The Swarming Bacteria
This is a great bit of research on bacterial biofilms, published open access in the Proceedings of the National Academy of Sciences journal. Researchers discovered that bacteria build an intricate network of tunnels within the biofilm that helps them to expand the colony, and that the coordinated self-organization required for this is facilitated by DNA.
• Even though bacteria are single celled organisms, under certain situations they can display multicellular behavior. Bacterial biofilms are multicellular communities of bacteria that live in a self-produced ‘gel’, made with various proteins and sugars and DNA.
• Biofilms can form on living or non-living surfaces; the bacteria ‘anchor’ themselves to the surface, and it is possible for a biofilm to contain many different types of microorganisms, with each group performing specialized metabolic functions.
• This self organization requires the bacteria to move in a unique manner in order to coordinate this process. Researchers captured this movement by carrying out a time lapse recording of Pseudomonas aeruginosa at one frame per 2 seconds, and visually inspected a 1000-frame time series. They also developed an automated cell-tracking algorithm to identify and track the movements of all individual bacterial cells.
• The researchers observed that the migration of cells at the very leading edge of an advancing population creates a “trail” that appears very bright when imaged using a technique known as phase-bright microscopy. The cells that follow are able to migrate along this bright trail left by the leading cells, and the following cells were confined to the trail.
• Intriguingly, these bright trails remained even after extensive washing, indicating that it was not composed of a bacterial “slime” substance. This led the researchers to believe that these trails were physical furrows, plowed into the media by the leading bacteria. To examine that, they used atomic force microscopy to analyze the surface of the media. Indeed, the bright trails were physical furrows caused by the leading bacteria, similar to the action of skis moving across snow.
• Further observations suggested that the coordinated movements by the bacteria requires a constant supply of cells to keep pushing the leading cells, to “bulldoze” their way across the lip of the furrow and migrate into uncharted territories!
• The researchers also discovered that this biofilm expansion requires extracellular DNA. By using an enzyme that digests DNA (DNase I) in the media, they demonstrated that biofilm expansion was significantly reduced (see movie here.)
• The extracellular DNA acts to prevent ‘traffic jams’ by aligning the cells at the leading edge; it is thought that this DNA physically interacts with the bacterial pili, which are hair-like appendages found on the surface of the bacterial cell.
• Bacterial biofilms are estimated to be responsible for 80% of all microbial infections in the body and are also involved in the development of antibiotic resistance. Therefore, gaining a deeper understanding of the intricacies of bacterial self-organization in the context of biofilms is useful for finding new ways of reducing the impact of biofilms on our health.