Posts tagged: sanitation

Cleaning a Dirty Sponge Only Helps Its Worst Bacteria, Study Says

Cleaning a Dirty Sponge Only Helps Its Worst Bacteria, Study Says
Image © (under license)

Since the advent of dishwashers, there are homes that don’t use sponges to clean their eating utensils and cookware anymore. However, in homes that do still use the old sponge, dishwashing soap, and running water, keeping your sponge “clean” is an anomaly in itself. Since the sponge is constantly washed with soap and water shouldn’t it be always clean? Wrong. A sponge is the perfect breeding ground for bacteria, which requires two important things to thrive: a (1) warm (check!) and (2) wet (check!) environment.

If you do a web search on how to sanitize your sponge, you will find various articles that cite microwaving it can kill off most of its bacteria. Michigan State University and the US Department of Agriculture actually encourage sanitizing sponges by microwaving, which reportedly kills 99.9 percent of bacteria living on it. However, a recent study by found that while microwaving, boiling, or other similar methods of cleaning the sponge may indeed kill off bacteria, it can only kill the weaker, heat-susceptible bacteria. The stronger strains of bacteria like the Moraxella osloensis lives on despite microwaving and will actually spread to the areas where the weak bacteria used to be. This is precisely why sponges will eventually start to smell or emit a foul odor despite sanitizing them regularly. [1]

The study was led by Markus Egert and his team and they analyzed the microbiome found in kitchen sponges. Driven initially by their interest in the microbial diversity of the organisms found in kitchen environments, the team actually found that kitchen sponges harbored abundant numbers of pathogens. Furthermore, they found that microwaving sponges affected the microbiome greatly… and (your health) negatively. [2]

The Pathogens In Your Kitchen Sponge

In any home, there are two places that are used the most: the kitchen and the bathroom. The researchers focused on these two because of their potential as “microbial incubators” because of how innately warm and moist they are. The kitchen sponge is actually a good representation of a kitchen or bathroom, because of its warm and moist environment. Furthermore, the sponge is constantly in contact with food and skin, which can carry pathogenic organisms that stick to and colonize in the sponge. The researchers were able to identify 362 different species of bacteria in 14 sponges, where there were roughly 82 billion bacteria in one cubic inch of sponge.

Analysis of kitchen sponges revealed three recurring bacteria: Acinetobacter, Moraxella, and Chryseobacterium species, all of which are pathogenic or pose a threat to human health. The second specie, Moraxella¸ was also commonly found on kitchen surfaces like the sink, faucet, refrigerator doors, and stoves — which exponentially increase human risk of being contaminated by them because those four things in the kitchen are the most touched and used (especially by kids!). Moraxella osleonsis is responsible for a number of infections that affect the skin and respiratory tract (including pneumonia). [2][3]

Because the population has become more conscious of our environment, you may turn your nose up at the thought of disposing of an item that can still be used and creating waste. However, your sponge is a lost cause. The more you try to clean it, the dirtier it actually gets! You can kill weak bacteria but that only allows the stronger, more dangerous pathogenic organisms to thrive all over the spaces the weak bacteria used to live. By trying to save your sponge, you are exposing yourself and your loved ones to the risk of a health-threatening infection. So think twice before tossing your dirty kitchen sponge in the microwave or washing machine to be “cleaned”. It’s safest just to get yourself a new sponge.

Sponges (the typical non-natural type) don’t biodegrade readily. For environmental reasons, look into natural or biodegradable sponges (not the easiest to find!) that will cut down on environmental pollution.


[1] Sanitizing kitchen sponges. Michigan State University.

[2] Cardinale, M. (2017). Microbiome analysis and confocal microscopy of used kitchen sponges reveal massive colonization by Acinetobacter, Moraxella and Chryseobacterium species.

[3] Maruyama, Y., et. al. (2017). Bacteremia due to Moraxella osloensis: a case report and literature review.