Most of our knowledge of bacteria comes from the study of bacteria swimming around in liquid but there is a growing consensus that how bacteria interact physically with each other and with surfaces is crucial to understanding their behavior. A new study appearing in the journal Science sheds some light on this consensus by showing that when interacting with surfaces, bacteria use nanoscale tentacles called pili as their sense of touch.

In the article, "Obstruction of Pilus Retraction Stimulates Bacterial Surface Sensing," a group of interdisciplinary researchers show that in the case of the bacterium Caulobacter crescentus, a model organism that splits its life between swimming and attaching to surfaces, physically blocking the retraction of its pili triggers the attachment to surfaces. Pili are ubiquitous microbial appendages that many types of bacteria possess. During their study, the scientists found a new technique to observe and film Caulobacter crescentus pili undergoing dynamic cycles of extension and retraction. They were also able to measure the forces exerted by the retraction of these nanometric tentacles and noticed that within seconds of contact with a surface the cycles ceased. This impediment of the retraction of the pilus coincided with the excretion of the adhesive holdfast, a chemical glue required for firm attachment. Therefore, bacteria need the resistance to pilus retraction that occurs upon contact with a surface in order to sense surfaces and commit to a tighter adherence. Therefore, bacteria need the resistance to pilus retraction that occurs upon contact with a surface in order to sense surfaces and excrete the glue that makes them firmly adhere.

Understanding this mechanism of surface sensing in Caulobacter crescentus "might help us understand how other bacteria sense surfaces and control their growth, either to improve it in the case of good bacteria, used for instance in bioreactors, or curb it as in the case of hospital pathogens growing on medical catheters," said Dr. Nicolas Biais, Assistant Professor of Biology at Brooklyn College and The Graduate Center of the City University of New York (CUNY).