Investigations on the structure and function of the microbiota in household washing machines, kitchen sponges, and on laundered textiles

Abstract

In industrialized countries, people spend up to 90 % of their lifetime indoors, rendering domestic hygiene an important issue for human health and well-being. Due to favourable growth conditions, such as moisture, warmth, and sufficient supply of nutrients, many domestic environments harbour dense and diverse microbial communities. However, it also has been hypothesized that typical domestic cleaning and sanitation measures might shape these communities in a way that is unfavourable for human health and well-being. In order to get a deeper understanding of how environmental factors shape microbial communities in domestic environments, the structure (community composition) and function (physiology) of the microbiota in washing machines, kitchen sponges, and on washed laundry were investigated and associated with environmental factors using a polyphasic approach of cultivation- dependent and independent methods. Washing machines are widely used tools for laundry cleaning, known to offer favourable growth conditions for microorganisms. Colonisation is further promoted by current washing trends such as short and water-saving programmes, low washing temperatures, and the use of bleach-free liquid detergent. Using 16S rRNA gene amplicon pyrosequencing and MALDI- TOF-based identification of isolated bacteria, a diverse microbial community was detected at three sampling sites of typical household washing machines (sump, rubber door seal, and detergent drawer) comprising more than 200 bacterial species. The composition of this microbiota was strongly site-dependent, with the highest bacterial diversity found inside the detergent drawer. No correlations between selected user data and bacterial community composition were found, except the fact that bacterial diversity was significantly higher in the detergent drawer of machines that are frequently used with washing temperatures of 60 °C and higher. Cell counts based on swab samples of detergent drawer and door seal showed a bacterial load of 21,000 colony-forming units (CFU) per cm2, when averaged over all sampling sites. The lowest bacterial counts were found in the upper area of the rubber door seal, probably due to lack of water. The other sampling sites (detergent drawer, detergent chamber, bottom part of the rubber door seal) revealed similar bacterial counts of approximately 104 CFU per cm2. These findings, together with the fact that about half of the most common bacterial species were classified as potentially pathogenic, show that washing machines are a domestic source of potential pathogens, malodour producers, and cross-contamination. In order to learn more about the metabolic activities (e.g., substrate use) of bacteria on laundered textiles, a metatranscriptomic analysis pipeline was established and applied for the first time to compare bacterial gene expression on laundered cotton and polyester fabrics. The analysis revealed that 17 genes differed significantly in their expression between the two tissue types, which are involved in several different biochemical pathways, e.g. amino acid transport and metabolism or bacterial carbohydrate metabolism. The data allow careful speculation that bacteria might feed on carbohydrates released from the cotton textiles. Knowledge about substrate utilization on washed laundry might help to identify novel strategies against microbial malodour production on washed textiles. Sponges are very popular cleaning tools in domestic kitchens. They pick up and spread microorganisms on a massive scale when cleaning dishes and other kitchen surfaces. Microwave treatment represents an effective and widely used technique to quickly reduce the microbial load of kitchen sponges. However, the long-term effects of such a treatment on the microbial community were largely unknown. When comparing the metagenome of 10 regularly microwaved and 10 untreated used kitchen sponges, microwave treatment showed a trend towards lower structural microbial diversity, while functional diversity increased. This finding clearly indicates that microwave treatment alters microbial diversity and genetic potential of resident communities in household kitchen sponges. However, further work is needed to clarify in more detail whether these changes are rather beneficial or adverse in terms of human health and well-being. In the future, the establishment and application of methods focusing on a more functional characterisation of the microbiota in domestic environments, such as stable isotope probing or metabolomic studies in addition to metagenomics or metatranscriptomics, will provide new and important insight into the genetic potential and metabolism of the domestic microbiota and its interference with domestic cleaning measures. Such knowledge might be useful to develop novel strategies to tackle hygienic problems such as malodour formation or the selection and enrichment of (potentially) pathogenic and antibiotic-resistant species in the domestic environment.