The more we know about how pathogenic bacteria arrive to and colonize our bodies, the better we can take measures to help our immune system combat them. While most bacteria that we coexist with are beneficial to us or at the very least, neutral, our body is constantly battling against microorganisms that may harms us.
In many infections, bacteria are distributed in clumps, or “aggregates”. The size of aggregates and their distance from each other can impact the development of an infection. The aggregate strategic map is also modified by cell-to-cell communication among bacteria. This communication can be within the aggregate, or between aggregates. Communication happens to coordinate an action, much like school of fish; when bacteria sense that they have a quorum (i.e., that their colony is big enough), they release a chemical signal that is “seen” by everyone in the group and which triggers a group response. Scientists call this mode of communication “quorum sensing”.
Scientists are trying to understand exactly how the microbial war plays out, to know with utmost precision how many bacteria settle, how fast they grow, how many explore the infected tissue to settle somewhere else and reproduce, and how each aggregate talk to each other. To accomplish this, scientists need to watch how bacterial aggregates behave in real time and on the battle ground. But taking samples of bacteria from infected organs, or keeping organs “alive” for long periods of time is essentially impossible. To get around this problem, Sophie Darch at Dr. Whiteley’s lab came up with an ingenious strategy.
Dr. Darch chose to study Pseudomonas colonization in cystic fibrosis patients. Cystic fibrosis is a genetic disease that causes persistent lung infections. A defective gene (which has to be present in both parents) causes the lungs to build up too much mucus. The mucus clogs the airways and traps bacteria, leading to infections, extensive lung damage, and eventually, respiratory failure.
To study Pseudomonas behaviour during lung infection, Dr. Darch and her collaborators created a system that mimics the lung environment, and used it to eavesdrop on how Pseudomonas aggregates communicate to become efficient colonizers. First, they had to create artificial sputum, which is a kind of mucusy substance secreted by infected lungs. Bacteria find this a very comfortable place to make their homes in. Once they figured out the proper consistency and composition of artificial sputum, they created micro-cages by leveraging a recently developed technique called micro-3D printing. These state-of-the-art micro-cages trap bacteria but let chemicals through, allowing chemical communication among bacterial aggregates.
What they wanted to know was the minimum number of Pseudomonas cells that needed to be in one aggregate to send a quorum-sensing chemical message to other aggregates, and how far away that signal could reach. How did Dr. Darch control the number of Pseudomonas in each caller group? As cells grow to fill out the cage space completely, they 3D-printed microcages of different sized to form aggregates with different cell numbers. The larger the cage, the more bacteria they had. Then, they squirted artificial sputum with “responder” Pseudomonas; cells that could “see” the chemical signal, but not produce it. They were able to see who responded, because they modified the cells to turn from red to green upon responding. Those that didn’t see or acknowledge the signal stayed red; those that did see the signal, turned green.
Dr. Darch found that the minimum number of signal-producing cells that generated a response in far-away aggregates was 2,000. Also, using a microscope, they were able to determine that the farthest “calling distance” of quorum signals arising from the caged aggregate that produced the signal was about 60 cells long (about a 5th of a millimeter, which is quite far away in the microbial world). Finally, they found that even though a colony of bacterial cells are essentially clones of each other and thus identical copies, each aggregate did not respond in an identical way to each other. The minimum concentration of the chemical signal needed for an aggregate to respond was variable. However, in general terms, the farther the signal was generated, the lesser the proportion of cells in an aggregate responded.
With this innovative approach, Dr. Darch is probing with incredible precision the dynamics of this complex system. A more precise understanding of where bacteria are hiding and how they respond to chemical signals and work together may help scientists better model infection patterns and find more specific approaches to fight infection.