U.S. scientists say chlorine bleach works as a household disinfectant because it attacks proteins in bacteria, causing them to clump together into a mass much like proteins do in a boiled egg.

The mystery behind the bacteria-killing power of bleach is revealed Thursday in the journal Cell, more than 200 years after the disinfectant was first developed.

Until now, scientists have never understood exactly how bleach kills bacteria.

A team of University of Michigan researchers led by molecular biologist Ursula Jakob found the answer by studying indirectly the effects of its active ingredient, hypochlorite, and they describe the process in their study.

"As so often happens in science, we did not set out to address this question," said Jakob, an associate professor of molecular, cellular and developmental biology at the University of Michigan, Ann Arbor. "But when we stumbled on the answer midway through a different project, we were all very excited."

Jakob and her team discovered that hypochlorite causes bacterial proteins to lose their structure and form what they call large aggregates. Hypochlorite has the same effect on proteins that heat stress or high temperatures do.

According to the researchers, the "denatured" proteins clump together into a mass of living cells and the process is irreversible.

Jeannette Winter, a researcher who was a postdoctoral fellow in the Jakob's lab, said the proteins usually cannot be dissolved and the stressed cells eventually die. The same process occurs when eggs are boiled because proteins in eggs, once boiled, never turn liquid again.

"At high temperatures, proteins begin to lose their three-dimensional molecular structure and start to clump together and form large, insoluble aggregates, just like when you boil an egg," Winter said.

Can mount a defence

The researchers discovered that the bacteria are not totally defenceless when attacked by bleach.

Under siege, a protein chaperone called heat shock protein Hsp33 comes into action to try to protect the proteins from the aggregation effect and to increase their resistance. The main job of protein chaperones is to help other proteins and protect them from negative interactions.

Jakob and her team were alerted to how bleach works by studying the protein Hsp33. The researchers found that the hypochlorite, instead of damaging Hsp33 as it does most proteins, actually causes it to jump into action. It works like an immune system.

"With Hsp33, bacteria have evolved a very clever system that directly senses the insult, responds to it and increases the bacteria's resistance to bleach," Jakob said.

The researchers found, however, that the resistance may be futile.

Marianne Ilbert, a postdoctoral fellow in Jakob's lab, said the hypochlorite attacks many of the proteins that are essential for bacterial growth, and despite resistance, "inactivating those proteins likely kills the bacteria."

Bleach, the researchers conclude, is effective because it works rapidly even in low concentrations.

And they say the findings are important, not only for understanding how bleach works to keep kitchens and bathrooms clean, but also because they may lead to insights into how people fight bacterial infections.

They said human immune cells produce significant amounts of hypochlorite as a first line of defence to kill invading bacteria.