How household bleach works to kill bacteria

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Public release date: 13-Nov-2008

http://www.eurekalert.org/pub_releases/2008-11/cp-hhb110708.php

Contact: Cathleen Genova

cgenova@...

Cell Press

How household bleach works to kill bacteria

Despite the fact that household bleach is commonly used as a

disinfectant, exactly how it works to fight bacteria remained an open

question. Now, a report in the November 14th issue of the journal Cell,

a Cell Press publication, provides an answer.

The researchers found that hypochlorous acid, the active ingredient in

bleach, causes the unfolding of proteins in bacteria in much the same

was that heat stress or fever does. Those denatured proteins then clump

together irreversibly into a mass in living cells, similar to what

happens to proteins when you boil an egg, according to the researchers.

The bacteria aren't totally defenseless, however. Under those

circumstances, a protein chaperone called heat shock protein Hsp33

springs to action, protecting proteins from the aggregation effect and

increasing the bacteria's bleach resistance. Protein chaperones are

generally defined as proteins whose function is to help other proteins.

" We found both in vitro and in vivo that bleach attacks proteins, " said

Ursula Jakob of the University of Michigan, Ann Arbor. " They lose

structure much like they would under high temperature. Under those

circumstances, the [Hsp33] protein is specifically activated to increase

resistance. " Jakob emphasized that this newly discovered mechanism is

clearly one way bleach kills bacteria, but it may not be the only way.

Why would bacteria have a system specifically designed to deal with bleach?

" Hypochlorous acid is an important part of host defense, " Jakob said.

" It's not just something we use on our countertops. "

In fact, the innate immune systems of mammals, and specifically immune

cells known as neutrophils, release high concentrations of hypochlorous

acid (aka bleach) upon recognizing microbial invaders. In addition,

Jakob said, some evidence suggests that enzymes that produce bleach may

help keep the bacteria in our guts in check.

The specific effects of hypochlorous acid on proteins help to explain

why hydrogen peroxide is an inferior antimicrobial agent even though

both chemicals are expected to act as strong oxidants, Jakob said.

Hydrogen peroxide doesn't do much for your countertops, she said,

because it doesn't provoke these effects on proteins.

Hsp33 also represents another example of an emerging concept in protein

biology: that some proteins actually become activated through the act of

partial unfolding. Indeed, chaperones react to stress by unfolding in

the same way that other proteins do. Far from leaving them useless,

however, that change in conformation is exactly what turns them on. "

Usually, we think proteins need structure to be active, but here they

must lose structure to be active, " Jakob said.

As for whether the findings will have any practical implications, Jakob

said she isn't yet sure. For instance, she has doubts that bleach could

be made to work any more effectively than it does, particularly given

that it works so rapidly and so well as it is even at low concentrations.

The findings in bacteria could perhaps offer new insight into the

damaging effects of bleach on our own proteins, she added, noting that

hypochlorous acid produced by the immune system has been suspected to

play a role in chronic inflammation. The protein unfolding seen in

bacteria might explain what the chemical agent is doing, perhaps

yielding clues about what might be done to stop it.

###

The researchers include J. Winter, M. Ilbert, P.C.F. Graf, D. Ozcelik,

and U. Jakob, of the University of Michigan, Ann Arbor, MI.

--

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