A kitchen sponge is not your enemy. But it can be very dirty. Scientists recently published a study revealing how densely packed your dirty kitchen sponge is with microscopic bacteria. After I wrote an article about their work, readers flooded my inbox with good questions, so I asked around for some answers.
First, let's examine what the study did and did not do.
The study was designed to establish improved measurements of the bacterial populations that live inside this common household item. Previous measurements had mainly looked at those from sponges dirtied in the laboratory, growing the bacteria in a petri dish.
But because not all bacteria will grow in that medium, their numbers may have been underestimated, said Professor Markus Egert, the microbiologist at the University of Furtwangen in Germany who led the study. "Our study was mainly thought to create awareness, and not fear," Prof Egert wrote in a follow-up e-mail.
But what they found alarmed many readers. Although not designed to evaluate disinfection methods, the researchers collected additional data from the sponge donors (a sample of 14 sponges, which the researchers concede was limited). And to their surprise, sponges regularly cleaned in soapy water or the microwave actually harboured more of a bacterium called Moraxella osloensis. This bacterium is generally common and harmless, but it can cause infections in people with compromised immune systems.
Nonetheless, Prof Egert suggested that in most cases, it may be best to throw away your sponge when it starts to stink - a sign that the nasty bacterium may be there - even if it may not harm you. This decision to toss, said Prof Egert, means balancing hygiene and sterility, thriftiness and a sustainable environment. The United States Department of Agriculture (USDA) also suggests buying new sponges frequently, as they are "difficult to clean". "You should not become hysteric and afraid of your kitchen sponge now," said Prof Egert in our original interview. Even sterile environments can make a person ill, he added. "But if you're already ill or have ill people at home, you should be more careful."
And that brings us to talking about risk, which the study was not designed to assess.
Kitchens are hot spots for cross-contamination, and immune systems differ. You could just as easily contract an illness from poorly prepared food or your cellphone as you could from a dirty sponge, many experts say. And two bodies' responses to the same pathogen can differ, just like a pothole might damage one car but not another, said Associate Professor Kevin Sauer of Kansas State University, who has studied cross-contamination in the kitchen.
If you are still worried, here are three tips from Dr Solveig Langsrud, a microbiologist at Nofima, an applied research institution in Norway, who has examined how different hygiene procedures can reduce bacterial contamination in kitchens.
Do not feed your sponge with dangerous bacteria.
Do not use your sponge to scrub off chunky food debris or wipe up fresh meat juices, dirt from fruits and veggies, unpasteurised milk stuff, vomit or your pet's droppings. Just use a paper towel, cleanser or running water. Keep sick people away from food preparation areas. (And for those who asked, a vegan kitchen full of raw vegetables is not immune.) To avoid cross-contamination, wash your hands (properly) and give different sponges their own jobs - like cleaning only your counter, floor or dishes.
A proper hand-washing means removing jewellery and using soapy water for 20 seconds before drying with a clean towel, said Mr Argyris Magoulas, an information specialist at the Office of Public Affairs and Consumer Education at the USDA Food Safety and Inspection Service.
Keep your sponge clean.
Dr Langsrud said that you should wash your sponge after each use, which does not quite jibe with Prof Egert's study. But Prof Egert does not think his donors gave their sponges a correct washing. With some effort, you can disinfect your sponges and get rid of most of their bacteria, although this may not be practical for many of us.
In a 2008 study, Dr Manan Sharma, a microbiologist who studies food-borne pathogens with the USDA, and his colleagues soaked sponges in ground beef at room temperature for two days to get them extra bacteria-y and then compared common cleaning methods. He found that microwaving and running them through the dishwasher were the most effective killers of some bacteria, mould and yeast.
But there were caveats: a synthetic, metallic or dry sponge can catch fire in the microwave. Microwaves and dishwasher models can vary - you must watch temperatures. Too little heat, time or steam can put your sponge in what Mr Magoulas calls "the danger zone", a place where bacteria proliferate. Also make sure your sponge is wet - the steam kills many microbes, experts say.
Dr Langsrud said drying is also "a simple, cheap, environmentally friendly and effective way to keep bacterial numbers down". That is because moisture-loving bacteria cannot multiply on a dry sponge - for the most part - which brings us to Dr Langsrud's final piece of advice and our original conundrum.
Do not be too attached to your sponge.
Even with prevention, washing and drying, some bacteria that live in kitchens can accumulate in the sponge, Dr Langsrud said. "These bacteria are tolerable to drying and protect themselves in food debris and a self-produced slime," she said. "They will be impossible to fight." She agrees with Prof Egert: Dispose of sponges at least once a week, or when they smell bad. And if someone is sick in your house, like with cancer, she says to throw away sponges daily. Reuse disinfected sponges in less hygiene-sensitive spots if you must.
This all may make you wonder if you even need a sponge, if some are better than others and if alternatives exist. Plenty of companies offer solutions - like bacteria-killing baths for sponges, water-repellent surfaces or antimicrobial materials.
But without peer-reviewed scientific studies, it is difficult to evaluate their effectiveness. Also, consider instead brushes, paper towels and washcloths (which are washed more often and used in restaurants).
"Tools that soak less water, dry faster, have smaller inner surfaces might indeed be better for regular cleaning," said Prof Egert.
Prof Sauer said the problem with sponges is that they are easy to ignore. They inhabit the sink. They stay wet. They get nasty. But can you really blame them? "A lot of us have been brought up to grab that sponge because it takes care of the surface, cleaning what we see," he said. "I don't think sponges are the enemy, but they provide a great medium to grow bacteria."