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Unwanted Housemates: Dishwashers Provide Habitat for “Extremotolerant” Fungi

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Dishwasher in a kitchenA dishwasher makes a nice addition to any home. But the appliances also make a nice home for a number of fungi, some of which are pathogenic, according to a new study.

A group of researchers from institutions in Slovenia, the Netherlands and China took samples from the rubber seals inside 189 dishwashers from 18 countries and found that 62 percent of them tested positive for fungi. The report will appear in a forthcoming issue of Fungal Biology.

The study’s authors note that no dishwasher-caused infections were reported in the studied households, but the fungi are not entirely benign, either. Fungi of the Exophiala genus, which were found in 35 percent of dishwashers tested, can colonize the airways of cystic fibrosis patients. Other, less prevalent fungi were also found that can cause infections in individuals with weakened immune systems. "Special attention to this habitat has to be paid particularly in hospital wards with immunocompromised patients," the researchers report.

On the one hand, the dishwasher would seem a nice place for fungi to dwell: it is moist and warm, and it has abundant organic matter to feed on in the form of food scraps. But dishwasher-dwelling fungi have to be of a hardy sort to handle the occasional burst of extreme heat as well as the alkalinity and salt content of dish detergents. In laboratory tests on the dishwasher-dwelling Exophiala dermatitidis and Exophiala phaeomuriformis, the researchers found that the fungi are tolerant to a wide range of temperatures, pH levels and salt concentrations, a degree of so-called polyextremotolerance that had not been found before in fungi. In that sense, the dishwasher fungi are something like domestic extremophiles, the life-forms that occupy seemingly inhospitable niches across the globe, from scorching hydrothermal vents in the deep ocean to frigid high deserts. Extremophiles are a popular subject for study because they demonstrate just how adaptable life is and offer hope that other planets, even those a bit different than Earth, could be inhabited.

The fungal prevalence in dishwashers varied widely from place to place, and the water supply appeared to play a role in the rate of colonization. The Exophiala fungi, for instance, were mostly found in places with hard or medium-hard water—that is, water high in dissolved minerals such as calcium and magnesium. Beyond that, the researchers do not go into much detail about why dishwashers in certain countries seem to be more hospitable than others. For whatever reason, North America was especially fungi-friendly, with all six U.S. dishwashers sampled testing positive for fungi, along with six of seven Canadian dishwashers. Many European countries faired much better: only one of 10 Italian dishwashers was infested, whereas all five Spanish appliances got a clean bill of health.

Photo credit: © iStockphoto/Jim Jurica

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  1. 1. BK505 12:11 am 06/22/2011

    I appears a very significant piece of data is missing from this article. Mentioned are the different countries sampled but nothing is mentioned about the water sources used by the sampled dishwashers. It seems that the water source would be a major factor in supporting fungi infestations. Also what percent of chlorination or other additives were used in the water that supplied these machines? It is possible that the water sources may be the source of the fungi. Lastly, what effect does a periodic cycle of vinegar and/or bleach have on the fungi? Those types of cycles are used to clean electric coffee makers, why not dishwashers?

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  2. 2. NedC 11:45 am 06/22/2011

    No mention if the study evaluated the composition of the rubber seals in the various dishwashers and whether this had any effect, Do different brands, models or types of dishwashers have different rates of fungal colonization? Does the quality of the local water supply or even local food sources have any effect?

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  3. 3. larrywac 3:53 pm 06/22/2011

    I am curious about how to clean out fungus perhaps after each use. Or use a special detergent? Or wash dishes by hand which I did in my youth

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  4. 4. EyesWideOpen 5:12 pm 06/22/2011

    Good article. I plan on treating my dishwasher with environmentally safe, non-toxic virucides such as DepHyze or Madacide, which are guaranteed to kill 99.99% of even the most invasive fungi, bacteria, and viruses (up to HIV and ebola). Google these virucides, and treat your dishwashers with them (including soaking the seals). These virucides are non-toxic and can be left to dry. During the dishwashing cycle, the water will completely evacuate any virucidal residues.

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