Let’s be honest: How often do you clean your washing machine? Do you occasionally wash laundry at high temperature programs? And do you still use bleach-containing laundry detergents instead of liquid products?

If your machine or freshly washed laundry occasionally smells bad, you should ask yourself these questions, says Professor Markus Egert of Furtwangen University: “We all want to wash our clothes as environmentally friendly and sustainably as possible. But unfortunately, low temperatures and gentle wash cycles counteract the necessary machine hygiene.”

Egert is a microbiologist and has long been concerned with the communities that microbes form in the human environment. In science, these communities are called a microbiome. Every person has their own individual microbiome, and even the objects in our environment are colonized by various communities of bacteria, fungi, viruses, and other microorganisms. However, this is not as disgusting as it seems: most of our fellow inhabitants are harmless to useful, and we even need some of them to be able to live healthily at all – such as the intestinal bacteria that help us with digestion.  

Biofilms with unknown contents

But what about microbial inhabitants inside the washing machine, an appliance that we usually associate with hygienic cleanliness? Most people know the slimy, persistent films that often form in the door seal or the rinse chamber: so-called biofilms in which bacteria and other microorganisms can survive quite well, protected from external influences. In detail, however, we know surprisingly little about which microbes live in washing machines and what they do there, says Egert: “Although it is known that washing machines are susceptible to contamination, there have hardly been any studies to date that have actually measured the microbial content at various sites in washing machines.”

To close this gap, Egert has launched the Washing Machine Microbiome Project. For a good three years now, his team has been working to shed light on the microbe communities in washing machines. For one thing, the microbial colonization raises health issues: some germs can definitely become a problem, especially for immunocompromised people. “However, these considerations mainly play a role in hospitals, retirement homes and other facilities where hygiene is particularly important,” Egert says.

Another aspect is much closer to the everyday lives of most people: our microbial roommates can cause bad odours. And smelly laundry is reason enough for Egert's team and the industrial partners in the project –including detergent manufacturer Henkel – to take a closer look at the washing machine microbiome.  

A medium-sized city on just one square centimetre

The project uncovered a few hard facts such as this: more than 200 different types of bacteria lived in the washing machines examined by Egert’s team – and viruses or fungi are not even included in this count. In the most contaminated areas, an entire microbial metropolis lives in a very small space: “We found peak values of up to 337,000 germs per square centimetre,” says Egert. “That's roughly equivalent to the population of Bielefeld.”

The WMP team examined three easily accessible sites of the machine that everyone can (and should) clean without difficulty: the sump, door seal, and detergent drawer. A bacterium called Moraxella osloensis, which is a well-known “stinker,” particularly likes to live in the door seal. The greatest diversity of species was found in the detergent drawer. And there is this finding also: about half of the most common species the WMP team found in the machines can potentially make people sick.

WMP is more than just a bacteria catalogue

However, merely cataloguing the washing machine microbiome was only the first step of the project and, according to Egert, is “almost a little boring”. So-called metataxonomic analyses in which certain marker genes are amplified from a sample, sequenced, and assigned to the different microbial species have long been standard scientific procedure. If you want to learn something about the activities of the microbes, too, then you have to look at other molecular levels and additionally analyse the mRNA, proteins, or metabolic products produced by a microbiome. This, however, is much more complex with regard to the methods, and the necessary equipment and procedures have only been established in recent decades.

Egert and his team have already published several studies on this subject. The most recent one, from July 2021, is a comparison of the bacterial transcriptome – that is, the entire bacterial mRNA from a sample – on cotton and polyester fibres after washing. According to Egert, it is the first study of its kind on laundry items: “Until now, metatranscriptome analyses have been done mainly on human body samples. However, there are far fewer microbes on textiles than in the human gut, for example,” he says. Therefore, the team first had to adapt the methods that were established for human samples to their own requirements and make them more sensitive – another important aspect of the work.

New approaches for more laundry hygiene

All in all, the Washing Machine Microbiome Project is pretty much basic research. But as abstract as the results may seem at first, they offer many new starting points for developing better hygiene products for laundry and machines, and thus for partially resolving the conflict between sustainability and hygiene. However, Markus Egert prefers to leave the development of new products that combat laundry odours and germy washing machines to his industry partners. Instead, the hygiene expert provides some tangible tips for washing machine owners: “Regularly wipe out the machine with all-purpose cleaner, as biofilms are best removed by mechanical cleaning,” he advises. “Leave the door and detergent drawer open to dry after each wash cycle. In addition, regular wash cycles at 60 degrees or more and with bleach-containing powder detergents are mandatory.”

Meanwhile, Markus Egert and his team will continue to get an overview of the immense diversity of our microbial household roommates and investigate the microbiomes of rinsing sponges, glasses, or other everyday items. Follow-up projects for the washing machine are also planned, of course, building on the WMP findings. And Egert is optimistic about at least one thing: Not all of our fellow inhabitants are bad for our health. Maybe some of them will even turn out to be quite useful? Egert intends to pursue this “probiotic” question – in his search inside washing machines and everywhere else where he encounters unknown microbial roommates.