The question of whether a dirty dishwasher can make you sick comes down to maintenance. While these appliances are engineered to clean and sanitize, a lack of regular care creates an environment where specific microorganisms can thrive, presenting a low but real health risk. Modern dishwashers operate in a constantly changing environment, fluctuating between wet and dry, hot and cool, and high and low acidity, which selects for hardy organisms. When maintenance is neglected, these microbes can accumulate, potentially compromising the cleanliness of dishware.
Identifying the Germs and Biofilm
The interior of a dishwasher, particularly the rubber seals, is an extreme habitat that fosters the growth of highly resilient microorganisms. These organisms often form a slimy protective layer known as a biofilm, which shields them from detergents, heat, and drying cycles. This complex, multi-species community adheres to surfaces and is difficult to eradicate using a normal wash cycle.
The microbial community in a dishwasher is dominated by bacteria and fungi that are tolerant of these harsh conditions. Bacterial genera frequently found in dishwasher biofilms include Pseudomonas, Escherichia, and Acinetobacter, which contain strains known as opportunistic pathogens. These bacteria are usually harmless but can cause infections, particularly in people with weakened immune systems.
Fungi are also highly prevalent, with species like Candida, Cryptococcus, and Rhodotorula commonly isolated from the rubber seals. More concerning is the frequent presence of heat-resistant fungi, specifically “black yeasts” like Exophiala dermatitidis. This species is rarely found in nature but thrives in dishwashers, and it is known to cause mycoses, or fungal infections, in vulnerable populations.
How Contamination Spreads
The presence of resilient microbes in the dishwasher creates several pathways for potential exposure to human health. The primary risk involves incomplete sterilization, where the washing cycle fails to fully eradicate the biofilm or the organisms it harbors, leaving them on the surface of dishes and utensils. This cross-contamination can occur when food particles and bacteria from dirty vessels are redeposited onto cleaned items during the wash or rinse phase.
Another significant vector is the aerosolization of pathogens, which occurs when hot steam is released into the air at the end of a cycle. Studies have shown that species isolated from dishwasher biofilms can be found in these hot aerosols, which can then be inhaled or settle on kitchen surfaces. Direct contact with contaminated components also poses a risk, particularly when touching the slimy rubber seals or the food-laden filter and then handling food or touching one’s mouth.
Additionally, standing water in the base of the machine, which may not completely drain, acts as a continuous reservoir for these microorganisms. This stagnant, nutrient-rich water allows the bacterial and fungal populations to multiply between cycles.
Preventing Pathogen Buildup
Regular, focused maintenance is the most effective way to disrupt the microbial ecosystems within a dishwasher and mitigate health risks. The most common source of buildup is the filter, which collects food particles and grease, providing a constant nutrient supply for biofilm formation. The filter, which is typically located at the bottom of the machine, should be removed and cleaned monthly using a scrub brush and warm, soapy water to physically remove the accumulated organic matter.
To sanitize the interior, running a self-cleaning or high-temperature cycle with a specific cleaning agent is recommended. For instance, placing two cups of distilled white vinegar in an open, dishwasher-safe container on the top rack and running a hot water cycle can help dissolve mineral buildup and kill certain types of bacteria. Alternatively, one cup of liquid bleach can be used in a bleach-safe bowl on the bottom rack, provided the machine’s interior is not stainless steel, as bleach can damage the finish.
Commercial dishwasher cleaning products are also formulated to break down biofilm and sanitize the machine. Regardless of the agent chosen, this deep cleaning should be performed monthly to prevent the establishment of dense, protective biofilms. Opening the dishwasher door after the cleaning cycle to allow the interior to air dry will further inhibit the growth of moisture-loving fungi and bacteria.