Dishwasher germs: three cleaning tricks for a cleaner machine

Many households place complete trust in their dishwasher. Put the plates in, start a cycle and the job is done - or so it seems. Yet studies indicate that virtually every appliance becomes home to bacteria, yeasts and even black yeasts, concealed within stubborn biofilms. A few straightforward habits can substantially lower this microbial load and make the kitchen noticeably more hygienic.

When the dishwasher becomes a home for germs

A widely cited 2018 study examined 24 ordinary domestic dishwashers in detail. Its findings were clear: dense biofilms - slimy layers made up of bacteria and fungi - were present on every rubber seal studied. Yeasts from the Candida genus were detected in every appliance in particular.

The researchers also identified familiar “problem bacteria”, including Pseudomonas, Escherichia - known for E. coli - and Acinetobacter. These microbes are considered opportunistic pathogens: they are usually harmless to healthy people but can pose considerably more risk to people with weakened immune systems.

“A dishwasher is meant to clean - but in reality, it often works with a hidden microbiome in the background that survives heat, detergent and salt remarkably well.”

Conditions inside the appliance resemble an extreme habitat: hot water, harsh detergents, salt, alternating humidity and periods of complete dryness. These are precisely the conditions that favour specialised microorganisms capable of adapting to this environment. They particularly tend to colonise:

• door seals and other rubber parts
• the base of the tub
• the filter insert and mesh
• spray arms and their fine nozzles

Why Eco programmes can become a problem

Many people now use energy-saving programmes at 30 to 45 °C almost exclusively. While this reduces energy use and costs, it provides little heat reserve against hardy fungi and bacteria. The biofilms remain in place, continue to grow and gradually become better adapted.

Several studies have found so-called black yeasts, such as Exophiala species, especially often on seals. These highly resilient fungi withstand major changes in temperature, humidity and pH. In some machines, they have been detected at very high concentrations.

For healthy people, this is generally not a major concern. However, for those with pre-existing conditions, chronic lung problems or severely compromised immune systems, such a germ reservoir may be relevant - especially when lightly soiled crockery repeatedly passes over the same surfaces.

What happens when you open the door

Anyone who opens the dishwasher immediately after a programme ends will recognise it: a rush of hot, humid air escapes. This cloud of steam can carry tiny droplets and particles with attached microorganisms into the room air.

A more recent study from 2024 suggests that spores and bacteria can spread around the kitchen through these aerosols. This does not mean every opened dishwasher automatically represents a health risk. It does show, however, that germs do not always remain where they developed.

“The warmer and more humid the interior is when the door is opened, the more easily germ-carrying microdroplets can enter the kitchen air.”

A similar pattern occurs with washing-up sponges and hand brushes: if they remain permanently damp, substantially more bacteria settle on them than on utensils that are allowed to dry properly between uses.

Three cleaning tricks that genuinely make your dishwasher cleaner

1. Treat rubber seals specifically

The rubber seals around the door are a favourite location for many yeasts and black fungi. Moisture collects there and food debris gets trapped there, creating an ideal habitat.

Proceed as follows:

• Preparation: Switch off the appliance, open the door fully and wipe away loose dirt with a cloth.
• Mix the cleaner: Combine roughly equal amounts of warm water and white household vinegar. If preferred, add a squirt of liquid soft soap or black soap cleaner.
• Scrub with a brush: Dip an old toothbrush into the mixture and thoroughly scrub all folds and edges of the seal. Include the underside, which is only visible when the rubber lip is lifted slightly.
• Wipe and dry: Go over the area with a damp cloth, then dry everything thoroughly with a dry microfibre cloth.

Acetic acid helps break down the alkaline protective layer in which many fungi thrive. The brush’s mechanical action loosens biofilms that can otherwise survive even powerful cleaning products.

2. Clean the filter and tub base every week

Grease, food scraps, eggshells and pasta - everything that should not enter the drain - ends up in the filter. In warm conditions, this mixture quickly becomes a buffet for microorganisms. Anyone who checks it only every few months risks rancid smells and higher germ levels.

A simple routine is enough:

• Remove the filter insert completely.
• Put food residue in the bin rather than the sink.
• Rinse the filter thoroughly under very hot water, using a brush if necessary.
• Wipe the base of the dishwasher with a cloth and remove any pools of water.
• Optionally, sprinkle a thin layer of bicarbonate of soda over the wet filter and the base area, leave it briefly, then reinstall the filter.

Bicarbonate of soda absorbs odours and may make conditions less favourable for mould-forming fungi. Above all, it is important not to leave anything rotting inside the warm, damp appliance.

3. Maintain spray arms and flush them regularly with heat

The spray arms distribute water throughout the dishwasher. If their small nozzles become clogged with limescale, grease residue or granules from the salt reservoir, water no longer reaches every area. Some zones are then consistently cleaned less effectively, giving biofilms an advantage.

Recommended procedure once a month:

• Remove the spray arms according to the manufacturer’s instructions.
• Rinse them under running water, then carefully clear the nozzles using a toothpick or thin needle.
• At the same time, add six tablespoons of citric acid powder to the empty interior, for example in the cutlery basket or on the base.
• Refit the spray arms and run a cycle of at least 60 °C without any dishes inside.

This hot empty cycle reduces limescale deposits and flushes loosened biofilms out of the pipework. If you normally use Eco programmes only, schedule such a hot cycle around every four weeks.

Simple everyday protection: drying, timing and hand hygiene

Alongside these three main measures, a few daily habits provide an extra layer of protection:

• Leave the door ajar after washing: Once the programme has finished, do not close the door completely; leave it slightly open instead. Residual moisture can escape, the interior dries faster and germs are less able to multiply.
• Do not leave the appliance full and closed for days: If you only run the dishwasher every few days, open the door in between so that a damp “fermentation chamber” does not form.
• Wash your hands after cleaning the filter: Removing food debris inevitably brings you into contact with high concentrations of germs. Brief handwashing afterwards reduces the chance of transferring them to your eyes, mouth or fresh wounds.
• Briefly rinse heavily soiled dishes: Remove larger residues such as lumps of sauce, spinach or squashed pasta first. This means less organic material ends up in the filter and corners.

How dangerous are the germs really?

The idea of a “bacterial habitat” inside the dishwasher sounds alarming. In everyday life, though, the risk to healthy people usually remains limited. Many of the detected microorganisms are already part of our surroundings and everyday contact - on chopping boards, in sinks and on hands.

The issue matters chiefly for people with severely reduced immunity, such as those who have had organ transplants, are receiving intensive chemotherapy or have serious lung disease. In these situations, specialists generally recommend a stricter hygiene routine throughout the household. This may include hotter wash cycles, more frequent empty cycles and particularly careful filter cleaning.

What terms such as biofilm and black yeasts mean

Many of the microorganisms mentioned are relatively vulnerable on their own. Within biofilms, however, they form a kind of protective community: a slimy matrix of sugars and proteins surrounds the cells, retains nutrients and partly shields them from chemicals and spikes in temperature.

“A biofilm is best compared with a multi-layered carpet - simply wiping the surface does not get rid of everything.”

Black yeasts are distinguished by their dark pigment. This pigment layer protects their cells from stress caused by heat, UV light and chemicals. That is why researchers frequently find them in extreme environments: on damp silicone seals in bathrooms, in saunas and on the rubber seals of washing machines and dishwashers.

In practical terms, this means that not every discolouration on a seal is immediately dangerous, but it does indicate that a persistent micro-ecosystem has become established there. Regular treatment with a brush, a little acid and drying removes the conditions these organisms need most for growth.

How often each dishwasher care step makes sense

Measure	Recommended frequency
Clean seals with a brush and vinegar mixture	every 2–4 weeks
Clean the filter and tub base	once a week
Run an empty cycle at ≥ 60 °C with citric acid	once a month
Leave the door ajar after the programme ends	after every wash cycle

Anyone who follows these intervals does not need expensive specialist cleaners. Household basics such as vinegar, citric acid, a little soft soap, some bicarbonate of soda, a brush and consistent routines are usually sufficient. The results are visible not just in the laboratory but in everyday use: fewer musty odours, cleaner seals and clearer glasses.