Hardly anything is really “clean” – many everyday objects are colonized by microbes. But what about a device that is actually supposed to clean – the washing machine? A study shows that many are teeming with microbes. These include harmless types of bacteria, but also potentially problematic and “stinkers”. Regular cleaning and occasional boil washes therefore make sense, say the scientists.
The washing machine is a symbol of cleanliness – but in the sense of sterility this does not apply to most devices, report the researchers working with Markus Egert from Furtwangen University. Moisture, warmth and a wide range of nutrients create ideal living conditions for microbes in the devices. In addition, current trends such as washing at low temperatures, water-saving programs and the use of gentle liquid detergents promote the growth of germs. “Although it is known that washing machines are susceptible to germs, there have been hardly any studies to date that have actually measured the germ content at different points on washing machines. We wanted to remedy this, ”says Egert.
For the study, his team examined randomly how many and which microbes are present in domestic washing machines. To do this, they took samples from various locations on the test equipment. The swabs were then smeared onto nutrient medium in the laboratory to encourage the microbes to grow. The bacterial colonies were then counted, which made it possible to draw conclusions about the population density of the sampled areas. In addition, the washing machine residents were characterized by molecular biological determination methods.
Potential bad guys and stinkers
As the researchers report, they found an averaged population density of 21,000 germs per square centimeter across all sampling points. But there were also distinct “deserts” and hotspots: “We found peak values of up to 337,000 germs per square centimeter,” says Egert. With 111 germs per square centimeter, the upper part of the porthole seal showed the lowest germ count. This place dries very quickly, so that the microbes here probably simply lack the basis of life, explain the scientists. “Our study proves the dense and hygiene-relevant contamination of washing machines, and underlines the importance of moisture in its formation,” says Egert.
More than half of the 40 types of bacteria identified in the study are known to be potential pathogens, the characterizations showed. They can pose a certain risk, especially for immunocompromised people, say the researchers. In everyday domestic life, however, other effects are more noticeable if the washing machine is too crowded: Some types of bacteria generate volatile substances that give the laundry a musty odor. The team was able to identify Pseudomonas oleovorans, Acinetobacter parvus, Moraxella osloensis and Rhizobium radiobacter as particularly typical washing machine residents. The researchers say that these microbes can be used to test the effectiveness of cleaning measures or surface coatings for washing machines.
“With our study, we make it clear that washing machines should be cleaned regularly – for example by wiping them out with an all-purpose cleaner, but also by washing them regularly at at least 60 degrees Celsius with a powder detergent containing bleach. In addition, the porthole and the detergent dispenser should be left open between washes so that they can dry out, ”recommends the scientist.
Source: Furtwangen University, specialist article: Microorganisms, doi: 10.3390 / microorganisms9050905