Molecular Switch That Controls Formation of Bacterial Biofilms Identified and Characterized

Infectious processes in which biofilms have been implicated include common problems such as urinary tract infections, catheter infections, middle-ear infections, formation of dental plaque, gingivitis, coating contact lenses, and less common but more lethal processes such as endocarditis, infections in cystic fibrosis, and infections of permanent indwelling devices such as joint prostheses and heart valves. More recently, it has been noted that bacterial biofilms may impair cutaneous wound healing and reduce topical antibacterial efficiency in healing or treating infected skin wounds.

Bacteria living in a biofilm usually have significantly different properties from free-floating bacteria of the same species, as the dense and protected environment of the film allows them to cooperate and interact in various ways. One benefit of this environment is increased resistance to detergents and antibiotics, as the dense extracellular matrix and the outer layer of cells protect the interior of the community. In some cases, antibiotic resistance can be increased by more than a thousand times.

Microbes form a biofilm in response to many factors, which may include cellular recognition of specific or nonspecific attachment sites on a surface, nutritional cues, or in some cases, by exposure of free-living planktonic cells to subinhibitory concentrations of antibiotics. When a cell switches to the biofilm mode of growth, it undergoes a phenotypic shift in behavior in which large suites of genes are differentially regulated.

Investigators at Newcastle University (United Kingdom) examined the mode of action of the SinR gene in the Gram-positive model organism Bacillus subtilis. They reported in the April 12, 2013, issue of the Journal of Biological Chemistry that the activity of SinR was controlled by its antagonists, SinI, SlrA, and SlrR. The interaction of these four proteins formed a switch, which determined whether SinR could inhibit biofilm formation by its repression of a number of extracellular matrix-associated operons.

To determine the thermodynamic and kinetic parameters governing the protein-protein and protein-DNA interactions at the heart of this molecular switch, the investigators analyzed the protein-protein and protein-DNA interactions by isothermal titration calorimetry and surface plasmon resonance. They also determined the crystal structure of SinR in complex with DNA, which revealed the molecular basis of base-specific DNA recognition by SinR and suggested that the most effective means of transcriptional control occurred by the looping of promoter DNA.

Senior author Dr. Richard Lewis, professor of structural biology at Newcastle University, said, “SinR is a bit like a rocker switch—a domestic light switch, for instance. In the "down" position, when SinR is bound to DNA, the proteins required to make a biofilm are turned off and the bacteria are free to move. In the "up" position, SinR is no longer bound to DNA and instead interacts with other proteins, and the biofilms genes are turned on.”

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Newcastle University