How Nasty is Your Kitchen Sponge, Really?

Every so often, there is a new scare about the countless number of germs that live among us. For decades, news crews have mined primetime gold by using black lights to reveal just how many “disgusting substances” (think bodily fluids) are lurking on hotel bedspreads. Now, savvy travelers remove the bed covering the moment they entered a hotel room. Likewise, reports of filthy offices resulted in hand sanitizer and disinfectant wipes popping up on desks everywhere. The latest scare is the kitchen sponge; perhaps you remember last year’s viral media stories that claimed your innocent sponge was far filthier than your toilet and that disinfecting actually made the sponge dirtier.

The origin of these worries was a study conducted by German scientists, who sequenced the microbial DNA of 14 used kitchen sponges. Five of the sponges had been “specially cleaned,” either by washing with detergent or microwaving, and the rest were not. The researchers found a total of 362 different species of bacteria on both the clean and unclean sponges. The density of those bacteria reached 45 million per square centimeter, comparable only to the density of bacteria in human feces. Hence, the comparison to the toilet.

The researchers also reported that, from a long-term perspective, the sanitized sponges did not appear to have a reduced bacterial load. They also had a higher prevalence of the bacterium Moraxella (which is associated with the odor of dirty laundry) and Chryseobacterium (a bacteria commonly found in nature but rarely associated with disease except for infections in infants or others with compromised immune systems). That’s the bit the media ran with: apparently, cleaning your sponge doesn’t kill bacteria!

NPR did a little investigative digging and concluded that journalists had missed some key details of the study. To get a better understanding of its scientific validity, they asked Jennifer Quinlan, a food microbiologist at Drexel University, to analyze the study. First, Quinlan noted that the study didn’t define how the sponges were “regularly cleaned.” Some were microwaved and some were put in hot, soapy water, but, Quinlan noted, putting a sponge in water would not disinfect it. Further, she noted that the study only looked at a sample size of five sponges that people “cleaned regularly.” Her conclusion: “We do not want to make public health recommendations based on five sponges from Germany.”

While previous studies had found pathogenic bacteria such as Salmonella and Campylobacter on sponges, these researchers reported finding none or very little. In order to keep it as sanitary as possible, Quinlan recommended keeping your sponge away from raw meat, replacing it every couple of weeks, and cleaning it in the dishwasher or the microwave.


Beyond the sponge

Just like humans, homes have their own unique microbiome. In our everyday lives, we are surrounded by bacteria, fungi, and other microbes; they are on walls, on our counters, on our furniture, and on us. As we move throughout our home, we pick up bacteria and shed our own, and, when we travel, we carry our microbiome with us to wherever we’re staying. This is all very natural, and the vast majority of bacteria around you are unlikely to make you sick. In fact, encouraging the growth of innocuous or beneficial bacteria can help crowd out pathogenic species—to some extent, bacteria to growing and prospering in your home may not be a bad thing.

Many household items contribute to the balance of bacteria inside the home. In 2015, a study looked at how plants contribute to the microbial abundance and diversity in built environments by analyzing the microbiome of the spider plant Chlorophytum comosum in relation to its surroundings. Scientists isolated the plant in a clean environment for six months and found that the bacterial diversity on surfaces in that environment increased significantly, while fungal diversity decreased.

The study demonstrated that plants can alter the microbiome of a built environment for the better, crowding out fungi that could cause mold and mildew and increasing the ecosystem’s stability. The authors of the study even hypothesized that plants could perform this function on manned space missions and in space stations, leading to more positive health outcomes for astronauts.

It’s possible that our kitchen sponges, laden with mostly innocuous bacteria, are a part of the same healthy indoor ecosystem as plants are. They wouldn’t be the only “dirty” thing in a house that contribute to a healthy home microbiome. Children who grow up in households with a dog have been found to have both fewer allergies and a decreased risk of developing asthma compared to children living in homes without dogs. Similarly, there’s ample evidence that good old dirt contributes to a healthy microbiome as well.