Science and your health

Dirty soap

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Just when we thought we could kill germs by regularly washing our hands we find out that soap can be a breeding ground for bacteria.

Yes that’s right. Whilst hand washing with soap and water is a universally accepted practice   to keep potentially harmful microorganisms at bay, a new study shows that washing with contaminated liquid soap actually increases the number of Gram-negative bacteria on hands

(Gram-negative bacteria are generally resistant to the effects of antibiotics or the actions of the body’s immune cells).

Furthermore, the results of the study, published by Carrie Zapka and colleagues in the journal Applied and Environmental Microbiology, directly demonstrate that bacteria from those contaminated hands can be transferred to other surfaces. The researchers conclude that washing with contaminated liquid soap not only defeats the purpose of hand washing but may contribute to the transmission of potentially harmful bacteria.

In the experiment the researchers intentionally loaded samples of soap with two species of bacteria that are most commonly found in contaminated soap. Both have caused infectious outbreaks in healthcare settings.

They tested a range of contamination levels. At the low end, concentrations hovered below 1,000 bacteria per milliliter-sized squirt of soap, which is generally the level considered acceptable. At the high end, some samples contained as many as 10 million bacteria per milliliter which was the level of the most contaminated soap ever identified.

When people washed with more highly contaminated the soap, more bacteria stuck to their hands after they were done washing. When there were more bacteria on people’s hands, more bacteria were transferred to the surfaces they touched.

An American elementary school that had a total of 14 open-style refillable soap dispensers was also examined. The school’s dispensers were never cleaned. All were contaminated with potentially harmful bacteria. Just as in the lab experiment, kids and staff members who scrubbed with the contaminated soap ended up with 26 times more bacteria on their hands after they washed.

The kinds of bacteria that contaminate soap, particularly in public bathrooms, can potentially cause health problems like rashes, urinary tract infections and eye infections. But, at the moment no one knows for sure whether washing hands with contaminated soap can make people sick.

Meanwhile, experts categorically state that regular hand washing is one of the most important things that people can do to stay healthy. What should be done however is to re-examine the kinds of soap dispensers used in public places and how they are cleaned. The school that was involved in the study replaced its dispensers with sealed versions. A year later, they were still contamination-free.

Read the full paper at

Nirmal Shah


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