Dishwasher Hygiene: Three Targeted Measures Against Germs and Biofilms

It may seem surprising that the appliance designed to clean our dishes can itself become a breeding ground for germs. Studies indicate that virtually every domestic dishwasher contains bacteria, fungi and persistent biofilms, particularly around rubber seals and in concealed areas. Three targeted steps can, however, substantially reduce this microbial load.

When a dishwasher becomes a home for germs

Conditions inside a dishwasher are extreme: very hot water, powerful detergents, fluctuating moisture levels, a high pH, salt and marked temperature changes. While these conditions are too harsh for many microorganisms, certain specialists cope with them remarkably well.

Researchers examined precisely these hardy survivors in more detail. A 2018 study analysed 24 household appliances. Biofilms were found on the rubber seals of every one: slimy layers of bacteria, yeasts and fungi that form a protective coating across the surface.

Biofilms act like a fortress: within their slimy layer, germs withstand heat, detergents and short wash programmes far more effectively.

Species from the genera Pseudomonas, Escherichia and Acinetobacter appeared particularly often. These groups also include what are known as opportunistic pathogens. Among fungi, yeasts from the genus Candida predominated and were detected in all of the machines examined.

Black yeasts: inconspicuous yet exceptionally resilient

The rubber seal around the door frame offers microbes an ideal habitat: it has a rough surface, remains continually damp and collects traces of detergent and food. So-called black yeasts also thrive there. One well-known species is Exophiala dermatitidis, a fungus capable of tolerating very hot, salty and alkaline environments.

Measurements show that as many as millions of colony-forming units of these fungi may be present on a single square centimetre of seal. The risk is generally low for healthy people. However, anyone with a severely weakened immune system should take particular care over hygiene inside the appliance and should not overlook very old, musty or visibly black deposits.

Why low temperatures make the problem worse

Many households now use energy-saving programmes at 30 to 45°C almost exclusively to reduce energy costs. Although this helps with electricity bills, it also makes biofilms more likely to survive because they are not exposed to sufficient heat.

At the same time, grease and food residues accumulate in filters, on internal walls and inside spray arms. These deposits provide food for microbes, allowing the slimy layer to grow and become more established with every cycle.

• Low temperatures: only clean dishes to a limited extent and have little effect on biofilms.
• Prolonged damp conditions: encourage bacteria and fungi to multiply.
• Food residues: provide an ideal nutrient source.

What happens when the hot dishwasher door is opened

Many people open the dishwasher immediately after a cycle, while steam is still rising from it. A burst of hot air and fine droplets then escapes from the interior. Researchers describe this as a possible “aerosol cloud”, which may carry spores and bacteria into the kitchen air.

A 2024 paper points to this distribution of microorganisms, although the specific health benefits of stricter hygiene measures still require further research. What is clear is that moist, warm surfaces - whether a sponge, washing-up brush or door seal - harbour considerably more germs than materials that are allowed to dry thoroughly between uses.

Leaving the door slightly ajar and allowing the dishwasher to dry out properly removes the basis on which many germs survive.

Three targeted measures for a much cleaner dishwasher

1. Clean the seals thoroughly

The rubber door seal is the key hotspot. This is where anything caught in grooves and folds tends to build up. A simple but effective routine is as follows:

• Remove visible crumbs and deposits from the seal.
• Dip an old toothbrush into a mixture of hot water and clear household vinegar.
• Scrub the seal firmly with the brush, including its underside and the corners.
• Dry it with a cloth so that no moisture remains in the folds.

Vinegar contains acetic acid, which attacks the alkaline film left by detergent residues and weakens fungal structures. Many people also add a little black soap or mild washing-up liquid to break down grease more effectively.

2. Clean the filter and base regularly

The bottom of the dishwasher, where the filter sits, is easily neglected - yet it collects all the dirt from a wash cycle. Failing to clean it gives microorganisms an abundant food source.

• Remove the filter once a week and discard any larger debris.
• Rinse the filter thoroughly under the hottest water possible, using a soft brush if necessary.
• Wipe away streaks and deposits from the bottom of the dishwasher tub.
• Finally, sprinkle a little bicarbonate of soda onto the base; it absorbs odours and has a mild effect against fungal growth.

These steps should be carried out more often if heavily soiled dishes are loaded regularly. This also reduces the likelihood of unpleasant smells coming from the appliance.

3. Spray arms and an empty hot cycle

The spray arms ensure that water and detergent reach every part of the dishwasher. However, their fine jets can easily become blocked by limescale, grease or small pieces of food. When this happens, dead zones remain inside the appliance, allowing germs to persist undisturbed.

A useful monthly routine includes:

• Remove the spray arms from the appliance according to the manufacturer’s instructions.
• Rinse them through under running water.
• Carefully clear blocked jets with a toothpick or thin piece of wire.
• Then run an empty cycle at at least 60°C, ideally with around six tablespoons of citric acid inside the dishwasher, to descale the pipes and metal components.

The higher temperature destroys many heat-sensitive bacteria. Meanwhile, citric acid loosens limescale deposits, where germs readily settle.

How often should each task be done? A quick guide

Task	Recommended frequency
Clean the seal	every 1–2 weeks
Clean the filter and base	once a week, more often with heavy use
Clean the spray arms	once a month
Empty cycle at 60°C or above	once a month, more often if low-temperature cycles are used frequently
Leave the door ajar after washing	after every wash cycle

Who should pay particular attention to dishwasher hygiene?

For most healthy people, the germ layer inside a dishwasher is not a serious everyday risk. Even so, consistent maintenance is worthwhile where young children, older people or those with serious pre-existing health conditions live in the household.

People with allergies or chronic lung conditions may also be more sensitive to high levels of mould and spores indoors. Visible black deposits on seals or a musty smell from the appliance should therefore be treated as warning signs.

Treating your dishwasher as a household appliance “with a maintenance schedule” lowers germ levels - without any specialist chemicals.

The role of detergent, salt and rinse aid

Modern tablets and powders generally clean reliably. Nevertheless, the dosage matters: too little detergent allows grease and deposits to adhere more easily, while too much can leave residues that provide biofilms with nutrients.

Dishwasher salt keeps the built-in water softener working properly, prevents limescale layers and therefore removes potential niches for germs. Anyone with extremely hard tap water should avoid letting the salt reservoir run empty. Rinse aid is mainly used to give glasses a shine, but it also helps water drain away, which in turn speeds up drying inside the appliance.

What many people forget: what to do after the wash cycle

What happens in the hours after a cycle is at least as important as the wash itself. If the dishwasher remains completely closed, warm, damp air becomes trapped inside - ideal conditions for microbes.

• Open the door slightly once the programme has finished so steam can escape.
• Do not leave cutlery and dishes wet in the dishwasher for days.
• Scrape off substantial food residues before loading, rather than allowing whole remnants of meals to be washed through the machine.

These straightforward habits significantly reduce the ecosystem inside the appliance. Many people notice the difference first by smell: the familiar “dishwasher odour” usually disappears after just a few weeks of regular maintenance.

Dishwasher hygiene does not need to become a major project. Keeping an eye on seals, filters and spray arms, running an occasional empty hot cycle and not closing the door tightly straight after washing all make biofilms less comfortable - and help ensure the appliance genuinely delivers clean dishes.