A closer look yields new clues to why bacteria stick to things

A bacterium’s ability to change its hairstyle may help in the effort to clean contaminated groundwater for drinking, according to Penn State researchers.

People are continually moving into places that are hot, sunny and arid where drinking water is in short supply, says R. Kramer Campen, Penn State graduate student in geosciences. “The imperative to find ways to clean groundwater is paramount,” he told attendees today (March 25) at the 225th American Chemical Society national meeting in New Orleans.

In the ocean, bacteria can be released into the water to clean up oil spills, carried to the target by the same currents that transport the oil. Groundwater poses a more difficult problem as these single-cell organisms tend to adhere to certain minerals in the soil preventing them from following the pollutant’s trail. Bacterial adhesion is also responsible for many medical problems such as tooth decay and artificial limb and organ rejection. “There is a growing awareness that you need a molecular level understanding,” says Campen. “At that level, the processes that cause a bacterium to adhere to a mineral in soil or to a tooth have to be the same.”

For many years, scientists have noted that bacteria stick to iron particles in soil, but not to sand grains. Until recently, this has been explained by invoking the same forces that hold a balloon to the ceiling after you rub it on your sweater. Researchers thought that the tiny, negative electrical charges on sand grains repelled the negatively charged bacteria, while the positively charged iron attracted them.

However, Campen and his adviser, James Kubicki, assistant professor of geosciences, think it is all about the hair. Bacteria are covered with atomic-scale chains of complex sugar molecules with “one end fastened into the cell membrane and the rest extending outward,” explains Campen. “The hair analogy is a good one.”

The hairs, actually polymers, present a problem for the charge-based explanation because the strength of the attraction (or repulsion) depends on how close the objects are to each other. Because the charges are so small, at a distance of one hair-length no attraction should be felt.

Electrical charges may still be important, just not for the reasons previously thought. Polymers come in two varieties – one with no charge and another with positive and negative charges distributed along its length. A single bacterium has both, and the aggregate is known as a polymer brush.

Campen put polymers similar to those on bacteria, both charged and uncharged, into a liquid solution with iron and sand-like particles. He discovered that both adhered to the iron, challenging the idea that electrical forces are the cause of stickiness.

The charged hairs may have another purpose. “If you’re a bacterium in a nutrient-rich environment you’d like to stick around for while,” says Campen. “If you’re in a nutrient-poor environment you’d rather decrease the chances that you’ll stick to surfaces.”

To accomplish this, he thinks the bacterium may rearrange the positive and negative charges along its charged polymers in such a way that they would extend, allowing the whole brush to expand, contact surfaces, and become stuck. Or, in a different arrangement the charged hairs would scrunch up, flattening the brush and allowing the bacterium to be carried away.

Developing a method to control this behavior would provide scientists with the means to send bacteria where needed, or prevent them from accumulating where they can do harm.

The National Science Foundation provided funding for this project.