An experimental study by biologists at the École Polytechnique Polytechnique de Lausanne (EPFL) shows how bacteria move across surfaces through twitching by extension and retraction of extracellular filaments (type IV pili) but also through a true sense of touch. These observations, which are the first of their kind and are made here on the bacterium Pseudomonas aeruginosa, not only shed light on how disease infections develop, but also suggest that by identifying the molecular mechanism underlying the bacteria’s tactile sensitivity, we may find a way to at least partially deprive them of the ability to spread.
Many pathogenic bacteria, such as Pseudomonas aeruginosa, crawl on surfaces thanks to a movement somewhat similar to muscle contractions: their type IV pili, or filaments, are known to cause these contractions, but the sensory signals that coordinate these movements are unknown. Here, a team of Swiss biologists has identified a mechanism similar to our sense of touch that allows bacteria to navigate on surfaces.
A new understanding of bacterial motility
Scientists know that human and animal cells can move on a variety of rigid and soft surfaces, but they do not know whether bacteria can actually control their movement. In fact, most research has focused on the mechanisms that cause bacteria to move in the direction of chemical compounds (or chemical stimuli) called chemotaxis.
In this study, the team focused instead on how bacteria sense, respond to and exploit mechanical forces (Mechanotaxis).
Previous studies have shown that the pili in Pseudomonas act like harpoons: Once the pilus has elongated and reached the surface, the pilus activates a molecular motor that pulls the filament back and propels the cell forward.
On the coordination of the motor activity of the pilus: here, the team is investigated how Pseudomonas bacteria move on the surface, for example at the bottom of a petri dish. The team suspected that a network of proteins called the “Chp” system might regulate the contractions. The team showed that bacteria lacking different components of the Chp system are no longer able to get around. Although they continue to move they are unable to orient themselves or move around an obstacle.
Finally, some bacteria, including Pseudomonas aeruginosa, use their own pili to regulate toxin secretion.
By combining fluorescent markers and microscopy techniques that allow the tracking of pili in living cells, the researchers were able to identify:
- A messenger protein that causes the pili to elongate, allowing bacterial cells to move forward.
- Another protein inhibits the formation of pili at the front of a moving cell to prevent it from stopping.
- When the bacteria come into contact with an obstacle, such as another cell, the inhibitor allows them to stop and change direction: in this way, the cells navigate according to what they see in front of them.
“It is a bit like a blind person using a white cane.” explains lead author Alexandre Persat, professor at the EPFL School of Biosciences. This ability to sense the environment is particularly useful when bacteria move in groups, as it allows them all to move in the same direction.
What are the consequences? The way bacteria move helps explain how they invade and colonize the surface. In the case of Pseudomonas aeruginosa, for example, this already has important consequences, as it is one of the main causes of disease. Pools of Pseudomonas bacteria form on surfaces such as catheters and ventilators and can be highly resistant to disinfectants and antimicrobials.
How bacteria translate mechanical stimuli into cellular responses remains to be elucidated. The molecular mechanisms underlying tactile sensitivity in bacteria could be a promising antimicrobial target.