Do Smelly Washing Machines Simply Contain More Bacteria? A Study of 16 Machines

A musty smell when you open the washing machine might make you imagine large numbers of microbes inside. But how do bacterial quantities and communities differ between machines that smell and those that do not?

A 2026 study examined 16 household washing machines in everyday use. Rather than a clear division between odorous and non-odorous machines, it found smaller differences that depended on the location inside each machine. The detergent drawer, in particular, offered clues to possible associations with odor.

Examining bacteria and odor from two angles

Washing machines contain moisture and residues introduced by laundry and other sources. Microorganisms can inhabit these environments and assemble on surfaces into communities called biofilms. Yet the relationship between finding bacteria and users perceiving a machine as smelly is not fully understood.

Sabasadat Seyednikkhoo and colleagues at Furtwangen University examined both which bacteria were present and how much bacterial material there was. They aimed to determine whether particular bacteria or overall bacterial quantity were associated with user-perceived odor.

The study involved 16 machines used by people associated with a German university and others: eight reported by their owners as odorous and eight as non-odorous. Classification started from household users’ experience, not trained sensory assessors or instrumental measurements of odor compounds.

Separating three locations within each machine

Samples came from three sites: the detergent drawer, the rubber door seal, and the sump, where drainage water collects at the bottom. The researchers collected 48 site samples from 16 machines and sequenced the 16S rRNA gene to help identify bacterial taxa.

They also quantified bacterial DNA in separately collected samples using qPCR. One machine in each odor group failed to yield the DNA concentration required for reliable quantification, leaving seven machines per group, or 14 in total, for this comparison. The analysis corrected for differences in sampled area and accounted for relationships among the three sites sampled within each machine.

DNA methods can detect bacteria that routine culturing may miss. However, the quantity measured is bacterial DNA, not a direct count of currently living, active bacteria or a measurement of the odor compounds they produce.

Bacterial composition depended strongly on location

The bacterial communities differed by sampling site. Moraxella was prominent on door seals, with Moraxella osloensis abundant at species level. The sump contained many bacteria from genera such as Pseudomonas, while detergent drawers contained a mixture of taxa.

Moraxella osloensis has been linked to unpleasant laundry odors in earlier research. In this study, however, it was abundant on seals in both odorous and non-odorous machines, without a significant difference in relative abundance. Finding a familiar bacterial name was not enough to explain users’ odor perceptions.

Differences between odor groups were limited for many dominant taxa. Nor were all differing bacteria more abundant in the odorous group. For example, Diaphorobacter nitroreducens was more abundant in the sumps of non-odorous machines.

Small differences clustered in detergent drawers

Some measures of bacterial richness and diversity were higher in detergent drawers of odorous machines. The same diversity measures did not show significant odor-group differences on door seals or in sumps.

Analyses of overall community composition also found group differences in detergent drawers, although statistical significance depended on the method. Door seals showed no difference in abundance-sensitive comparisons but did show a difference when the analysis focused on which taxa were present. Sumps showed no clear separation in either type of comparison.

Some low-abundance taxa also had higher relative abundance in the odorous group. The authors draw attention to these finer compositional differences and to bacterial activity. Differences that are hard to see by counting only dominant bacteria may be concentrated in particular locations.

Bacterial quantity was higher in the odorous group, but highly variable

Median bacterial DNA quantity was higher in the odorous group at all three sites. However, variation among machines was large, with considerable overlap between groups.

The statistical analysis found a significant effect of sampling location, but the odor-group effect did not reach the study’s significance threshold. Its p-value was 0.067. This does not prove that bacterial quantities were equal; rather, the study could not clearly detect a group difference given its size and variability.

The authors emphasize that this was an exploratory study of a small number of machines, with limited power to detect small effects. More machines would need to be examined to clarify the relationship between odor and bacterial quantity.

The next question is what the bacteria are doing

Beyond bacterial types and quantities, the findings raise questions about which materials bacteria use and which substances they produce at each site. The same bacteria may behave differently under different moisture and residue conditions.

The study did not measure bacterial volatile compounds or metabolic activity. The suggestion that detergent drawers may be important is an association-based lead for further research, not evidence from an experiment showing that cleaning them will necessarily eliminate odor.

The authors propose combining trained sensory assessment with measurements of bacterial composition, metabolites, and activity. Recording detergent use, washing frequency, and household circumstances could help clarify how the environment inside a machine relates to its odor.

Summary

In this study of 16 household washing machines, bacterial composition differed by location, and differences between odorous and non-odorous groups were relatively small and site-dependent. Associations involving diversity and community composition made detergent drawers a candidate for closer investigation.

Bacterial DNA quantity, measured in 14 machines, tended to be higher in the odorous group, but the group difference was not statistically significant. Rather than judging odor from total bacterial quantity or the names of prominent bacteria alone, it may be more informative to ask where communities live, which organisms they contain, and what they produce. Understanding washing-machine odors requires these complementary perspectives.

A co-author is a researcher at Henkel Consumer Brands. Funding came from the NASH project of Germany’s Federal Ministry of Research, Technology and Space. The authors declared no competing interests.

References

• Seyednikkhoo, S., Weide, M., & Egert, M. (2026). Bacterial communities in domestic washing machines associated with user-perceived odours. Applied Microbiology and Biotechnology, 110. DOI and original article. See the corrected open full text, which includes the added Figure 5.