Science Questions

Are bacteria intelligent?

Sun, 6th May 2007

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Tony, Westcliffe asked:

Do bacteria have intelligence? How do they find their food?


Bacteria have no brain, but on the surface of a bacterial cell there are receptors for different chemicals.  Salmonella bacteriaThis means that they can tell which way to travel by comparing, chemically, how many of those receptors have things that they like - or dislike - attached.  They assume that the side with the most ‘good’ receptors filled is closest to their food.  They use this as a guidance mechanism to control where they go. 

Bacteria travel towards desirable chemicals, or away from toxic ones, using a tail called a flagellum.  This is like a propeller, powered by a protein ‘motor’. 

When the ‘motor’ burns energy it causes the protein to change shape, quickly spinning the long ‘tail’ part. 

This lets bacteria move so quickly that one species - Bdellovibrio - are officially the world's fastest swimmers, travelling up to sixty times their body length in each second!


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Communities of bacteria also employ decision-making processes involving quorum sensing, where each bacterium produces and emits signaling molecules, and in turn, senses those signaling molecules from other bacteria and modifies its gene expression and/or behavior based on the levels that it senses. In this way, bacteria can behave as one organism based on the size of their community.

For example, a few bacteria inside a human body would not be effectively virulent and would be defeated if they attacked the body. However, if they waited for sufficient numbers, then they could produce a sufficient virulence to overwhelm the body's defenses and complete their objective.

I crudely described it here, so perhaps someone else could give a better or more specific example. Lmnre, Mon, 18th Nov 2013

As so often happens, it all depends on your definition of intelligence.

If we take the common definition of the ability to use information to your advantage, then responding to a chemical gradient is certainly an intelligent action.

If we use a more sophisticated definition, as the ability to surprise a human, then I guess the answer is still yes. Individual bacteria may appear fairly mechanistic but the ability of a swarm to evolve or re-order their environment is at least as surprising to an intelligent human as the behaviour of human crowds is to an economist.  alancalverd, Mon, 18th Nov 2013

Chemotaxis is sometimes described as simple stimulus response mechanism. A causes B. But it actually is more complex.
Bacteria have two types of movement depending on the direction cilia beat. They either move in a strait line or twiddle (tumble). They cannot actually control the direction in which they move. Increasing concentrations of a nutrient increase the probability of straight line movement. Decreasing concentrations increase the probability of twiddles, which will send them off in a different direction.

What's interesting, though, is because it is not the absolute value that triggers this but increases or decreases in the concentration gradient, the bacteria has mechanisms to "remember" or compare successive measurements. I don't know if chemotaxis represents intelligence, since there is not much flexibility in response, but it does seem like the most basic form of memory. To me it's more impressive than "the memory" of immune function, which is really more of a selection process. cheryl j, Tue, 19th Nov 2013

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