Due to extreme environmental factors prevailing inside dishwashers, such as high temperature, oxidative stress, elevated salt concentrations (NaCl), mechanical shear forces, high organic loads and occasional dehydration, microorganisms colonizing dishwashers merge into survival structures - biofilms. They consist of mixed assemblages of various bacteria and fungi. The mycobiota of the rubber seals was globally consistent and it comprised 6 species of opportunistic pathogens such as E. dermatitidis, E. phaeomuriformis, Candida parapsilosis, Fusarium dimerum and Saprochaete/Magnusiomyces complex. Besides rubber seals, fungi populated also doors, walls, drains, sprinkler, side nozzle, rack for cutlery, rinse-air dispenser and detergent dispenser. Bacterial flora of dishwasher rubber seals was comprised of the genera Bacillus, Gordonia, Exiguobacterium, Acinetobacter, Stenotrophomonas and Micrococcus. Surprisingly, bacteriome of rubber seals could include also opportunistic pathogens, such as E. coli, P. aeruginosa, S. maltophilia and E. faecium. The initial settlers of the dishwasher seals were yeasts of the genus Candida, that offer a biotic surface for further establishment of microbes. Extensive synergies were observed among bacteria isolated from dishwashers. The species A. junii and P. aeruginosa were the best producers of biofilms in dishwashers and significantly contributed to the observed synergy in biofilms and to the total increase of biomass. Mixed bacterial biofilms included also the opportunistic pathogenic black yeast E. dermatitidis, enabling an easier settlement of rubber seals of dishwashers and other artificial surfaces (eg polypropylene). Changes of microbiome over time depended on the years of operation and frequency of use, while water hardness essentially influenced the fungal community. Tap water, connected to the household appliances, especially dishwashers, was the main entry point of fungi and to a lesser extent for bacteria. Additional sources of fungal and especially bacterial contamination were inhabitants, pets and food leftovers on dirty dishes. Fungi could be released from the dishwasher to the external environment at the end of the washing cycle via waste water and aerosols. Colonization of kitchen surfaces with fungi was influenced by the presence of dishwashers, resulting in higher incidence of black yeasts, especially E. dermatitidis, that were considerably less frequent in kitchens without dishwashers, where opportunistic pathogenic white yeast C. parapsilosis prevailed.
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