Today we have another in our series of guest posts by participants in the upcoming meeting on indoor evolution at the National Evolutionary Synthesis Center in June. Rachel Adams is a post-doctoral researcher at the University of California at Berkeley who studies the dispersal of fungal spores into homes.
In the early 1940s, the promise of the drug penicillin far exceeded its production. Scientists were on a quest to find a strain of the penicillin producing fungus, Penicillium, that would produce more of the “mold juice.” In the most rotten citizen science project ever to be staged, researchers at the then-named Northern Regional Research Laboratory in Peoria, Illinois, requested moldy fruits and vegetables be mailed from around the world. Military personnel were asked to collect soil samples from far-flung locations. The scientists even tasked a lab employee named Mary Hunt with collecting spoiled foodstuffs around town, a task that earned her the nickname “Moldy Mary.” Eventually, a golden-hued mold now identified as Penicillium rubens growing on a cantaloupe in Peoria emerged as the magic strain for producing larger quantities of penicillin.
Few of us can claim to have discovered a mold in our homes that saved millions of lives, but we’ve all had molds in our homes – and, in most cases, the evolution of those mold species remains mysterious. These mysteries call on scientists and citizens to scour the world for answers about environments as ordinary as countertops and basements.
Some molds, such as Penicillium, are extremely common in soil and can become airborne easily, so in most cases they probably just enter our homes through open windows and doors, though it is also possible that the golden fungus, for example, came in on the skin of the cantaloupe itself. But the story of how many other molds in our homes have come to live alongside us is less clear. Understanding the changes that can occur during the shift from natural habitats to human-made ones can give us important clues into the character – and perhaps even consequences – of our unintended housemates. This broader topic is the focus of an upcoming working group on Evolution In the Built Environment, and has inspired me to reflect upon what we do and do not know about the sneaky, scrabbling species we live alongside.
“Mold” is a generic term for furry fungi that grow on moist surfaces. For critters that often make their living breaking down plant and other biological material, many parts of our houses are ripe for growth. An undefended cantaloupe is just as inviting a treat when on a vine in nature as it is on our countertop. Plus, many of the materials we use to build our homes, materials like wood and drywall that do not seem very appetizing, become edible to fungi when wet. But then there are more unusual places in our homes, places that seem hostile to life and devoid of food, where mold can also be found.
For instance, Exophiala dermatitidis is a black yeast that all over the world can be found growing in dishwashers, saunas, and steam baths. Amidst the high temperatures, large amounts of water, and soapy washes, Exophiala clings to the walls, and to life. You just have to wonder – what was this yeast doing, and where was it doing it, that made this part of the built environment an alternate habitat?
In other words, what is the natural counterpart of a dishwasher?
Researchers from the Netherlands and Thailand teamed up to investigate this question. To find E. dermatitidis they did what fungal biologists seem to do when confronted with the question of where a household fungus has come from: they gathered samples, thousands of them. From temperate and tropical climates, from natural areas, like rain forests, and from human-made settings, like bathrooms and railroad ties. Across all these samples, E. dermatitidis was fairly common in steam baths and around railway stations, but was only very rarely found in nature. It was never found in cool habitats, never for example in outdoor habitats in the Netherlands. Where it was found was in animal feces, plants, fruit, and soil, and only in tropical areas. Based on this pattern of occurrence, the authors proposed that the original life cycle of this fungus is tied to that of fruit-eating animals in tropical rain forests.
Perhaps the abilities to adhere to fruit skins, withstand high tropical temperatures and pass through the digestive tracts of animals are pre-adaptations to growing on the walls of steam baths and dishwashers. (For the record, this also means that E. dermatitidis is the answer to that classic barroom question, “What do your dishwasher and fruit bat’s colon have in common?”)