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There are washing machines in 96% of all private households in Germany. However, what many consumers do not know is that washing machines, which are designed to clean textiles, can become contaminated with bacteria. Furtwangen University has now published a cultural study on the number of germs in washing machines.
Moisture, warmth and a large supply of nutrients create ideal living conditions for the growth of microbes such as bacteria or fungi. Current trends such as washing at low temperatures, water-saving programmes and the use of bleach-free liquid detergents, further promote the growth of germs.
“Although it is well known that washing machines are susceptible to contamination, there have hardly been any studies to date that have actually measured the microbial content in various parts of the washing machines. We wanted to remedy this situation,” explains the research leader, Prof. Dr. Markus Egert, who teaches microbiology and hygiene at Furtwangen University. The focus was on four easily accessible areas: the water inlet compartment and detergent dispenser drawer, as well as the upper and lower parts of the seal around the porthole door.
For the study, 10 household washing machines from the Villingen-Schwenningen area were sampled with swabs. From the swabs, microorganisms were cultured on agar plates, counted and extrapolated to the sampled area. The isolated bacteria were subsequently identified based on protein spectra using MALDI-TOF analysis.
Averaged over all sampling points, 21,000 germs per cm2 were determined.
"Peak values of up to 337,000 germs per cm2 were found. This is roughly equivalent to the population of Bielefeld," said Professor Egert, the research leader. With 111 germs per cm2, the upper part of the porthole seal showed the lowest germ counts. This area dries very quickly, so the microbes presumably simply lack a basis for life here.
More than half of the 40 species of microorganisms identified in the study are classified as potentially pathogenic and can cause illness, especially in immunocompromised people. Bacteria in washing machines are also a known source of bad odours. Four bacterial species were found that had already been identified as dominant in washing machines in an earlier, purely molecular biological study by the Egert group: Pseudomonas oleovorans, Acinetobacter parvus, Moraxella osloensis and Rhizobium radiobacter. This confirmation marks them as important washing machine bacteria that can now be used, for example, for practical testing of antimicrobial cleaning measures or surface coatings for washing machines.
"Our small study impressively confirms the dense and hygiene-relevant contamination of washing machines, and underlines the importance of moisture. Washing machines should be cleaned regularly, e.g. by wiping them out with all-purpose cleaner, but also by washing them regularly with a powder detergent containing bleach at a minimum of 60°C. To dry out, the porthole and water inlet compartment should be left open between washes," summarizes Professor Egert.
The new study was carried out by a team of researchers from Furtwangen University, Giessen University and Henkel AG & Co. KGaA, Düsseldorf, and funded by the German Federal Ministry of Education and Research (grant number 13FH197PX6). It was published in the journal Microorganisms with the title "Cultivation-based quantification and identification of bacteria at two
hygienic key sides of domestic washing machines".
Prof.Dr. Markus Egert
Susanne Jacksch et al. (2021). Microorganisms 9(5), 905;
https://doi.org/10.3390/microorganisms9050905
Susanne Jacksch, First Author, and Prof. Dr. Markus Egert, Research Leader
Hochschule Furtwangen
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