Kitchen sponges are a breeding ground for bacteria, with a 2017 study by German microbiologist Markus Egert finding up to 54 billion bacteria per square centimetre in used sponges. This density is comparable to that found in human stool samples. The sponge's structure, with its many pores and pockets, provides ideal niches for microbial communities to thrive.
However, experts say the risk to healthy individuals is low. Egert's study identified that five of the ten most common bacteria species in sponges are related to pathogens that can cause infections in people with compromised immune systems, but none were linked to food poisoning or severe illness in healthy adults. A 2017 US study found only 1-2% of household sponges contained food-poisoning bacteria, and in small amounts.
Cleaning sponges by microwaving or rinsing with hot soapy water may not eliminate the risk, as it can select for more resistant bacteria. A 2022 Norwegian study found that washing-up brushes harbour similar harmless bacteria to sponges, suggesting they may be a safer alternative. For most people, the bacteria in kitchen sponges are not harmful, but those with weak immune systems, the elderly, or children should take extra precautions.
