Kitchen sponges should be replaced every single day, according to Dr Primrose Freestone, Associate Professor in Clinical Microbiology at the University of Leicester, who says she has settled a long-running online debate about how often the cleaning tool should be swapped out.
"Kitchen sponges really should have a single day of use," Dr Freestone said. "They can take up bacteria really quickly, which then can over time grow on the food residues and moisture within the sponge interior."
The question of sponge lifespan has divided social media users for years. Some replace their sponge multiple times a week, while others keep using the same one until it visibly falls apart.
On Reddit, one commenter said they would get a new sponge "twice per week because it gets wet and I don't want any bacteria on my dishes." Another wrote: "Sponges are foul after a couple days, and nothing you do is going to get them clean enough to be used to wash your dishes."
On the other side of the argument, one commenter said: "I replace it if it's ugly or stinks." Another said they wait until the sponge "has fallen apart" before throwing it out.
The 'sniff test' does not work
According to Dr Freestone, using smell or appearance to judge when to replace a sponge will not protect against harmful bacteria. Researchers from the German Federal Institute for Risk Assessment grew bacteria from used sponges in a lab and examined them under microscopes.
They found that kitchen sponges harboured food-poisoning pathogens, including Salmonella enteritidis, E coli and Staphylococcus aureus, deep within their structure. The researchers also found that smell and colour did not correlate with how many bacteria were present, meaning there is no way to visually judge when a sponge needs replacing.
Still, if a sponge does start to smell, the advice is to throw it away immediately. "Bad odour usually means microbial growth at a high level and you really would not want this sponge to clean your worktop or wash up your pots!" Dr Freestone said.
A perfect breeding ground
Dr Freestone said kitchen sponges act as a "reservoir" for bacteria that can spread infections around the kitchen. Sponges are almost constantly wet, packed with food residue and left in warm kitchens, creating conditions in which bacteria can multiply.
"The longer you leave the sponge, and use it, the more germs and food bits it will pick up," Dr Freestone said. "The sponge microflora will eat the food residues in the sponge and start increasing in numbers."
The porous structure that makes sponges effective at cleaning also makes it difficult to remove bacteria once they take hold. Researchers found that bacteria form sticky, protective layers that shield them from the environment, and the longer they were allowed to grow, the less effective disinfectant became, eventually reaching a point where the sponge was not clean even after being soaked in bleach.
The illness-causing bacteria were even able to survive after a sponge had been completely dry for up to two weeks, according to the research. And the danger is not confined to the sponge itself.
"Bacteria inside sponges grow and can be transferred to any surface the sponge touches," Dr Freestone said. "If there are food poisoning bacteria in the sponge, these pathogens could then be spread onto whatever that contaminated sponge touches."
Other ways to avoid food poisoning
Beyond replacing sponges regularly, several other steps can reduce the risk of food poisoning, according to guidance cited alongside the research.
- Keep a clean workspace, since germs can survive on kitchen surfaces and hands.
- Avoid cross-contamination by using separate cutting boards and plates for raw meat, poultry, seafood and eggs, and storing them separately in the fridge, as recommended by the CDC.
- Use a food thermometer to make sure food reaches a safe internal temperature, since this varies by ingredient.
- Refrigerate perishable food within two hours of purchase, and keep the fridge below 40°F.
- Do not rely only on expiration dates. If food has an unusual smell or colour, throw it out.
- Thaw frozen food in the refrigerator, in cold water, or in the microwave rather than on the counter, since room-temperature thawing lets bacteria multiply quickly.
