The dishwasher rubber seal acts as a reservoir of bacteria in the home environment

In modern lifestyles, people make their everyday tasks easier by using household appliances, for example dishwashers. Previous studies showed massive contamination of dishwasher rubber seals with fungi, thus bacterial community, able to survive under harsh conditions, remain undetermined.

Bacteria that colonise the extreme environment of household dishwasher rubber seals were investigated using cultivation-dependent and metagenomic approaches. All bacterial isolates were tested for resistance to seven selected antibiotics. Same time bacterial diversity of tap water, connected to the dishwashers was investigated.

All 30 dishwashers investigated were colonised by various bacteria. Cultivation approaches resulted in 632 bacterial isolates in total, belonging to four phyla, eight classes, 40 genera and 74 species. The majority were Gram-positive, as solely Firmicutes (dominated by the Bacillus cereus group) and Actinobacteria. Gammaproteobacteria were primarily represented by Stenotrophomonas maltophilia, Pseudomonas aeruginosa and Escherichia coli. Metagenomic assessment of the bacterial biodiversity of the dishwasher rubber seals confirmed the predominance of Gram-positive bacteria, as primarily Actinobacteria, followed by Proteobacteria dominated by Gammaproteobacteria, and by pathogenic species such as Escherichia sp., Acinetobacter baumannii, Pseudomonas sp., Stenotrophomonas maltophilia, and Enterobacter sp.. Metagenomic assessment of bacterial biodiversity in the tap water connected to dishwashers revealed predominance of Gram-negative bacteria, in particular Proteobacteria, mainly represented by Tepidimonas sp.. Actinobacteria showed low numbers while no Firmicutes were detected in the tap water. The bacterial diversity of tap water was also lower, 23 genera compared to 39 genera on dishwasher rubber seals. Only 13 out of 49 genera identified by metagenomics approach was found in both environments, of those Gordonia was enriched while half of 13 genera were depleted in dishwashers compared to tap water.

These data indicate that colonisation of dishwasher rubber seals probably depends primarily on the bacterial input from the dirty vessels, and much less on the bacteria in the tap water. Based on the antibiotic resistance data, the dishwasher rubber seal bacterial isolates do not represent a serious threat for the spread of antibiotic resistance into the household environment. Nevertheless dishwashers cannot be ignored as potential sources of human infections, in particular for immuno-compromised individuals.