The microcosm of a kitchen sponge: Bacterial contamination is not always visible or detectable by smell

According to the World Health Organisation (WHO), an estimated 600 million cases of foodborne illness occur worldwide each year. Outbreaks involving multiple people that are traced back to commercial kitchens or catering establishments are particularly well documented. However, experts point out that home kitchens are also a frequent source of infection. This can happen, for example, when bacteria are spread throughout the kitchen due to poor hygiene while preparing contaminated food such as raw chicken.

Study: Kitchen Sponges as Reservoirs of Foodborne Pathogens: Microbial Growth Dynamics, Surface Cross-Contamination, and Hygiene Implications

Once in use in the kitchen, sponges serve as an ideal reservoir for bacterial pathogens due to their porous surface and the moisture they contain. Food residues trapped in the sponges provide the nutrients that enable bacteria to grow and multiply. This explains why kitchen sponges are typically colonised by a wide variety of bacteria shortly after first use. Previous research has also shown that the bacterial load in kitchen sponges is generally higher than on other kitchen surfaces, such as sinks or chopping boards.

In the current study, the researchers showed that pathogenic bacteria can proliferate in kitchen sponges. To this end, they applied solutions containing Escherichia coli and Salmonella – two major foodborne pathogens – in varying concentrations to commercially available kitchen sponges. In addition, some sponges were treated with a solution containing Staphylococcus aureus. This bacterium can produce toxins that may cause food poisoning. The study found that all pathogens tested multiplied significantly within a few days, even when the initial pathogen counts were relatively low. In addition, the pathogens survived even after the sponges had dried out for several days. In further experiments, the researchers demonstrated that even light pressure was sufficient to transfer the pathogens from the sponges to other surfaces. As a result, pathogenic bacteria can be transferred to food through direct contact and – if the food is consumed without being properly heated – may cause foodborne illness; in some cases, even very low pathogen counts are sufficient to do so.

Many consumers decide when to replace their kitchen sponges based on how they look or smell. However, this is an unreliable indicator, as the researchers observed no noticeable changes in the test sponges, even when bacterial counts were high. How often kitchen sponges should be replaced depends, among other things, on how they are used. For example, if a sponge has been used to clean surfaces that have come into contact with raw meat, it should be discarded afterwards. Alternatively, the bacterial load can be reduced by treating the sponge for at least two minutes with water at temperatures above 70 °C. Kitchen sponges should be replaced more frequently particularly in households with vulnerable individuals – such as people with weakened immune systems, young children or older adults. Brushes or microfibre cloths can be used as alternatives to kitchen sponges as previous studies have shown that they harbour lower bacterial counts, dry more quickly and can be cleaned in the dishwasher or washing machine. To effectively reduce the bacterial load, dishwasher and washing machine programs should be run at a temperature of at least 60 °C.

Further information on kitchen hygiene and foodborne infections

FAQ Foodborne infections in private households – identifying sources and avoiding risks
https://www.bfr.bund.de/en/service/frequently-asked-questions/topic/foodborne-infections-in-private-households-identifying-sources-and-avoiding-risks/

Consumer advice: Protection against foodborne infections in private households
https://www.bfr.bund.de/en/publication/consumer-advice-protection-against-foodborne-infections-in-private-households/

Topic page foodborne infections
https://www.bfr.bund.de/en/food-safety/assessment-of-microbial-risks-in-foods/food-hygiene/foodborne-infections/

About the BfR

The German Federal Institute for Risk Assessment (BfR) is a scientifically independent institution within the portfolio of the German Federal Ministry of Agriculture, Food and Regional Identity (BMLEH). It protects people's health preventively in the fields of public health and veterinary public health. The BfR provides advice to the Federal Government as well as the Federal States (‘Laender’) on questions related to food, feed, chemical and product safety. The BfR conducts its own research on topics closely related to its assessment tasks.