Waking sleeping bacteria

There are quite a few bacteria that, when times are hard, are able to put themselves into a state of metabolic inactivity. They are still technically alive - just not really eating, growing or changing in any way, just waiting until conditions become more favourable so that they can regain normal active behaviour. The bacteria used most frequently to explore this behaviour is B. subtilis which frequently form spores in response to adverse conditions, such as lack of nutrients or extremes of temperature.

Getting into a dormant state is fairly understandable, bacteria have lots of sensing systems which can tell whats going on in their surrounding environment, and dormancy is just a response to that. However getting out of a dormant state is a little more complicated. Once the cell is dormant hardly any of it is active, whatever signal system is used to activate the cells, it has to require very few cellular componants.

In some bacteria (particularly E. coli persisters) exit from dormancy seems to be a fairly random, stochastic effect. Every now and then, one of the cells will simply switch back to being active. If conditions are good it will replicate and re-colonise, whereas if conditions are bad it can either switch back to dormancy or, if it dies, be later replaced as its fellow bacteria come back. It's not a wonderfully good strategy for individuals, but for the species, and for the genes, it works very well.

One possibility the paper in the reference was looking at is that bacteria exit dormancy in response to growth signals from other surrounding bacteria. The idea behind this is that once bacteria start growing and dividing they start secreting signaling molecules such as muropeptides from the cell wall. If these muropeptides are present, it means that bacteria are able to grow and survive, and this might serve as a signal to other bacteria: "conditions are good enough for us!"

Diagram from the reference. a) bacteria switching from dormancy (light brown) to growth (green) in response to environmental signals. b) growing bacteria releasing signals to activate surrounding bacteria.

Addition of amino-acids and other required nutrients can activate B. subtilis spores, but the concentrations of amino-acids needed are so high that this is unlikely to be a natural response. On the other hand, addition of even small amounts of muropeptide leads to spore activation, strongly suggesting that it may be a relavent signal.

With this and the data from stochastic E. coli persisters it is tempting to see this as a full mechanism: exit from dormancy is random, but once one bacteria has grown and survived it can signal to the others that conditions are safe for growth. The only problem with this is that it has the potential to produce large amounts of bacterial growth independent of any signal from the actual environment. It has also been found (via "unpublished observations") that germinating spores don't release the same kind of murapeptide signal, and are unable to activate surrounding spores.

It was also found that the murapeptide signal was not really species specific, which is fascinating from the point of view of bacterial ecology. Growing bacteria, especially Gram-positive species, release large quantities of muropeptides as they grow, and it is a very well conserved molecule, found in the cell wall of other bacteria. A dormant bacteria might therefore not even need one of its own species to report back on conditions outside the cell, it can rely on signals from other bacteria in the surrounding environment.

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Dworkin J, & Shah IM (2010). Exit from dormancy in microbial organisms. Nature reviews. Microbiology PMID: 20972452

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