It may be a jungle out there, but it’s no less a jungle in here as well. Fungal organisms, although technically not plants, are hardy little buggers that are abundant in the environment everywhere, a veritable fungal jungle – even if you can’t always see them. As a group, they can survive almost anywhere – some better than others, of course – under extremes of conditions; they are capable of drawing their nutrition from the soil and dead/decaying organic matter, and can grow on almost any substance, provided there is a degree of moisture (even in form of humidity) present. Many fungi are known to live in symbiotic or parasitic relationships with plants and animals.

Fungal organisms may reproduce and/or disseminate in form of spores. This is most commonly observed in the group of fungi known as molds (variant spelling: moulds). These often have a thread-like (a.k.a. filamentous) body – along, or at the tips of, which they produce single-celled, seed-like spores. Mold spores are reproductive units, highly resilient structures which can survive unfavorable conditions; once formed, they can be dispersed by air currents – even imperceptible disturbances in the air that may be generated by ordinary activities – and may travel great distances by hitching a ride on various surfaces, such as clothing, fur, and so forth. When they eventually settle down on a surface that can sustain fungal growth, they re-grow into the parental form.

Of course, producing spores is not a prerogative of molds only. The other group of fungi, known as yeasts, do that too. While yeasts in general reproduce by budding off daughter cells from a mother cell, under certain conditions they make spores. For instance, the baker’s yeast (Saccharomyces cerevisiae) respond to the condition of nutrient depletion by forming spores, and surrounding each of them by a multi-layered spore wall that confers resistance to various environmental stressors. The disease-causing yeast, Cryptococcus, makes spores both as a result of sexual reproduction, and as a response to nutrient limitations, in either case the ultimate goals being survival, dispersal, and germination into new organisms under conducive conditions.

Did I mention they are resilient? It appears that many, many substances – even seemingly dead, inorganic ones – can sustain the growth of spores in presence of minute quantities of water; all they need is love cellulose, a polysaccharide (or ‘complex sugar’) that is a structural component of the wall of plant cells. Consider the black mold that grows on buildings, inside and out: any plant-derived, cellulose-containing material, such as plywood or regular wood, cardboard, cellophane, paper layers attached to drywalls as backing, carpet fibers, cotton fabric, et cetera can be a haven for the mold spores. Add a little moisture – in form of high humid conditions, water leaks, even moisture build-up via normal breathing and perspiration – and boom! Let there be mold infestation. These molds can also feed on non-dietary, often invisible, organic materials common in households, such as soap, dust particles, and – eww! – shed skin cells trapped in dust, fabric, or carpet fibers.

Fungal organisms generally like slightly warmer temperatures to grow, optimally around 80˚F/27˚C; however, don’t let that fool you. After all, bread mold (often a species of Rhizopus) and mold on cheese (often Penicillium) grow fine at the humid 40-50˚F/4-10˚C of the refrigerator. Yeasts such as species of Cryptococcus are ubiquitous in the non-arid environments of tropical and subtropical countries, with average temperatures ranging between 50-64˚F/10-18˚C. Disease-causing molds (such as Aspergillus) and yeasts (such as Cryptococcus, and pathogenic Candida) grow inside the body at 98.6˚F/37˚C. Under laboratory conditions, on artificial media, yeasts and molds grow at room temperature (generally, 68-77˚F/20-25˚C) or at 95-99˚F/35-37˚C (though sometimes at different rates). I have observed Aspergillus at room temperature on an aqueous solution of a highly alkaline chemical substance (for those who know what it means, it was a pH 9.0 buffer). Fungi, molds and yeasts, have this amazing ability to adapt to their immediate environment, because their enzymes have evolved in such a manner as to remain active over a wide range of external temperatures and chemical conditions.

Nevertheless, in our daily experiences, we have grown used to seeing molds grow – with their fuzzy-wuzzy exteriors – on external surface, more so than the yeasts. Therefore, it may come as a bit of a surprise that a type of thermophilic (high temperature-loving) yeast, called Exophiala dermatidis (a.k.a. the Black Yeast because of its color) – commonly observed in various hot and wet environments, such as the tropical rain forests – should be found on the surface of fruits (such as mango and pineapple) that grow in the tropical climates; in the natural hot water springs in Thailand; humidifiers; Japanese house baths; public Turkish steam baths; and… [drum roll] dishwashers!

Exophiala dermatidis, a related species Exophiala phaeomuriformis, and a mold known as Magnusiomyces capitatus have all been recovered from dishwashers. What is it about dishwashers that makes them such a favorite playground for fungal spores? Scientists surmise that the dishwashers, especially the rubber seals of their doors, may offer a conducive, none-too-harsh environment for spore survival and growth. The internal environment of a dishwasher is rich in organic nutrients in a high pH (alkaline) condition due to regular use of detergents; the temperatures vary intermittently during various cycles, often reaching as high as 60-80˚C, in presence of high humidity arising from processing large volumes of water. High concentrations of salt, used to prevent calcareous accumulations on the inner surface, possibly makes the environment more comfortable for Exophiala, since species of this microbe can grow even at a salt (Sodium Chloride) concentration of 17%, almost 20 times the amount contained in mammalian blood and body fluids.

Wangiella (a.k.a. Exophiala) dermatitidis
Exophiala dermatitidis colony (left) & spores (right);
courtesy: © 2007

In a 2011 study, a group of European investigators sampled 189 dishwashers from various localities in Europe (Slovenia, Austria, Belgium, Croatia, Denmark, Germany, the UK, France, Italy and Spain), South Africa, North America (USA and Canada), South America (Brazil), Israel, Far East Asia (Japan and China), and Australia. The presence of some mold or yeast could be detected in about 62% of these samples; the major culprits were the black yeasts Exophiala, and one white yeast called Candida parapsilosis, followed by low level occurrences of red yeast Rhodotorulla, as well as molds Magnusiomyces and Fusarium – apart from a few other fungi. The researchers made the additional discovery that the hardness of water seemed to play a role in the persistence of the black yeasts in the dishwashers, corroborating earlier evidence that local calcium ion concentrations influenced their growth form and virulence. Not only that, using scanning electron microscopy, they demonstrated that the black yeasts excreted polymeric substances, forming on the rubber seals a tough biofilm within which the microbe was protected from excess heat and mechanical damage.

In a recent study, to be published in July 2013, Turkish investigators sampled 153 dishwashers, in addition to other appliances, from household in the Mediterranean city of Mersin in Turkey, and made similar observations. Almost one in five dishwashers yielded fungi, a whopping 86% of which were the black yeasts, followed by a few white yeast and the mold Magnusiomyces. The researchers also studied the genetic signatures (or genotypes) of these black yeasts, and found that the genotype A appeared to be the most successful in invading urban human habitats and creating a niche for itself in households.

Imagine! Fungal bodies growing, lurking within you dishwasher. In many households, people sometimes leave cleaned utensils, plates and vessels of regular use in the dishwasher for convenience. The implications are astounding!

Earlier studies have demonstrated that high humidity areas in a household, such as kitchens, bathrooms, and steam rooms may offer a haven to fungal organisms, especially those that make sticky spores (in fancy science-y words, “mucuous propagules”), such as the black, white and red yeasts, and the mold Fusarium. Dishwashers may be offering a similar environment within an enclosed, relatively small space.

And why is this important? Well, the potential health hazard, of course. Let’s look at the organisms. Species of Exophiala, earlier known to cause opportunistic lung disease in immune-compromised patients, have been recovered from clinical samples in the US. Magnusiomyces capitatus (alternatively known as Geotrichum capitatum, Dipodascus, Blastoschizomyces or Saprochaete in literature – isn’t fungal taxonomy fun?) has been associated with fatal infections in patients with hematological malignancies, especially leukemia. Both the red yeast, Rhodotorula, and the white yeast, Candida parapsilosis, are clinically significant emerging opportunistic pathogens of debilitated patients, especially those who have indwelling catheters attached for various reasons.

To clarify, there is yet no direct relationship established between dishwashers and human fungal infections. [Note: I am emphasizing this point, because our ever-vigilant Community Manager Khalil brought up an important point in the comments. Cleaning the rubber seals of the dishwasher regularly is an effective deterrent to fungal growth. An acidic solution such as vinegar may help, but the important idea is not to allow moisture to accumulate. If the door is periodically left open, moving air currents are generally effective as a desiccator, which prevents spores from growing. In addition, fungi are ubiquitous in the environment and cannot be avoided; many kitchens, washrooms, boiler-rooms et cetera have yielded similar fungal microbes in the past. Yet, we don’t always fall ill with clockwork regularity, because our immune systems are efficient at combating such external malefactors. As medical science progresses, we have better medications as well to combat such diseases. So… Don’t Panic. no, really. Don’t.]

Nevertheless, it is important to keep in mind that times they are a-changing; both apparently healthy individuals and infirm patients, in today’s world, may have varying degrees of immune suppression due to environmental stressors, disease and/or medical intervention. There is evidence of such an association between mold-infested ‘sick’ buildings and human disease, especially respiratory illness. The recent outbreak of serious meningitis due to medication contaminated by a black mold (Exserohilum rostratum) – traced to poor sterility practices in a Massachusetts pharmacy – was an eye-opener to environmental fungal hazards. Therefore, a continued co-existence with the fungal jungle within one’s own household may one day become a serious public health concern.

UPDATE: An abridged and slightly modified version of this write-up was published in The Conversation UK magazine.

Main Papers cited:

  • Zalar P, Novak M, de Hoog GS, & Gunde-Cimerman N (2011). Dishwashers–a man-made ecological niche accommodating human opportunistic fungal pathogens. Fungal biology, 115 (10), 997-1007 PMID: 21944212
  • Döğen A, Kaplan E, Oksüz Z, Serin MS, Ilkit M, & de Hoog GS (2013). Dishwashers are a major source of human opportunistic yeast-like fungi in indoor environments in Mersin, Turkey. Medical mycology : official publication of the International Society for Human and Animal Mycology, 51 (5), 493-8 PMID: 23167707
  • Horré R, Schaal KP, Siekmeier R, Sterzik B, de Hoog GS, & Schnitzler N (2004). Isolation of fungi, especially Exophiala dermatitidis, in patients suffering from cystic fibrosis. A prospective study. Respiration; international review of thoracic diseases, 71 (4), 360-6 PMID: 15316209
  • Straus DC (2009). Molds, mycotoxins, and sick building syndrome. Toxicology and industrial health, 25 (9-10), 617-35 PMID: 19854820
  • Shoham S, & Marr KA (2013). Treatment of iatrogenic fungal infections: a black mold defines a new gray zone in medicine. Annals of internal medicine, 158 (3), 208-10 PMID: 23147215