Dishwasher germs: three cleaning tricks for a cleaner machine

Many households place unquestioning trust in their dishwasher. Load the plates, start a cycle and job done – or so it seems. Yet studies indicate that virtually every appliance harbours bacteria, yeasts and even black yeasts, concealed within persistent biofilms. Establishing a few straightforward routines 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 investigation examined 24 ordinary household dishwashers in detail. Its findings were clear: researchers found dense biofilms – slimy layers formed by bacteria and fungi – on every rubber seal. Yeasts from the Candida genus were especially prevalent, appearing in all of the appliances.

Typical “problem bacteria” were also present, including Pseudomonas, Escherichia (known through E. coli) and Acinetobacter. These microbes are regarded as opportunistic pathogens: they are generally harmless to healthy people, but can pose a considerably greater risk for those with weakened immune systems.

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

The conditions inside the appliance resemble an extreme habitat: hot water, harsh detergents, salt, fluctuating moisture and periods of complete dryness. This is exactly the kind of environment that favours specialised microorganisms able to adapt to it. They particularly tend to colonise:

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

Why Eco programmes can become a problem

Many people now use economical programmes at between 30 and 45 °C almost exclusively. While this saves energy and money, it provides little heat reserve against resilient fungi and bacteria. Biofilms remain in place, continue growing and gradually become better adapted.

Several studies have found so-called black yeasts, such as Exophiala species, particularly often on seals. These exceptionally hardy fungi tolerate major changes in temperature, humidity and pH. In some appliances, they were detected at very high densities.

This is usually not a major concern for healthy people. However, for people with pre-existing conditions, chronic lung problems or severely compromised immune systems, such a germ reservoir may be relevant – especially if slightly soiled crockery repeatedly passes over the same surfaces.

What happens when you open the door

Anyone who opens the dishwasher immediately after a cycle will recognise it: a rush of hot, humid air escapes. Tiny droplets and particles carrying attached germs may enter the room air within this cloud of steam.

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

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

A similar pattern is seen with washing-up sponges and hand brushes: if they stay wet all the time, they become home to far more bacteria than utensils that are allowed to dry properly in between.

Three cleaning tricks that genuinely make your dishwasher cleaner

1. Treat rubber seals specifically

Door seals are a favourite location for many yeasts and black fungi. Moisture collects there and food residues get trapped there – creating an ideal habitat.

Here is how to do it:

• Preparation: Switch off the appliance, open the door fully and wipe away any coarse dirt with a cloth.
• Mix the cleaning solution: Combine roughly equal parts warm water and white household vinegar. If desired, 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 every fold and edge of the seal – including the underside, which is only visible when you gently lift the rubber lip.
• Wipe again: Go over it with a damp cloth, then dry everything thoroughly with a dry microfibre cloth.

Acetic acid helps to break through the alkaline protective layer in which many fungi thrive. The mechanical action of the brush loosens biofilms that can otherwise withstand even powerful cleaners.

2. Clean the filter and dishwasher tub base every week

The filter catches grease, food remnants, eggshells and pasta – everything that should not reach the drain. In warm conditions, this mixture quickly becomes a feast 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 residues 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 standing water.
• Optionally, sprinkle a thin layer of bicarbonate of soda onto the wet filter and base area, leave it briefly, then refit 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 decaying permanently in the warm, damp interior.

3. Maintain spray arms and flush them through with heat regularly

Spray arms distribute water around the appliance. If their small nozzles become blocked by limescale, grease residues or grains from the salt reservoir, water no longer reaches every area; some zones are then consistently cleaned less effectively, which benefits biofilms.

Recommended method once a month:

• Remove the spray arms according to the manufacturer’s instructions.
• Rinse them under running water and carefully clear the nozzles with a toothpick or fine 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 an empty cycle at at least 60 °C.

This hot empty cycle reduces limescale deposits and flushes loosened biofilms from the pipes. Anyone who normally uses only Eco programmes should schedule such a hot cycle about every four weeks.

Simple everyday protection: drying, timing and hand hygiene

Alongside these three core measures, a few daily habits provide an additional layer of protection:

• Leave the door ajar after washing: Once the programme has finished, do not close the door fully; leave it slightly open instead. This lets residual moisture escape, helps the interior dry more quickly and makes it harder for germs to multiply.
• Do not leave the appliance full and shut for days: If the dishwasher is only run every few days, open the door in between so that a damp “fermentation chamber” does not develop.
• Wash hands after cleaning the filter: Removing food debris inevitably brings you into contact with high concentrations of germs. A brief hand wash afterwards lowers the chance of transferring them to your eyes, mouth or fresh wounds.
• Quickly rinse heavily soiled dishes: Remove large residues such as clumps of sauce, spinach or crushed pasta beforehand. This means less organic matter reaches the filter and corners.

How dangerous are the germs really?

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

The issue matters most for people with severely reduced immune defences, such as after an organ transplant, during intensive chemotherapy or with serious lung disease. In these situations, specialists generally recommend a stricter household hygiene regime. This may include hotter wash cycles, more frequent empty cycles and especially thorough filter cleaning.

What terms such as biofilm and black yeasts mean

Many of the microorganisms mentioned are relatively sensitive when alone. In biofilms, however, they form a type of protective community: a slimy matrix of sugars and proteins surrounds the cells, retains nutrients and partly shields them from chemicals and temperature peaks.

“A biofilm is best compared with a multi-layered carpet – if you only wipe the surface, you will not remove nearly everything.”

Black yeasts stand out because of their dark pigment. This pigmented coating protects cells from stress caused by heat, UV light and chemicals. That is precisely 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 everyday terms, this means that not every discolouration on a seal is immediately dangerous, but it does show that a lasting micro-ecosystem has established itself there. Regularly tackling it with a brush, a little acid and drying deprives these organisms of their best growing conditions.

How often each maintenance task makes sense

Measure	Recommended frequency
Clean seals using a brush and vinegar mixture	every 2–4 weeks
Clean the filter and dishwasher tub base	once a week
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 products such as vinegar, citric acid, a little soft soap, some bicarbonate of soda, a brush and clear routines are generally sufficient. The effect is apparent not only in the laboratory, but in practical terms too: fewer musty smells, cleaner seals and clearer glasses.