Washing machines (WMs) have become essential appliances in modern households, providing convenience in daily life (Bae et al., 2020). However, the presence of microbial fouling (biofilm formation) and the generation of malodors often lead to an unpleasant user experience with household WMs. The most commonly identified biofilm-producing microorganisms in WMs include Acinetobacter, Bacillus, Brevundimonas, Micrococcus, Staphylococcus, and Pseudomonas, as well as Candida, Fusarium, Aspergillus, and Trichosporum (Van Herreweghen et al., 2020). These microorganisms can form biofilms on various surfaces and locations within WMs (Jacksch et al., 2020). The composition of the bacterial community within WMs exhibits significant variation depending on the specific site, with high diversity observed in the detergent drawer, followed by the sump and door seal (Jacksch et al., 2020). Nix et al. investigated the microbiota present in the biofilm of detergent drawers and door seals of household WMs and found that the microbial community within WMs is highly diverse, with Proteobacteria as the dominant prokaryotic organisms, and Basidiomycota and Ascomycota as the primary fungal colonizers (Nix et al., 2015). The formation of biofilms by microorganisms inside WMs is a key factor contributing to washing machine fouling (Gattlen et al., 2010). Biofilms aid microorganisms in adhering strongly to the surfaces of WMs, making them difficult to clean (Bockmuehl et al., 2019). Comparative analysis of biofilm-forming capabilities has shown that the majority of isolates from WMs form more biofilms than their reference strains. Notably, Pseudomonas putida has been identified as a prolific biofilm producer, requiring higher detergent concentrations for effective removal compared to typical strains (Gattlen et al., 2010). Microorganisms within WMs can survive and thrive by utilizing the available nutrients in the stagnant water, facilitated by conditions such as humidity, warmth, and residual nutrients (Stapleton et al., 2013). The symbiotic relationship among multiple microorganisms within biofilms, along with cross-feeding interactions, can contribute to malodor production(Luo et al., 2022).
Front-loading washing machines, due to their sealing design, are more prone to odor generation than top-loading machines(Abney et al., 2021). Takeuchi et al. identified 4-methyl 3-hexenoic acid (4M3H) as the primary malodor emitted by unwashed clothes, described as a wet, dirty, dusty cloth-like stench or a sour, sweaty odor (Takeuchi et al., 2012). Subsequent research indicated that the main contributors to malodor and laundry odors are 3-hydroxy-3-methylhexanoic acid, 3-mercapto-3-methylhexan-1-ol, and 4-methyl-3-hexenoic acid, along with other C7 compounds (Takeuchi et al., 2013). Analysis of volatile organic compounds (VOCs) from 66 microbiological isolates obtained from WMs and fabrics revealed 10 identified VOCs, including dimethyl disulfide, 3-methyl-1-butanol, 2,4-dithiapentane, dimethyl trisulfide, 2-tridecanone, indole, 2-phenylethanol, isovaleric acid, isobutyric acid, and 1-undecene (Stapleton et al., 2013). Furthermore, Moraxella osloensis, isolated from WMs, was found to be responsible for the production of 4M3H (Kubota et al., 2012). Moraxella is a member of the human microbiota and can utilize human metabolites for growth and reproduction.
Microbial contamination in WMs not only damages the appearance and components of the machines but also results in the release of unpleasant odors. These volatile compounds and bacteria can potentially transfer from contaminated WMs to laundry fabrics, leading to cross-contamination and even the risk of disease transmission (Bockmuehl et al., 2019). For example, Babic et al. identified opportunistic pathogens in the detergent drawer and rubber door seals of 70 residential WMs sampled in Slovenia (Kubota et al., 2012). Silver and isothiazolinone are common antibacterial agents, among which silver is usually used in the WM structure bacteriostasis and isothiazolinone is a component of laundry detergent (Wang et al., 2021).The main objective of this study is to identify the predominant microorganisms and major odorants present in washing machines in China and evaluate the inhibitory effects of commonly used WM cleaning methods, such as drum self-cleaning, addition of silver ions, and the use of antimicrobial agents, on pre-existing biofilms.