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Bacteria and fungi colonize the rubber seals of dishwashers (2018-05-04)

Bacteria and fungi colonize the rubber seals of dishwashers in biofilms—which are thin, slimy, hard-to-eradicate films of these microbes that adhere to surfaces.

Limiting factors, such as high temperatures, high and low pH, high NaCl concentrations, presence of detergents and shear force from water during washing cycles define the microbial survival in this extreme system.

Investigators have now shown that the diversity of species in the biofilms correlates most strongly with the age of the dishwasher, its frequency of use, and the hardness of the tap water.

The types of bacteria most commonly found in the samples were groups that include opportunistic pathogens, notably Pseudomonas, Escherichia, and Acinetobacter.

The most prevalent fungal groups, Candida, Cruyptococcus, and Rhodotorula, are also known for opportunistic pathogenicity, said coauthor Nina Gunde-Cimerman, PhD, professor of Microbiology and Mycology, University of Ljubljana, Slovenia.

The investigators obtained the bacteria by swab sampling rubber seals on 24 household dishwashers, followed by extracting the environmental DNA from each.

Next generation sequencing was then used to determine the species present in the biofilms.

The investigators also used a molecular tool called “network interaction” to determine positive and negative correlations among different microorganisms.

“Positively correlated microorganisms contribute to the development of the biofilms on the rubber seals,” said Dr. Gunde-Cimerman.

The investigators used “predictive modeling” to determine which microbes are the early colonizers of the biofilms on the seals.

The investigators also performed statistical tests on the information they obtained from the householders on age of the dishwashers, hardness of the tap water, and the condition of the seals, as well as the microbial diversity in each dishwasher, to determine the relative contributions of each factor to the colonization process.

The genesis of the study was a moment of insight in the latter 2000s.
For nearly two decades, Dr. Gunde-Cimerman had studied fungi living in extreme environments.

Dishwashers, she realized, are artificial extreme environments. She took a swab sample.

“The analysis in the lab revealed that my dishwasher was full of the black yeast, Exophiala dermatitidis,” she said, noting that it was known almost exclusively from patients.

“When I asked my coworkers to sample their dishwashers, we confirmed my results,” said Dr. Gunde-Cimerman.

The study expanded to Slovenian students and later to collaborators around the world, and in almost all cases, the original finding was confirmed,” she said.

That research was published in 2011. “The finding was surprising, particularly in light of the extreme conditions within the dishwasher, and the consistency of the results,” she said.

“We now know that black yeasts on dishwashers’ rubber seals are a global phenomenon which might be connected to increased incidence of mycoses in immunocompromised people who spend a lot of time indoors,” said Dr. Gunde-Cimerman.

The new research extends the work to bacteria.

The research is published January 12th in Applied and Environmental Microbiology, a journal of the American Society for Microbiology.

For more information
Applied and Environmental Microbiology
Microbiome in Dishwashers: Analysis of the microbial diversity and putative opportunistic
pathogens in dishwasher biofilm communities

American Society For Microbiology