Your Washing Machine Isn’t As Clean As You Think

By Pooja Toshniwal PahariaReviewed by Lauren HardakerNov 5 2025

Your washing machine might be home to a community of microbes uniquely tied to you. Researchers have shown that washing machines can harbor microbes from you, your clothes, and your home.

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A recent study published in Frontiers in Microbiology reveals that household washing machines are microbially active environments. Researchers found diverse, user-specific microbes, including opportunistic pathogens, with front-loading models harboring the highest loads. 

Household washing machines are essential for hygiene but can also serve as reservoirs for a wide range of microbes, including potentially pathogenic organisms. Their warm, moist, and nutrient-rich environment fosters microbial growth, which can vary depending on machine design, usage, and detergent type. Genera such as Pseudomonas, Acinetobacter, and Candida have been detected on the surface of washing machines and laundry, posing a risk of infection.

However, gaps remain in understanding the influence of user behavior, machine type, and drying processes on the microbiome, and contribute to cross-contamination.

Inside The Household Washing

In the present study, researchers examined the microbial landscape of household washing machines to understand their potential role in cross-contamination. They used culture-based and molecular techniques, including 16S ribosomal ribonucleic acid (rRNA) sequencing and internal transcribed spacer (ITS) metabarcoding, to characterize microbial richness and bioburden. They applied two complementary sampling approaches: one to identify microbial “hotspots” across different washing machine designs, and another to evaluate microbial transfer between the machine surface and laundry during wash and dry cycles.

The study included 10 washing machines, five of each top-load and front-load type, sourced from volunteer homes in New York and New Jersey via convenience sampling. The team collected surface swabs from a standardized 2” × 2” area at key high-contamination sites, including the drum wall, detergent drawer, and rubber door seals in front-loaders. In top-loaders, they collected samples from the agitator, drum rim, and softener reservoir.

The researchers used sterile cotton washcloths (12 × 12 inches) to assess microbial spread under two washing conditions: a cold-water cycle with detergent and sterile washcloths (Run 1) and a cycle that included soiled household laundry (Run 2). One washcloth was collected wet post-wash, and another after drying for 30 minutes at high heat.

The team determined viable bacterial and fungal counts using Tryptic Soy Agar and Malt Extract Agar (with chloramphenicol), respectively. After incubation, they centrifuged liquid samples, preserving the pooled pellets in a stabilizing solution to maintain nucleic acids for sequencing.

Data analysis included relative abundance and microbial diversity, using the Shannon index for alpha diversity and Principal Coordinates Analysis for beta diversity. Amplicon sequence variants (ASVs) were classified using the Microbe Directory, assigning pathogenicity scores from 1 (non-pathogenic) to 3 (highly pathogenic).

Microbes Thrive Despite Washing

The analysis revealed that household washing machines contain a mixture of bacterial and fungal microbes, including conditionally pathogenic organisms, indicating their potential as habitats for microorganisms with implications for hygiene and public health. Front-loading machines exhibited significantly more microbes than top-loading models, with bacterial counts reaching up to 6.50 log₁₀ colony-forming units (CFU)/swab compared to 3.79 log₁₀ CFU/swab in top-loaders.

In top-load machines, the drum gasket exhibited the highest microbial recovery (3.79 log10 CFU/swab). In contrast, the door seals and detergent drawers in front-loaders showed the highest bacterial concentrations (6.50 log10 CFU/swab). This difference likely reflects design characteristics that favor water retention and biofilm formation in front-loading systems.

However, microbial community profiles were highly user-specific, suggesting that human-associated factors, such as lifestyle, diet, and product use, shape washing machine microbiomes more strongly than machine design or sampling site. Beta diversity analyses further supported this observation, revealing distinct clustering patterns based on machine ownership.

The presence of soiled clothing during washing markedly altered the microbial structure. The bacterial class Clostridia and fungal class Malasseziomycetes were more abundant in soiled clothes, indicating that clothing acts as a vector for microbial transmission from skin and the environment into washing systems.

Although the team found no highly pathogenic organisms, potential disease-causing microbes were identified, including Pseudomonas aeruginosa, Stenotrophomonas maltophilia, and Staphylococcus epidermidis-hominis. Common fungal taxa included Cladosporium, Aspergillus, Penicillium, and Malassezia globosa, the latter of which was detected in nearly 60% of the washcloth samples.

Machine drying had a minimal effect on microbial reduction or diversity, suggesting that residual microorganisms can persist after laundering. Alpha diversity analyses (using the Shannon index) revealed no significant differences in the microbiota between machine types or sampling locations.

Rethinking Laundry Hygiene

The study findings demonstrate that household washing machines, particularly front-loading models, can host multiple and resilient microbial populations, including potential pathogens. However, no highly pathogenic species were detected, and the health risk for most users remains low.

Standard washing and drying cycles may not eliminate these organisms, highlighting the need for better hygiene measures such as regular cleaning, high-temperature or disinfectant cycles, and tailored routines for vulnerable households.

Future research involving larger, more varied populations should assess the impact of demographics, laundering behaviors, and water quality on microbial persistence to inform evidence-based public health guidelines for safer, more hygienic laundering practices.

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Journal Reference

Whitehead, K., Eppinger, J., Srinivasan, V., & Ugalde, J. A. (2025). Microbial cross-contamination in household laundering and microbial ecology of household washing machines. Frontiers in Microbiology, 16:1667606. DOI: 10.3389/fmicb.2025.1667606. https://www.frontiersin.org/journals/microbiology/articles/10.3389/fmicb.2025.1667606/full