Experiment confirms that microbes swim slower in saliva

Human swimmers would probably have a tough time swimming in saliva, though nobody knows for sure. But now scientists have confirmed that microbes do. Believe it or not, this settles years of debate.

When your primary way of moving through the world is wriggling pathetically, the medium through which you wriggle is very important. Sadly, if all you can do is wriggle, you don't often get to pick the medium in which you've evolved to live. While some microorganisms move through elastic fluids - fluids that pull back together after being pushed apart - and some move through non-elastic fluids, they don't often switch back and forth. They can't compare which is better.

Scientists can't compare either. They debate whether non-elastic fluids or elastic ones allow microorganisms to swim faster. Elastic substances are harder to push through, but they provide a more solid support to push off of. Either opinion on the subject was tough to test, since changing the consistency of the fluid through which the microorganisms swam often killed them off. (Researchers have, however, settled the question as to whether microorganisms can swim when they're dead. The answer is 'no'.) At last scientists came up with a way to increase the elasticity of the fluid - polluting it with polymers.

Polymers are little strings of molecules. They can be organic or inorganic. (DNA can be used as a polymer, for instance. So can shavings of plastic.) These strings increased the elasticity of the substance without increasing the viscosity. (Elasticity and viscosity are often considered the same thing. Actually, elasticity is the tendency of a substance to return to its former shape when stretched or hit. Viscosity is resistance to flow in any direction, including in a direction that would take it back to its former shape. All fluids have some degree of viscosity but only certain fluids have elasticity. Human mucus and saliva, under the right conditions, manage to have both.) They then constructed tiny swimming racetracks and recruited nematode worms to swim the course. They watched through a microscope to see how the worms did. As the elasticity increased, the worms slowed down. Elasticity, it seems, is not for wrigglers.

Via Physical Review Letters.