When you step on some things, like a banana, they squish and flatten to the ground. But when you step on other things, like a rock, they maintain their shape and aren't affected. So what happens when you step on bacteria? Are they squishy?
While we work with microorganisms and other cells as chemical and biological engineers, neither of us has actually tried squishing them in the lab. One of us has a background in physics and studies mechanical forces in biology, while one of us genetically engineers microorganisms and cultivates them to make biofuels and other chemicals.
Forces and pressures
Let's think about what happens when you step on something. Any time you push or pull on an object, you exert a force on it. What happens after that depends on how much force you're exerting and the properties of the object.
The force your footstep exerts comes mainly from your body weight. Also important is the area over which that force is distributed on your foot, which creates pressure. That part about the area is important – it's why you can walk on snow with snowshoes, but you'd sink with regular shoes.
You can calculate pressure by dividing the weight of an object by its area. If your foot is approximately a rectangle of 7 inches (about 18 centimeters) in length and 4 inches (10 cm) in width, it has a surface area of 28 square inches (180 square cm). And if your weight is 110 pounds (50 kilograms), the force you exert per square inch is roughly 3.9 pounds per square inch.
For comparison, the pressure of the air, or atmospheric pressure, on your body at sea level is 14.7 pounds per square inch. The atmosphere exerts significantly more pressure on you than your footstep does on the ground – you just don't feel it because it is balanced by the internal pressure of the air inside your body.
Stepping on a microbe
Now what happens to a bacterial cell when you apply the force of your footstep on it?
Bacteria have different shapes, ranging from spheres to rods and spirals. Bacterial cells have walls that protect their gel-like insides from the environment. How strong is a bacterial cell wall, and can it withstand the force of your footstep?
Scientists have studied the strength of bacterial cell walls for several reasons, including to find out whether high pressure can kill bacteria. People in the food industry use high pressure to make food such as milk safe for us to consume.
To determine the toughness of bacterial cell walls, researchers use a variety of tools to measure their ultimate tensile strength, which is the maximum pressure an object can withstand before breaking. This can be done, for example, by putting them in a sealed container and rapidly lowering the pressure until they explode. A 1985 study found that it would take nearly 1,500 pounds per square inch to make the bacterium Salmonella explode, and later experiments showed it would take about 1,900 pounds per square inch for the common soil bacterium Bacillus subtilis to explode. That's 400 to 500 times more pressure than your sneaker is going to have on the sidewalk and any microbes lying about.
To understand these numbers in a different way, imagine a bacterium large enough for a person to stand on top of it. If it had the same cell wall strength as Salmonella, it could support over 350 people of 110 pounds each standing on it at the same time. While high pressures can kill bacteria in some applications such as food processing, one person standing on them won't work.
Slipping through the cracks
It's clear that bacterial cell walls are very strong. But there's an added complication that makes it even harder to squish bacteria: They're incredibly small. The average bacterium is only about 1 to 5 microns or millionths of a meter (smaller than ten-thousandth of an inch) in size. In comparison, the tip of a common pin is about 130 microns in diameter.
The surface of your skin has fine grooves called sulci cutis that are, on average, tens of microns deep. The soles of your shoes also have grooves that are much deeper than the ones in your skin. As a result, whether you are stepping on bacteria with your bare feet or while wearing shoes, most of the cells will slide into one of those grooves and escape from the full pressure you exert on the ground.
Standing on a pin
How could you increase the pressure your feet exert on a bacteria cell to squish it?
One theoretical way would be to change the bottoms of your shoes from flat to very pointy, with the bottom of the point having a diameter as wide as the tip of a pin. While walking on these shoes would be impossible, a 110-pound person would exert a pressure of 5.6 million pounds per square inch. That is enough to smash any known bacteria.
While people can't actually do this, it turns out that some insects can. Cicada wings have tiny molecular structures that look like needles. These needle-like structures are only nanometers in size, a thousand times smaller than most bacteria, and are called nanorods.
When a bacterium lands on the surface of the cicada wing, it makes special chemicals that help it stick to the surface. When the bacteria divides, it produces tiny forces that allow the new cells to separate from each other. These small forces are magnified into enormous pressures when they push against the nanorods on the cicada wing, puncturing the bacteria and killing it.
Cicadas, dragonflies and many other flying insects have similar wing surfaces that are naturally bactericidal, meaning bacteria killing. Bioengineers are taking inspiration from nature and trying to make surfaces with needle-like structures that kill bacteria in a similar way.
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