Program Affiliation: Molecular Biology & Genetics | Microbiology

Research Groups Affiliation: Genetics | Microbiology

Phone: 812/856-2523
Fax: 812/855-6705
Email Dan

Multicellular behavior and bacterial domestication.  Bacteria were long thought to take up nutrients, grow, and divide as individual unicellular organisms.  However, this strict unicellular view has been challenged by the discovery of bacterial cell-to-cell communication systems and the observation of rudimentary multicellular behaviors.  Perhaps one reason that bacterial multicellularity had gone unnoticed was the fact our cultivation techniques have selected against cell-cell interactions in favor of rapid growth as dispersed cells.  Such is the case for our model system of choice, Bacillus subtilis.  Laboratory strains of B. subtilis have been selected for ease of manipulation to create a powerful model genetic system at the expense of more complex biology.  Our research returns to the study of wild strains of B. subtilis and two of the multicellular behaviors that were lost during laboratory strain domestication: swarming motility and biofilm formation.  Each behavior appears to be an opposing and exclusive multicellular state in which swarming favors population dispersal while biofilm formation favors persistence.

Swarming motility.  Swarming motility is a social form of migration in which cells associate in multicellular clusters to cooperatively propel a population over solid surfaces.  Swarming is similar to swimming motility in that both motile behaviors are powered by rotating flagella.  However, unlike swimming, swarming requires a solid surface, surfactant secretion, and a critical cell density.  We have identified a variety of mutations that specifically impair swarming motility and many of these mutations are within genes of previously unknown function.  Our lab uses a combination classical genetics, molecular genetics, and biochemistry to dissect the function of these unusual swarming specific genes.  Through the study of such genes we have discovered that B. subtilis grows a heterogeneous mixed population of short motile cells and long non-motile chains.  This heterogeneity is a form of development that is governed by a bistable regulatory switch that turns motility gene expression ON in motile cells and OFF in chains.  Proteins required for swarming motility, SwrA and SwrB, bias the switch in favor of the motile ON state.  SwrA is of particular interest as the swrA gene is mutated in laboratory strains and contributes to the inability of domesticated strains to swarm.  Future work will focus on investigating the molecular mechanisms of SwrA, SwrB, and the bistable switch that controls motility.  In addition, we would like to explore the distribution of swarming motility genes in other wild isolates to assess the prevalence of swarming in the environment.  By determining the presence and functionality of such genes in different natural isolates, we hope to gain insight into the evolutionary benefits and ecological roles of swarming motility.

Biofilm formation.  B. subtilis also creates architecturally complex, sessile aggregates called biofilms and like swarming motility, biofilm formation is robust in undomesticated strains but severely attenuated in laboratory strains.  Of critical importance to biofilm architecture is the master transcriptional regulator SinR.  SinR appears to mediate the transition from motility to biofilm formation by directly repressing genes required for the synthesis of a biofilm-stabilizing extracellular polysaccharide while simultaneously activating motility by an unknown mechanism.  To mechanistically understand the transition between these two alternative behaviors, we seek to identify the motility-related target of SinR regulation. 

Goal. The overall goal of the lab is to identify, characterize, and understand new genetic components of multicellular behavior in undomesticated B. subtilis.  With this information, we hope to create a larger model that explains how swarming motility and biofilm formation interact and in which environments each is favored.

Chu, F, DB Kearns, SS Branda, R Kolter, and R Losick.  2006.  Targets of the master regulator of biofilm formation in Bacillus subtilis.  Mol Microbiol 59:1216-1228.

Kearns, DB, and R Losick.  2005.  Cell population heterogeneity during growth of Bacillus subtilis.  Genes Dev 19:3083-3094. 

Kearns, DB, F Chu, SS Branda, R Kolter, and R Losick.  2005.  A master regulator for biofilm formation by Bacillus subtilis.  Mol Microbiol  55: 739-749.

Kearns, DB, F Chu, R Rudner, and R Losick.  2004.  Genes governing swarming motility in Bacillus subtilis and evidence for a phase variation mechanism controlling surface motility.  Mol Microbiol  52: 357-369.

Kearns, DB, and R Losick.  2003.  Swarming motility in undomesticated Bacillus subtilis.  Mol Microbiol  49: 581-590.