The O’Toole lab is interested in how bacteria transition from a free-swimming (planktonic) lifestyle to life on a surface in a biofilm.  Biofilm formation can be thought of as a bacterial group behavior.  Our recent studies indicate that in or model microbe, Pseudomonas aeruginosa, biofilm formation is co-regulated with a second group behavior known as swarming motility.  Swarming motility is a form of movement that allows bacteria to move across surfaces.  We have shown that the co-regulation of these group behaviors relies on an interesting intracellular signaling molecule called cyclic-di-GMP.  High levels of c-di-GMP favor a sessile lifestyle, while low levels of this signal favor motility.

On the left are motile bacteria, including P. aeruginosa swarming across an agar plate.  The right side of the figure shows biofilm formation.  The lower right panel is an EM of a Pseudomonas biofilm showing the typical polysaccharide matrix.  In the center of the figure is shown the structure of the signaling molecule cdiGMP.

                                                            From O’Toole, Microbe, 2008.

The ability to control these two group behaviors relies on the opposing activities of two different classes on enzymes.  The diguanylate cyclases synthesize c-di-GMP, and are characterized by a so-called “GGDEF” domain.  Phosphodiesterases degrade c-di-GMP, and these enzymes are often associated with an “EAL” domain.  These reactions are depicted in the figure above.

We have spent the past several years studying a variety of proteins involved in c-di-GMP metabolism.  This work is summarized in the figure below.  We have found that c-di-GMP controls the synthesis of an important exopolysaccharide (EPS) called Pel AND function of the flagella, possibly via the chemotaxis-like cluster IV (CheIV).  When c-di-GMP levels are high, this results in more EPS production, decreased flagellar function and the promotion of biofilm formation.  In contrast, low c-di-GMP is associated with decreased EPS production and stimulation of flagellar function, which results in swarming motility.  Understanding how biofilm formation and swarming are coordinately regulated is one key goal of our group.

The SadC enzyme synthesizes cdiGMP and the BifA enzyme degrades this signal.  Levels of cdiGMP control two different outputs: EPS production, via the Pel proteins, and motility, via the CheIV chemotaxis-like system and the flagellum.

                                                            From O’Toole, Microbe, 2008.

Check out some recent publications from the lab describing our work:

Merritt JH, Brothers KM, Kuchma SL, O'Toole GA. 2007. SadC reciprocally influences biofilm formation and swarming motility via modulation of exopolysaccharide production and flagellar function. J Bacteriol. 189:8154-64.

Kuchma SL, Brothers KM, Merritt JH, Liberati NT, Ausubel FM, O'Toole GA. 2007. BifA, a cyclic-Di-GMP phosphodiesterase, inversely regulates biofilm formation and swarming motility by Pseudomonas aeruginosa PA14.  J Bacteriol. 189:8165-78.

Toutain CM, Caizza NC, Zegans ME, O'Toole GA. 2007.  Roles for flagellar stators in biofilm formation by Pseudomonas aeruginosa.  Res Microbiol. 158:471-7.

Caiazza NC, Merritt JH, Brothers KM, O'Toole GA. 2007. Inverse regulation of biofilm formation and swarming motility by Pseudomonas aeruginosa PA14.  J Bacteriol. 189:3603-12.

Monds RD, O'Toole GA. 2009. The developmental model of microbial biofilms: ten years of a paradigm up for review.  Trends Microbiol. 17:73-87.