The microbial world inside your dishwasher

Many studies have shown that bacteria and fungi colonize a wide range of everyday objects: from the coffee machine to the washing machine, microorganisms are everywhere. That is why, an international team focused on the microorganisms present in the dishwasher and their potential impact on our health.

 

The dishwasher is part of our everyday life and hosts a microbial community which has hardly been studied. Yet inside it, germs grow in hostile conditions, thus raising concerns regarding a possible contamination of the different items that have been washed, and a potential impact on our health. To assess this impact, researchers extracted the rubber seals from 24 dishwashers of private homes. Usage frequency, age of the appliance and water hardness are factors that have an impact on germs and that were taken into account during the analysis of the different species present.

Life in extreme habitats

Despite extreme conditions, the most robust “harmful” bacteria and fungi have developed on the metal and plastic components and rubber seals of the dishwasher. They had to survive dry periods alternating with humid ones, quick changes in temperature during washing cycles reaching up to 74ºC, contact with potent alkaline detergents, or even the shear stress caused by water jets.  How do they survive this flood? Thanks to germ resistance acquired through the formation of a protective layer, called “biofilm”. Some microbes are found in all samples: most of them are typical of extreme environments (resistance to UV radiation, high temperature and pH, presence of heavy metals such as arsenic); and able to survive by degrading large molecules that are found, among others, in detergents.

Disease-causing bacteria?

The researchers also found microorganisms that are part of the human microbiota (intestines, skin), which probably originate from the people using the appliance, and whose aptitude to live in an extreme environment comes from the formation of a protective biofilm.  Among them, sample analysis revealed several pathogenic germs: the fungus Candida is the most abundant one, and bacteria Escherichia, Shigella and Pseudomonas were found in near two thirds of dishwashers. All these potentially pathogenic germs, which belong to our intestinal flora, can be involved in intestinal diseases,  which are sometimes severe. Although their harmfulness was not assessed in this study, their presence could be a potential source of domestic infection.

 

Sources:

Raghupathi, P. K. et al. Microbiomes in Dishwashers: Analysis of the microbial diversity and putative opportunistic pathogens in dishwasher biofilm communities. Appl. Environ. Microbiol. (2018). doi:10.1128/AEM.02755-17