Bacteria have invented a potentially global language - quorum sensing. Kim Janda of the Scripps Research Institute in La Jolla, US, translates

Over the course of history, humans have developed countless ways to communicate with each other, and over 6800 languages have been catalogued. Despite these advances, our verbal relations with other species remain somewhat limited to gestures and shouts, as can be seen in the case of dogs and their owners. On a microscopic level, bacteria can communicate with one another through a different language, one based on small molecules, using a mechanism known as quorum sensing (QS). In contrast to humans' limited verbal communication capacities, QS allows communication and interaction with other bacterial species, and even other organisms such as mammals.

Bacterial language relies on the exchange of small chemical signals, called autoinducers. Through this exchange, bacteria monitor their density and regulate gene expression in a population-dependent manner. This allows them to coordinate their behavior and function, equipping the bacterial communities for competition or cooperation with multicellular organisms. A classic example is the symbiosis between the Hawaiian bobtail squid Euprymna scolopes and the luminescent bacterium Vibrio fischeri. In this relationship, the bacteria provide the squid with luminescence, allowing it to blend in with the moonlight while feeding, and so avoid casting shadows on the sea floor which would alert both predators and prey. At the same time, the bacteria also benefit, as they receive nutrients and safety.

QS has traditionally been referred to as a communication mechanism between bacteria within one species. However, research is emerging that implicates a role for QS in interspecies communication and competition, and such systems have been proposed to exist in a wide variety of bacteria. Particularly relevant to interspecies communication is the autoinducer 2 (AI-2)-based QS system, which has been suggested to function in over 50 bacterial species. Recently, it was shown that Actinomyces naeslundii and Streptococcus oralis, two bacteria responsible for oral plaque formation, require AI-2 production to initiate plaque development.

But communication amongst bacterial species is not always so cooperative; certain autoinducers and their by-products have been shown to have cytotoxic effects on other bacteria. Pseudomonas aeruginosa is especially adept at this intercellular competition, in that at least two autoinducer-derived molecules exhibit detrimental effects towards other bacteria, most notably Staphylococcus aureus. This activity may give P. aeruginosa a competitive advantage over S. aureus in the lungs of cystic fibrosis patients, a clinical setting plagued by infections due to these two pathogens.

In addition to helping bacteria organise their behaviour and functions, recent research suggests QS is a means for bacteria to interact with other organisms. Similar to bacterial interspecies relations, QS systems may mediate this interkingdom signalling either through host cell recognition of bacterial signals or through the unregulated action of an autoinducer on the host cell. Several studies have detailed the effects of AHL (N-acylhomoserine lactone)-based signalling molecules on human cells - the responses ranging from immuno-activation to cell death.

A potential communication language between humans and Escherichia coli has also been described. E. coli responds to two human-derived small molecule signals, adrenaline and noradrenaline, to regulate virulence expression. For this same purpose, E. coli also employs a small molecule of its own production, termed AI-3. Based on the role of E. coli in the gastrointestinal tract, and the overlap between bacterial recognition of AI-3 and adrenaline, AI-3 has been suggested to play a role in maintaining intestinal homeostasis.

Because QS can mediate so many relationships, it may represent a global language that spans every kingdom of life. Human interpretation may impart a deeper knowledge of bacterial lifestyles and provide the opportunity for an appropriate response, at least one of which would be developing pharmacological interventions for bacterial infection.

Read more in the tutorial review 'Interspecies and interkingdom communication mediated by bacterial quorum sensing' in issue 7, 2008 of Chemical Society Reviews.