in their environment. This so far unknown mode of bacterial swimming may rely on functional interaction of polar and lateral flagella and, thus, may represent a novel mechanism of exploring new habitats under certain circumstances. Additionally, the discovery of dual flagellar systems in a growing number of bacteria inhabiting aquatic environments may point towards similar flagellar functions in other organisms. A second representative of the family of Shewanellaceae, Shewanella oneidensis MR-1, uses a single polar flagellar filament consisting of two highly homologous flagellin subunits, FlaA and FlaB, and a dual torque-generating stator system to effectively move at different sodium ion concentrations. Posttranslational modification of flagellins is essential for both the assembly of the flagellar filament and subsequent motility. Sophisticated MS-analyses in accordance with single-residue substitutions performed in this study identified at least 4 sites of O-glycosidic modification for both FlaA and FlaB. Detailed structural analyses revealed that the modification likely consists of a basal 274 Da pseudaminic-acid (Pse) derivative and a directly attached unknown moiety of 264 Da. A S. oneidensis MR-1-specific operon, designated as sfmABCDE, is involved in flagellin glycosylation. This operon is arranged within a gene region comprising glycosylation-related genes that is partially conserved among other Shewanella species implicating a conserved glycosylation mechanism using variable glycan moieties in a species-specific manner for these Shewanella strains. In this study, I could demonstrate that a synchronously working dual flagellar system, as well as flagellin glycosylation is crucial for effective swimming of S. putrefaciens CN-32 and S. oneidensis MR-1, respectively. In summary, studying mechanisms of flagella-mediated motility in these two different Shewanella species may expand our knowledge about bacterial motility and has implications for the evolution of highly homologous protein complexes and interacting bacterial regulatory control circuits in response to changing environmental conditions.