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I don’t know what worried me more about this article in The Registrar – the implication that my dishwasher & its fungal denizens might be out to get me (which I suppose could necessitate returning to Plan B: the Significant Other; after all, I do the cooking, so he can wash up!), or the rather piss-taking tone of the story. I mean, how else to take the headline: The Killer Mutant Fungus in Your Dishwasher: don’t approach without a biohaz suit and a flamethrower ?

On the other hand, it did spur me into going to look for the original article :-) And now I know that the ‘interesting’ black stuff that sometimes springs up (not literally!) around the seals is probably a living organism & not necessarily due to the family’s regrettable inability to rinse dishes before loading. (The authors of the article don’t actually say whether their investigation was initiated after observing similar black mouldy bits, but I can’t help wondering…)

Now, purveyors of various household cleaning agents would have us believe that the kitchen is home to a range of nasty microbes, which can be held at bay only by spraying or wiping with various anti-microbial or antiseptic products. (I wonder how my family remains so healthy, in the absence of many of these wondrous chemicals.) But you’d think something like a dishwasher would be hygienically clean – after all, anything that goes through the wash cycle has been exposed to high temperatures and a fairly alkaline (high pH) environment (although that may be changing, as we move to less-harsh detergents and cooler temperatures in attempts to use less energy and release fewer wastes).

Not according to Zalar and colleagues. Noting that our knowledge of organisms that live in extreme environments (extremophiles) is expanding, they decided to look away from the hot pools and volcanic vents and into a more mundane environment – the domestic dishwasher. Anything that can colonise & survive in that machine’s hot, alkaline conditions could also rightly be described as an extremophile – one with a ready source of nutrients from all those messy food smears. So the team took samples from the inside surfaces of dishwashers: specifically, the rubber seals, as their surface would be easier to colonise than slick metal.

They ended up sampling 189 machines from private homes: 102 from Slovenia, 42 from elsewhere in Europe, and the rest from North & South America, Africa, Australia, Israel, and Far-East Asia. Because they were interested in the possibility of dishwashers harbouring human pathogens, they incubated their samples at 37oC, before going on to test the ability of some of them to grow at temperatures closer to what you might find in an operating dishwasher.

The results: a range of fungi, including Aspergillus (which can cause quite significant disease), Candida (aka ‘thrush’) and Penicillium, with the most commonly-found species – in around a third of dishwashers - being the ‘black yeasts’ (Exophalia spp.) They also found quite a bit of variation in terms of how ‘infected’ the machines were, with those from North America having the most fungal species while those from Spain were all devoid of fungal life. However, I think the numbers are a bit low to draw much from that, with only 13 from North America and 5 from Spain.

 Exophalia is ‘known to cause systemic disease in humans’ and is a common pathogen in the lungs of cystic fibrosis patients. Some of the Exophalia strains survived in temperatures up to 47oC, although I do wonder how they could hang on given that dishwasher temperatures can exceed 60o and get up to 80oC on occasion. The authors don’t propose any survival mechanisms, & I’d like to hear more about that. 

However (before you rush out & get rid of the dishwasher) they found no evidence of fungal illnesses that could be attributed to the ‘dishwasher’ fungi in the homes where they obtained their samples. So while the possibility is there for the home dishwasher to be a hotbed of infection, in practice no link has yet been observed. And this rather gives the lie to the somewhat hysterical tone of the Register report. We’re not yet at the point of needing haz-mat suits to wear while doing the dishes. Still, I suppose that approach wouldn’t sell so many papers…

But it’s also rather cool to think that extremophile organisms may be living much closer to home – no need to head off to the slopes of Erebus or the edge of a boiling soda spring to spot them.

I must go & get the rubber gloves and baking soda :-)

P.Zalar, M.Novak, G.S.de Hoog, & N.Gunde-Cimerman (2011). Dishwashers – a man-made ecological niche accommodating human opportunistic fungal pathogens Journal of Fungal Biology : 10.1016/j.funbio.2011.04.007

 (Hopefully Siouxsie will cover this one too, from the perspective of a microbiologist.)