While a chattering crowd of various species of bacteria is essentially a microbial tower of Babel, certain snippets of their chemical conversation are almost universally understood. Howard Hughes Medical Institute (HHMI) researchers have found that bacteria of different species can talk to each other using a common language – and also that some species can manipulate the conversation to confuse other bacteria.
The interspecies crosstalk and misdirection could have important consequences for human health, said Bonnie L. Bassler, an HHMI investigator at Princeton University whose study was published in the September 29, 2005, issue of Nature. "The ability of cells to communicate with one another and the ability to interfere with the communication process could have consequences in niches containing competing species of bacteria or in niches where bacteria associate with humans," Bassler said. "In the gut, you can imagine how the normal microflora might interfere with cell-cell communication to thwart bacterial invaders."
Using a chemical communication process called quorum sensing, bacteria converse among themselves to count their numbers and to get the population to act in unison. A synchronized group of bacteria can mimic the power of a multi-cellular organism, ready to face challenges too daunting for an individual microbe going it alone. Swelling populations trigger their quorum-sensing apparatuses, which have different effects in different types of bacteria. One species might respond by releasing a toxin, while another might cut loose from a biofilm and move on to another environment.
Each species of bacteria has a private language, but most also share a molecular vernacular that Bassler's lab discovered about 10 years ago. A chemical signal called autoinducer-2 (AI-2), originating from the same gene in all bacteria, is released outside the cell to announce the cell's presence. Nearby bacteria take a local census by monitoring AI-2 levels and conduct themselves as the circumstances warrant.
Researchers have speculated that AI-2 is a universal language, and the new study from Bassler's lab is the first to show those conversations taking place – and producing consequences -- between co-mingling species.
Postdoctoral fellow Karina Xavier mixed E. coli, beneficial bacteria that live in the human gut, with Vibrio harveyi, a marine species that naturally glows in the dark in the presence of a crowd. In the test tube, AI-2 production by either species turned up the marine bacteria's light and turned on the quorum-sensing genes in E. coli. That confirmed what the scientists already suspected: the linguistic versatility of AI-2.