Germs in the dishwasher: how to remove biofilms effectively

The dishwasher is often seen as the kitchen’s hygiene hero: load the plates, press a button and expect everything to come out clean. Yet both recent and older studies paint a different picture. Stubborn biofilms made up of germs and fungi develop in virtually every household appliance. The encouraging part is that a few targeted measures can substantially reduce the microbial load.

What researchers found inside dishwashers

A frequently cited 2018 study took a closer look at 24 domestic dishwashers. Samples were taken primarily from the rubber seals around the door, where moisture, food debris and detergent residue collect.

"The researchers found dense biofilms of bacteria and yeasts on the rubber seals in every appliance examined."

These slimy layers included germs from the Pseudomonas, Escherichia and Acinetobacter genera. Many of these are considered opportunistic pathogens: they are generally harmless to healthy people but may be problematic for people who are severely debilitated.

Yeasts from the Candida genus were particularly common, having been detected in every dishwasher included in the study. So-called “black yeasts” were also present. These dark-coloured fungi are exceptionally resilient and can withstand high temperatures, detergents and salt.

Why a dishwasher is an ideal habitat for germs

At first glance, the interior of a dishwasher seems inhospitable: it is exposed to hot water, detergent, salt and considerable shifts between wet conditions and drying. However, these very extremes make it attractive to certain microorganisms.

• Warm and damp: Residual water and condensation provide germs with a permanently moist environment.
• Alkaline conditions: Dishwasher detergent raises the pH. While many everyday germs cannot cope with this, specialised species can.
• An endless supply of nutrients: Food particles, fats, starch and protein provide a continuous source of nourishment.
• Protected areas: Rubber seals, corners, filters and spray arms are not fully rinsed through.

Microorganisms exploit these conditions to create biofilms. These are slimy layers in which bacteria and fungi protect themselves with their own matrix. Within this matrix, they exchange nutrients and signals, becoming markedly more resistant to heat and chemicals.

When economy programmes become a hygiene issue

Many households now rely almost exclusively on Eco or short programmes running at 30 to 45 °C to save water and electricity. This may reduce bills, but it does not necessarily improve hygiene.

"Low temperatures reduce energy consumption, but leave biofilms inside the appliance largely unaffected."

Temperatures that are too low do not reliably kill heat-resistant germs. They merely rinse away loose layers. Even fungi that can cause illness in people with weakened immune systems may remain in the deeper layers of the film.

What enters your kitchen when you open the dishwasher door

Anyone who opens the dishwasher immediately after a programme ends will know the cloud of hot steam that escapes. A 2024 study indicates that this release may contain more than water vapour: tiny droplets carrying spores and bacteria can be swept out too.

These aerosols briefly disperse through the kitchen air. For healthy people, this is usually not a serious risk. However, in households where someone has a severely weakened immune system, such as after chemotherapy or with severe lung disease, it is worth paying closer attention to the appliance’s hygiene.

There is another consideration: washing-up sponges and brushes left damp beside the machine also provide ideal surfaces for germs. They often stay wet for days and can then develop biofilms similar to those inside the dishwasher.

Three targeted steps that properly clean your dishwasher

A deep clean combined with a few small routines can considerably lower the germ load in a dishwasher. The key is to deal with the critical areas regularly.

1. Scrub the rubber seals thoroughly

The door seal is a hotspot for fungi and bacteria. Wiping it with a damp cloth is not enough.

• Open the door fully and carefully fold back the entire seal so that every crevice is visible.
• Dip an old toothbrush into a mixture of warm water and white household vinegar.
• Scrub the seal firmly, section by section, including folds and corners.
• If deposits are particularly greasy, add a little washing-up liquid or black soap cleaner.
• Wipe over with clean water afterwards and allow it to dry thoroughly.

Acetic acid lowers the pH locally, disrupting the protective layer of many fungi. The physical friction from the brush loosens the film that chemical cleaners alone cannot break through.

2. Clean the filter and sump every week

The filter collects everything washed off the dishes: food scraps, bones, eggshells and coffee grounds. This mixture decays, smells and acts as a buffet for microbes.

• Unscrew or unclip the filter according to the manufacturer’s instructions.
• Rinse it thoroughly under very hot water and remove larger debris.
• Scrub the mesh and edges with a soft brush or sponge.
• Wipe the area beneath it in the sump with a cloth.
• Sprinkle a little bicarbonate of soda into the filter area to reduce odours.

"A clean filter protects spray arms and pumps from blockages while substantially reducing odours and the germ load."

3. Maintain the spray arms and run an empty hot cycle

Blocked spray arms distribute water unevenly, leaving parts of the appliance permanently damp. Biofilms thrive particularly well in these areas.

Once a month, a short maintenance session is worthwhile:

• Remove the spray arms, where possible, following the instruction manual.
• Rinse them under running water and clear every nozzle opening with a toothpick or fine wire.
• Refit the arms and make sure they click securely into place.
• Run an empty programme at a minimum of 60 °C.
• Put six tablespoons of citric acid powder into the empty interior to dissolve limescale.

After an empty cycle like this, the inner surfaces are noticeably smoother, limescale deposits are removed and germs have less to cling to.

How to prevent new biofilms from forming

A thorough deep clean only works if everyday use supports it. Small habits slow down the formation of new microbial films.

• Use a high-temperature setting occasionally: One hot cycle a week delivers a thermal hit against many germs.
• Leave the door ajar after a wash cycle: The interior dries more quickly and damp areas are reduced.
• Scrape off larger food residues first: Briefly scrape plates rather than fully pre-washing them; this is kinder to both the environment and the machine.
• Dose salt and rinse aid correctly: Too little can weaken cleaning performance, while too much can put strain on seals and metal parts.
• Wipe the interior occasionally: Go over the walls with a damp cloth and mild cleaner, particularly in the lower section.

Who should pay especially close attention to dishwasher hygiene

For most people, the dishwasher remains a low health risk despite being a reservoir for microbes. The appliance cleans intensively, and clean dishes usually dry well afterwards.

The situation is more sensitive in households with:

• transplant recipients
• people with severely weakened immune systems
• chronic lung diseases
• premature babies or seriously ill infants

In these circumstances, a strict cleaning schedule is worthwhile, potentially supplemented by advice from doctors. Some clinics recommend higher temperatures, more frequent empty cycles and especially consistent filter care in such situations.

Understanding common terms relating to germs in the dishwasher

Many technical terms sound alarming, although their everyday implications are often less dramatic than expected. This brief overview helps put them into context:

Understanding these terms makes it easier to plan hygiene measures realistically and avoid excessive panic without downplaying the risks.

How often is a dishwasher deep clean really worthwhile?

The right frequency depends heavily on how the appliance is used. In a single-person household running two cycles a week, a deep clean every two to three months is usually sufficient. Families that use the dishwasher daily and prepare plenty of starchy or protein-rich foods should maintain it considerably more often.

A practical guide:

• Filter and sump: once a week
• Rubber seals: brush thoroughly every two to four weeks
• Spray arms and an empty hot cycle: roughly once a month

This keeps the dishwasher functioning properly as well as making it more hygienic. The dishes handled every day will not merely feel clean; they will have gone through a significantly lower-germ route.