Preventing Biofilm Formation; In a Friendly Way

Cell-to-cell communication is possible in many species of bacteria via a process called quorum sensing. Within a bacterial population the cells are constantly secreting small molecules called autoinducers. These can either diffuse across the cell wall (gram-negative) or bind with a surface receptor (gram-positive) of other cells within the community. When the population gets to a certain size, the concentration of secreted autoinducers is sufficient to trigger the expression of certain genes and cause a major phenotypic shift. Quorum sensing in certain species is a concern as it can regulate antibiotic production, swarming and biofilm formation.

Disrupting quorum sensing is a growing area of research as an alternative to using antibiotics. Many human pathogens cause disease when they establish a biofilm. For example, Pseudomonas aeruginosa causes pneumonia when growing as a biofilm in the lungs. Quorum sensing disruption has also been considered for preventing biofouling of marine and fresh water structures from biofilms. A number of marine bacteria have been studied for their ability to prevent quorum sensing by secreting secondary metabolites.

In a study by Teasdale et al (2009), a marine bacterium called Halobacillus salinus was examined. This particular strain had been isolated from a sea grass that was free from any biofilms growing on its surface. To test H. salinus, the authors grew it in a cocultivation with Vibrio harveyi. V. harveyi has a quorum sensing controlled bioluminescence ability which made it a good test strain for such experiments. Both strains were grown on agar plates with dialysis membranes separating the two. Although growth of V. harveyi was not effected there were ‘dark zones’ where no bioluminescence could be seen surrounding the H. salinus cultures. A likely explanation for this was a compound H. salinus was secreting which was then diffusing across the membrane.

The authors isolated and identified two non-toxic compounds in the extracellular media of H. saline. The term non-toxic referred to the fact that they did not kill or inhibit growth of V. harveyi but only prevented community changes that were triggered by quorum sensing. These two compounds were tested further on two more species of gram-negative bacteria. These were Chromobacterium violaceum that produces a dark violet pigment called violacein and a mutant strain of Escherichia coli that produces green fluorescent proteins, both controlled by quorum sensing. Once again the phenotypic changes that would normally start once the population had reached a certain density failed to do so. Bacterial growth was also not affected.

The mode of action these two compounds were having were believed to be a mimicking effect. Their structure was very similar to the acylated homoserine lactone (AHL) autoinducer used by many gram-negative bacteria, including the three test strains used in this study. By binding the AHL receptor proteins in adjacent cells it prevents binding of the AHLs and prevents up regulation of key genes.

Although there have been a few studies over the last few years looking at inhibiting quorum sensing, furanones for example, this appears to be the first discovery of quorum sensing inhibition between species that were not closely related, as of 2009 anyway. They also claim this to be the first bacterium from the bacilli class that produces secondary metabolites capable of inhibiting quorum sensing. Novel antimicrobial drugs and antifouling products may also be a possibility.

A Review of:
Teasdale ME, Liu J, Wallace J, Akhlaghi F and Rowley DC (2009) Secondary Metabolites Produced by the Marine Bacterium Halobacillus salinus That Inhibit Quorum Sensing-Controlled Phenotypes in Gram-Negative Bacteria. Applied and Environmental Microbiology. 75: 567-572.