Irreducible complexity. From ScienceDaily:
Although bacteria have no sensory organs in the classical sense, they are still masters in perceiving their environment.
Swimming Caulobacter bacteria have a rotating motor in their cell envelope with a long protrusion, the flagellum. The rotation of the flagellum enables the bacteria to move in liquids. Much to the surprise of the researchers, the rotor is also used as a mechano-sensing organ. Motor rotation is powered by proton flow into the cell via ion channels. When swimming cells touch surfaces, the motor is disturbed and the proton flux interrupted.
The researchers assume that this is the signal that sparks off the response: The bacterial cell now boosts the synthesis of a second messenger, which in turn stimulates the production of an adhesin that firmly anchors the bacteria on the surface within a few seconds. “This is an impressive example of how rapidly and specifically bacteria can change their behavior when they encounter surfaces,” says Jenal. Paper. (paywall) – Isabelle Hug, Siddharth Deshpande, Kathrin S. Sprecher, Thomas Pfohl, Urs Jenal. Second messenger–mediated tactile response by a bacterial rotary motor. Science, 2017; 358 (6362): 531 DOI: 10.1126/science.aan5353 More.
From the paper:
Elucidating a bacterial sense of touch
Bacteria can adhere to surfaces within the host. This leads to tissue colonization, induction of virulence, and eventually the formation of biofilms—multicellular bacterial communities that resist antibiotics and clearance by the immune system (see the Perspective by Hughes and Berg). Hug et al. show that bacteria have a sense of touch that allows them to change their behavior rapidly when encountering surfaces. This tactile sensing makes use of the inner components of the flagellum, a rotary motor powered by proton motif force that facilitates swimming toward surfaces. Thus, the multifunctional flagellar motor is a mechanosensitive device that promotes surface adaptation. In complementary work, Ellison et al. elucidate the role of bacterial pili in a similar surface-sensing role.
From the Abstract:
When bacteria encounter surfaces, they respond with surface colonization and virulence induction. The mechanisms of bacterial mechanosensation and downstream signaling remain poorly understood. Here, we describe a tactile sensing cascade in Caulobacter crescentus in which the flagellar motor acts as sensor. … Thus, the bacterial flagellar motor acts as a tetherless sensor reminiscent of mechanosensitive channels.