Myth or Fact: Is the Kitchen Sponge Really the Dirtiest Thing in Your Kitchen?

What scientists found inside

In 2017, microbiologists from Furtwangen University and Justus Liebig University Giessen in Germany analysed 14 used kitchen sponges and published the results in Scientific Reports (Cardinale et al.). Under the microscope, bacteria reached local densities of up to 54 billion cells per cubic centimetre. Five of the ten most common types were closely related to species in "risk group 2", a classification for organisms that can cause illness, mainly in people with weakened immune systems.

The study's lead scientist, Markus Egert, told NPR the team found 362 different species of bacteria, and that there is hardly any habitat on Earth with similar densities apart from the human gut (NPR).

One finding surprised people. Sponges that their owners said they cleaned regularly did not have a healthier microbiome, and two of the risk-group-related types were actually more common in them. The authors suggested replacing sponges regularly, for example weekly, rather than relying on cleaning alone.

Does cleaning a sponge work at all?

It depends on the method. In 2007, the US Department of Agriculture's Agricultural Research Service soaked sponges in a mix of ground beef and growth medium to make them very dirty, then tested common household methods (USDA ARS). Microwaving a sponge for one minute killed 99.99999% of bacteria, and a dishwasher cycle with drying killed 99.9998%. Soaking in a 10% bleach solution, lemon juice or water killed between 37% and 87%, which still left enough bacteria to potentially cause illness.

If you microwave a sponge, make sure it is thoroughly wet and contains no metal. A dry sponge can scorch, and metal scourers can spark.

Your 10-minute checklist before guests arrive

• Swap the sponge. If it smells or is falling apart, put out a new one. Otherwise microwave it wet for one minute or run it through the dishwasher.
• Change the tea towel and hand towel. Guests will dry their hands on them.
• Wipe the handles. Fridge, oven, tap and cupboard handles are touched far more often than the counters.
• Clean the sink and the area around the drain with hot soapy water and a fresh cloth.
• Keep cloths separate. Use one for counters and paper towels for raw meat juices.

The bottom line

A kitchen can look spotless and still be wiped down with a dirty tool. Treat the sponge as something you replace often, sanitise it properly in between, and it will stop spreading what you're trying to remove.

Sources

1. Cardinale, M. et al., "Microbiome analysis and confocal microscopy of used kitchen sponges reveal massive colonization by Acinetobacter, Moraxella and Chryseobacterium species", Scientific Reports (2017)
2. NPR, So Your Kitchen Sponge Is A Bacteria Hotbed. Here's What To Do (2017)
3. USDA Agricultural Research Service, Best Ways to Clean Kitchen Sponges (2007)