Novel research has elucidated the role of pili, ubiquitous microbial appendages, in bacterial surface sensing. Once bacteria recognize surface contact, they have been shown to change their behavior and form biofilms that resist antibiotics and clearance by the immune system.
The study was led by Indiana University Distinguished Professor of Biology Yves Brun and published in Science yesterday.
The team showed that bacteria use pili that extend from the cell and retract dynamically to feel and stick to surfaces and ultimately produce biofilms. The pili stop moving after sensing a surface, after which the bacteria start producing bioadhesive, that drives attachment to surfaces and biofilm formation.
To fool the bacteria into sensing a surface, Brun's team attached a large maleimide molecule to the pili to effectively block the hair-like structures' movement.
"It's like trying to pull a rope with a knot in the middle through a hole—the maleimide molecule can't pass through the hole the cell uses to extend and retract the pili," said Courtney Ellison, the study's lead author and a Ph.D. student in Brun's lab.
"These results told us the bacteria sense the surface like how a fisherman knows their line is stuck under water," Brun added. "It's only when they reel in the line that they sense a tension, which tells them their line is caught. The bacteria's pili are their fishing lines."
"By using florescent dyes to label these microscopic structures, we're able to produce images that show the first direct evidence of the role that pili play to detect surfaces," Brun said.
In order to observe the movement of pili, the IU team had to visualize the extremely thin structures and their movement. They did this by substituting a single amino acid within the chain of amino acids that comprise the pili with cysteine. The maleimide, which delivered the florescent dyes to the pili proteins, binds to the cysteine. The maleimide is also the molecule used to deliver the large molecule to the cysteine in the pili protein to physically block the pili movement.
"It's like switching on a light in a dark room," Ellison said. "Pili are composed of thousands of protein subunits called pilins, with each protein in the chain composed of amino acids arranged like a tangled mess of burnt-out Christmas lights. Swapping out a single light can illuminate the whole string."
Next, Brun and colleagues hope to unravel precise mechanisms that link pili movement and bioadhesive production, as the two processes appear related but the exact nature of the connection remains unknown.
Image: Pili (green) in cells from the bacterium Caulobacter crescentus (orange). IU scientists used a fluorescent maleimide molecule to stain pilin proteins that contained a cysteine molecule, which was introduced in place of one of the pili's amino acids. Image courtesy of Courtney Ellison, Indiana University.