Free-floating DNA molecules strengthen and help to confer helical shapes within the extracellular polysaccharide matrix of Pseudomonas aeruginosa biofilms, according to microbiologist Daniel Wozniak of Ohio State University (OSU), Columbus, and his collaborators. Thus Psl, the main polysaccharide of the matrix, forms helical structures that stabilize cell-cell interactions within the biofilm and also its attachments to underlying and adjacent surfaces.  "We assumed the polysaccharide was randomly distributed on the surface, but instead there's a definite pattern," he says. "We think of DNA as an information storage element in cells, but here it clearly has a structural role." Details appeared in PLoS Pathogens online (27 March 2009).  

 
Wozniak and his colleagues use confocal scanning laser microscopy to monitor biofilm development in P. aeruginosa and in several biofilm-impaired mutants. When individual cells begin to aggregate to form a biofilm, they deposit Psl molecules along their outer surfaces, and those molecules form into sticky, corkscrew assemblages that grow into a matrix. Pieces of DNA reinforce the Psl corkscrew structure without integrating into it. DNA and Psl molecules do not overlap, but apparently act separately but cooperatively, while encasing the underlying bacteria within the biofilm, according to Wozniak. "No one had ever seen the matrix at different time points during biofilm development," he says.

"One reason the biofilm matrix is so enigmatic is that it is normally invisible," says Phil Stewart, director of the Center for Biofilm Engineering at Montana State University, Bozeman. "This gorgeous visualization of a specific matrix constituent is an essential step in learning how biofilms are built and held together."

P. aeruginosa biofilms eventually take a mushroom shape. When the biofilm reaches a critical size, some of the cells within it die, creating a cavity in the Psl matrix at the stalk end of the "mushroom," from which individual bacterial cells depart to form new colonies. Membrane-associated proteins called holins apparently control the apoptotic-like process when cells die within the biofilm, presumably by lysing their membranes and releasing DNA molecules into the extracellular space, where they help to stabilize the matrix, Wozniak says. "We're the first to show this in Pseudomonas."

A major reason that Wozniak and his collaborators at OSU focus on P. aeruginosa biofilms is that these opportunistic pathogens cause chronic and sometimes deadly infections in the lungs of cystic fibrosis patients. Excess mucus in the lungs of such patients promotes biofilm formation, leaving cells within the biofilm resistant to antibiotics and the host immune system. Since Psl is a major contributor to the matrix, inhibiting its formation or genetically knocking it out, at least in theory, could interfere with biofilm development, a strategy for combating this pathogen that Wozniak and his collaborators are exploring. Weakening the matrix by eliminating the polysaccharide component of biofilms might also render these bacteria susceptible to conventional antibiotics, he points out.

Carol Potera
Carol Potera is a freelance writer in Great Falls, Mont.

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