How chlorine bleach overwhelms germs

Study finds one way clinical disinfectant may resist microbial resistance

December 4, 2008 (published)
By Edie Lau

In a world of ever-evolving microbial might, chlorine bleach is a knife that never dulls. Its ability to wipe out most important pathogens has made bleach a mainline disinfectant in veterinary clinics and animal shelters.

How it works has been, surprisingly, something of a mystery. But a new study from the University of Michigan published last month in the journal Cell helps explain how bleach maintains its potency.

The team, led by molecular biologist Ursula Jakob, found that contact with hypochlorous acid -- the active ingredient in bleach -- causes bacterial proteins to unfold and collapse into clumps. Like elaborate origami, most proteins need to be in three-dimensional form to function properly. Losing their shape can be catastrophic to the proteins, and ultimately, to the cell.

“Bleach is so very reactive and will react with cellular components as soon as it hits the cells,” Jakob said in an interview. “So it is very difficult for bacteria to combat bleach when it is used at high enough concentrations.”

Jakob compares the protein breakdown to an egg fried: Heated in a pan, a raw egg quickly clumps up, taking on an altogether different form.

Some bacteria do have a weapon against the unfolding action of bleach. It’s called heat shock protein 33, or Hsp33, and it’s one in a class of “chaperone” molecules that protect cells from stress caused by insults such as heat.

Researchers in Jakob’s lab found that Hsp33 is actually activated by contact with hypochlorous acid. When Hsp33 unfolds, it creates a platform to which proteins damaged by bleach can bind.

“The binding of damaged proteins to Hsp33 reduces the amount of aggregating protein in the cells, which appears to be directly correlated to the increase in bacterial resistance toward bleach,” Jakob said.

Does that mean bacteria eventually can withstand bleach? Jakob doesn’t think so. The level of hypochlorous acid her team studied in the lab was extremely dilute -- about 1/10 the concentration of bleach in a bottle of Clorox, she estimated.

“At the high concentrations that we use bleach on our countertops, the presence of Hsp33 will not make any difference in the survival of bacteria,” she said.

Other research has found that chlorine bleach is effective in killing viruses and some fungi as well as bacteria. “Bleach is one of the things that will kill most of the things we need to (kill),” said Dr. J. Scott Weese, a veterinary internist with expertise in infectious diseases and infection control at the University of Guelph.

But using it effectively takes a bit of finesse. Because it is highly reactive, chlorine bleach will act on whatever it comes in contact with, whether a germ or dirt. And if it encounters dirt before the germ, that may render it ineffective as a germicide.

“You kind of think of it as being used up,” said Dr. Melissa Kennedy, a clinical virologist and associate professor of veterinary medicine at the University of Tennessee. “So you have to remove the organic matter first and then come in with disinfection. There’s a difference between cleaning and disinfection.”

Cleaning with soap or detergent and water alone can kill some pathogens. Germs known as envelope viruses, including the feline and canine herpes viruses and canine distemper virus, have a lipid membrane that’s destroyed by simple soap.

“Non-envelope” viruses that have a protein shell around their genetic material are hardier. Those require an oxidant such as chlorine bleach to kill, Kennedy said.

But again, washing away dirt, feces, mucous and other organic matter is an essential first step. “The first line of defense is just good, common-sense cleaning,” Kennedy said. “Get out that fecal material with lots of virus in it. If the cat’s been sneezing in the cage, just getting out that material alone is as important as anything. Then you tie up loose ends, as it were, with disinfection.”

Another thing to remember is that chlorine bleach needs a certain amount of contact time to work. “It's not instantaneous,” Kennedy said. It may take from a few seconds to a few minutes.

But if wiping off the bleach immediately isn’t advised, neither is leaving it on indefinitely. That’s because chlorine bleach will react with inert materials such as metal, causing corrosion.

Some forms of bleach are less harsh than others. Kennedy and colleagues found in a study comparing the virucidal efficacy of four new disinfectants that a product containing chlorine dioxide was comparable to sodium hypochlorite. Both are types of chlorine bleach, but the manufacturer of the chlorine dioxide product claims it is non-toxic, hypoallergenic and less corrosive for steel instruments and surfaces.

The same study, which was published in May/June 2002 edition of the Journal of the American Animal Hospital Association, found that another oxidizing disinfectant, containing the active ingredient potassium peroxymonosulfate, also was a good substitute for standard bleach.

Not so good for attacking the hardier, non-envelope viruses, the researchers found, were a quaternary ammonium compound and a product made from grapefruit extract.

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