Dishwasher germs: how to remove biofilms effectively

A dishwasher is often seen as the kitchen’s hygiene hero: load the plates, press a button, and everything comes out clean – or so we hope. However, 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 news is that a few targeted measures can significantly reduce the microbial load.

What researchers found inside dishwashers

A frequently cited 2018 investigation examined 24 domestic dishwashers in detail. Samples were mainly taken from the rubber seals around the door – precisely where moisture, food residues and detergent deposits collect.

“Researchers found dense biofilms of bacteria and yeasts on the rubber seals in every appliance examined.”

These slimy layers contained, among others, germs from the genera Pseudomonas, Escherichia and Acinetobacter. Many are considered opportunistic pathogens: they are usually harmless to healthy people but can potentially cause problems for those who are severely weakened.

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

Why dishwashers are an ideal habitat for germs

At first glance, the inside of a dishwasher appears inhospitable to life: hot water, dishwasher detergent, salt, and sharp shifts between wet and dry conditions. Yet these very extremes make it appealing to certain microorganisms.

• Warm and damp: Residual water and condensation provide germs with a continually moist environment.
• Alkaline conditions: Dishwasher detergent raises the pH level. While many everyday germs cannot cope with this, specialised species can.
• An endless nutrient supply: Food remnants, fats, starches and proteins continually provide fresh nourishment.
• Protected areas: Rubber seals, corners, filters and spray arms are not completely rinsed through.

Microorganisms exploit these circumstances 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 considerably more resistant to heat and chemicals.

When economy programmes become a hygiene issue

To save water and electricity, many households now use almost exclusively Eco or quick programmes at 30 to 45°C. This may ease the pressure on household bills, but it does not necessarily support good hygiene.

“Low temperatures cut energy use, but leave biofilms inside the appliance largely unaffected.”

Temperatures that are too low do not reliably kill heat-resistant germs. They merely wash away loose surface layers. Even fungi capable of making people with weakened immune systems ill can remain deep within the film.

What enters your kitchen when you open the door

Anyone who opens the dishwasher immediately after a cycle will recognise the cloud of hot steam that escapes. A 2024 study suggests that this is not just water vapour: tiny droplets carrying spores and bacteria may also be released.

These aerosols spread briefly through the kitchen air. This is generally not a serious risk for healthy people. However, in households where someone has a severely weakened immune system – for example, following chemotherapy or with serious lung disease – it is worth paying closer attention to the appliance’s hygiene.

Another consideration is that washing-up sponges and brushes left damp beside the machine also provide ideal surfaces for germs. They often remain wet for days and can form biofilms similar to those inside the appliance.

Three targeted steps that genuinely clean your dishwasher

A thorough clean alongside a few small routines can noticeably reduce the germ load in a dishwasher. The key is to address the critical areas regularly.

1. Scrub the rubber seals thoroughly

The door seal is a hotspot for fungi and bacteria. Simply 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 its folds and corners.
• For heavy greasy deposits, add a little washing-up liquid or soft soap cleaner.
• Then wipe over with clean water and allow it to dry thoroughly.

Acetic acid lowers the pH locally and can disrupt the protective layer of many fungi. The mechanical friction of the brush dislodges the film, which chemical cleaners alone cannot break through.

2. Clean the filter and sump weekly

The filter collects everything rinsed from the dishes: food scraps, bones, eggshells and coffee grounds. This mixture decomposes, smells unpleasant and serves as a buffet for microbes.

• Twist out or unclip the filter according to the manufacturer’s instructions.
• Rinse it thoroughly under very hot water and remove larger debris.
• Use a soft brush or sponge to scrub the mesh and edges.
• 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 greatly reducing odours and the germ load.”

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

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

A small monthly service 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 thin wire.
• Refit the arms and make sure they click securely into place.
• Run a hot cycle at a minimum of 60°C with no dishes inside.
• Add six tablespoons of citric acid powder to the empty interior to remove limescale.

After such an empty cycle, the internal surfaces are noticeably smoother, limescale deposits disappear, and germs have fewer places to cling to.

How to stop new biofilms from forming

A deep clean only works in the long term if everyday habits support it. Small changes can slow the return of microbial films.

• Use the high-temperature setting occasionally: One hot cycle a week delivers a thermal shock to many germs.
• Leave the door slightly ajar after a wash: The interior dries more quickly, reducing damp areas.
• Scrape off large 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 affect seals and metal components.
• Wipe the interior occasionally: Use a damp cloth and mild cleaner on the walls, particularly around the lower section.

Who should take dishwasher hygiene particularly seriously

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

The situation is more sensitive in households with:

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

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

Understanding common terms relating to germs in dishwashers

Many technical terms sound alarming, but in everyday life they are often less dramatic than they seem. This brief overview can help put them into perspective:

Term	Meaning	Relevance in the home
Biofilm	A slimy layer of microorganisms and a protective matrix	Forms on seals, filters and spray arms – difficult to remove
Opportunistic bacteria	Mainly become dangerous for severely weakened people	Found in many damp areas of the home
Yeasts	Single-celled fungi, sometimes part of normal flora	Widespread in dishwashers, usually unproblematic for healthy people
Black yeasts	Particularly resilient, darkly pigmented fungi	May inhabit appliance biofilms and respond poorly to mild cleaning

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

How often is a thorough dishwasher clean really worthwhile?

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

A practical guide:

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

This helps the dishwasher remain not only functional but also more hygienic – and the dishes handled every day will not merely feel clean, but will have taken a considerably lower-germ route through the machine.