Bacteria need a quorum, too

many institutions formed by human societies are designed so that they can function only when a prescribed minimum number of members are present. Familiar instances are parliaments and boards of management. The idea, presumably, is to ensure that a handful of individuals do not hijack the institution. Curiously, cellular and animal societies, too, contain examples of quorum-based behaviour.

In higher animals, during the formation and development of the embryo, the process of differentiation (wherein cells commit to a specialised pathway) appears to require more than one cell to be in step at the same time: differentiation is inherently a 'group process'. Interestingly, the one known exception is the case of aberrant differentiation that we recognise as a cancer; that seems to be the result of a heritable change that occurs in a single cell.

In recent years, much to the surprise of most biologists, a number of experiments have shown that such group behaviour is also found in bacteria, along with the concomitant prerequisite of an ability to sense whether a quorum is present or not. The surprising thing is that bacteria are believed to be quintessentially single-celled creatures who have nothing to do with one another except for the occasional exchange of dna .

A well-studied example of 'quorum sensing' in bacteria is light production by the luminous bacterium Vibrio harveyi. Individuals of this species live in pouches in fish and some other marine organisms and emit light -- but only when their number exceeds a threshold. The plausible explanation is that when they are in a small number, they assume that there is enough room for them to keep growing. But when it starts to become crowded, they realise that the food supply may be running out. Then they produce enough light for their host to see in the dark, catch prey and indirectly provide them with more food.

The basis of quorum sensing is the production of a hormone-like molecule known as an auto-inducer. This molecule acts as a stimulant to a sensory element in the bacterium. And when its concentration reaches a high enough value, it causes the bacterium to change its behaviour by activating a specific set of genes, for example, to produce light. In short, quorum sensing is a form of regulating gene expression in response to changes in bacterial cell density, and it works via a sensor auto-inducer pair.

A recent report by M G Surette and colleagues of the University of Calgary in Canada adds to the surprise. The researchers claim that a whole host of other bacteria possess molecular components of quorum similar to that of the bacteria Vibrio fischeri ( Proceedings of the us National Academy of Sciences , Vol 96, p 1639-1644).

Surette and his colleagues began with the knowledge that the V fischeri bacterium has two auto-inducers, called ai -1 and ai -2, each of which functions along with its own sensor. While the molecular nature of ai -1 has been deciphered, details of ai -2 are not known. Using standard techniques, the researchers constructed V fischeri strains that were capable of detecting either ai -1 or ai -2, and used these strains to test whether other bacteria produced one or the other auto-inducer. They discovered that strains of two common bacterial species that live in our gut, Escherichia coli and Salmonella typhimurium , both appeared to be capable of manufacturing ai -2. They have named the genes responsible lux (for light). The lux proteins appear to be quite similar between one species and another, as they ought to be if they can function across species.

What might be the role of the auto-inducer system in E coli and S typhimurium ? Why might quorum sensing be important to these bacteria? One hint comes from a finding that a strain of E coli that has been 'domesticated' in the laboratory for many hundreds of generations has also lost its previously functional lux gene. It would appear that there is some role that these genes carry out in real life, which is redundant in the laboratory. The authors suggest that the ai -2 might be important for regulating a change in the lifestyle of E coli or S typhimurium , forcing them to become pathogens from a benign state.

There is an observation to support this hypothesis: an extract from E coli in which ai -2 was concentrated behaved similar to known virulence-targets. If the guess turns out to be true, one would have an entirely new insight into the phenomenon of virulence, which indicates that like any sensible invader, virulent bacteria too make sure that there are enough of them before they think of initiating an attack.

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