Bacterial communities in domestic washing machines associated with user-perceived odours

Washing machines are known to contain significant amounts of microorganisms with negative implications for laundry and machine hygiene. In particular, the formation of machine and laundry odour and the potential role that microorganisms might play are still far from being understood. Here, we used Oxford Nanopore-based sequencing of full-length 16S rRNA gene amplicons and quantitative PCR to qualitatively and quantitatively investigate the bacterial community composition of different parts (detergent drawer, door seal, sump) of machines categorized as odorous (n = 8) and non-odorous (n = 8) by their users. DNA was extracted from swab samples taken in a standardized manner from each machine. In total, full-length 16S rRNA gene sequences (~ 1.5 kb) were obtained from the investigated machines and assigned to 1492 distinct species. The observed community compositions corroborated previous investigations using shorter gene sequences for identification. For instance, odour-related Moraxella osloensis showed the highest relative abundance in door seal samples. However, only a few statistically significant differences were found in overall community composition (alpha- and beta-diversity) between odorous and non-odorous machines. To investigate whether odorous machines might contain a higher microbial load, bacterial DNA was quantified by qPCR. Odorous machines showed higher median bacterial DNA concentrations across compartments, but odour category had no significant effect on bacterial abundance in a mixed-effects analysis. In summary, odorous and non-odorous machines did not differ greatly in bacterial community composition. Differences in bacterial activity or subjective odour perception may be responsible for perceived odour. Hence, future studies should include metatranscriptomic analyses and larger sample sizes.