A recent article in New Scientist entitled Why microbes are smarter than you thought looks at six behaviours that seem remarkably intelligent for single celled organisms. Single-celled organisms don’t have nervous systems, let alone brains, but they could be viewed as “biological computers” with internal machinery that can process and respond to information.
On MicrobiologyBytes I’ve often discussed bacterial communication – the ways in which bacteria talk to each other using chemical signals. If Bacillus subtilis cells are growing in a nutrient-poor area, they release chemicals into their surroundings which tell their neighbours “There’s not much food here, so clear off or we’ll both starve.” In response to these chemical messages, the other bacteria move away, changing the shape of the colony.
Many single-celled organisms can work out how many other bacteria of their own species are in their vicinity – something known as “quorum sensing“. Each individual bacterium releases a small amount of a chemical into the surrounding medium. If there are lots of other bacteria around, all releasing the same chemical, levels can reach a critical point and trigger a change in behaviour of the whole population. This “voting system” can be used to decide when to launch an attack on a host. Once they have grown to sufficient numbers to overwhelm the immune system, they collectively launch an assault on the body. Jamming these signals might provide us with a way to fight back.
Bacteria form communities known as biofilms, familiar as the thin layer of slime that coats the insides of water pipes, or surgical implants. Many different species live side by side in these “bacterial cities”, consuming each other’s wastes, cooperating to exploit food sources, and safeguarding one another from external threats such as antibiotics.
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Many microbes can accelerate the rate at which their genes mutate. This allows them to obtain new abilities that may be helpful when conditions get tough. Escherichia coli mutates more rapidly when under stress (Stress-induced mutagenesis in bacteria. Science. 2003 300(5624): 1404-9), and yeast can perform the same trick (Adaptive mutation in Saccharomyces cerevisiae. Crit Rev Biochem Mol Biol. 2007 42(4): 285-31).
Microbes are also pretty good at navigation. The single-celled algae Chlamydomonas swim towards light, but only if it is of a wavelength that they can use for photosynthesis. Some bacteria move according to the presence of chemicals in their environment – a behaviour called chemotaxis. Another group of bacteria align themselves to the Earth’s magnetic field, allowing them to head directly north or south, and more importantly, up or down for optimum photosynthesis.
When the amoeba Dictyostelium searches the surface of a Petri dish for food, it makes frequent turns. But it does not do so randomly. If it has just turned right, it is twice as likely to turn left as right on its next turn, and vice versa. It remembers which direction it last turned.
Remarkable though these behaviours are, we have probably only scratched the surface of what single-celled organisms can do. With so many still entirely unknown to science, there must be plenty more surprises in store.