Sponge Disinfection Experiments

11/14/2016

I don't use a kitchen sponge because I've always thought they make great homes for bacteria. They hold onto food and moisture and have a welcoming surface. However, lots of people like to use a sponge to wash dishes. They often keep that sponge for a week or two until it STINKS! You can google and find several methods for disinfecting sponges. However, I wanted to test them myself to really know which disinfecting methods are sponge worthy. (Yes, I am quoting Elaine from Seinfeld). I went around to my neighbors and collected their stinky kitchen sponges and put them to the test. If you don't want to read this entire page, use the Table of Contents to skip to the results summary. 

Microwave

Microwaving a sponge is fast and easy. But how well does it actually kill germs? To test this, I pressed a damp, dirty sponge onto an agar plate.

If you would like to repeat any of my experiments, you can buy your own agar plates here on amazon

. These are the same agar plates that I buy.

Then I got the sponge wet and microwaved it for 2 minutes on high.

I let the sponge cool for a few minutes in the microwave, and then I pressed it onto another agar plate.

The plates were incubated for about 48 hours in my warm incubator (about 90 degrees F). 

Results

If you don't have much experience looking at bacteria on agar plates, let me explain. The white/yellow dots are colonies (or piles) of millions of bacteria. I single bacterium on the plate will double and double and keep doubling while in the warm incubator. After 24-48 hours you can visually see the pile of bacteria. Not every kind of bacteria will grow on these plates, and viruses don't grow on these plates. So, just because a plate looks clean, doesn't necessarily mean that no germs were present. I'm also not determining what kinds of bacteria are present. That is too much work and requires more equipment than I have. It is likely a mix of harmless and harmful bacteria. 

As you can see, the microwaving killed some bacteria but not enough in my opinion. I repeated the experiment with another sponge and used my neighbor's microwave and got similar results. 

Just for comparison, I pressed a brand new sponge that I wet with tap water onto an agar plate. I also rubbed some tap water onto an agar plate as a negative control. 

As expected, nothing grew.

I had previously tried very hard to sterilize Norwex cloths using the microwave and did not have much luck. You can see those results here. So, I vote NO on using the microwave to sterilize sponges.

Dishwasher 

I have also heard of people putting their sponges in the dish washer. To test this I used more of my neighbor's dirty, stinky sponges. I'm so lucky to have such great neighbors who put up with my research. Although, I admit that I sometimes confiscate biohazard sponges without even telling the owner. First, I pressed the wet dirty sponge onto an agar plate. 

Then I put the sponge into my whirlpool dishwasher in the silverware section. 

I used Palmolive Eco Dishwasher Detergent

 and the heaviest cycle that I had with high temp wash, sani rinse and heated dry. The cycle was 4 hours. I doubt that the type of detergent plays a significant role in reducing germs. I haven't found any laundry detergents yet that make a significant impact on the amount of bacteria. 

When the dishwasher was done, I pressed the sponge onto a clean agar plate. The plates were incubated for about 48 hours in my warm incubator. Here are the results.

As you can see, the dishwasher did a great job killing bacteria in the sponge. No colonies at all grew on the plates from the sponges that had been in my dishwasher. However, please remember that some types of bacteria don't grow on these plates and viruses don't grow on these plates. So, just because a plate looks clean, doesn't necessarily mean that no germs were present. Also, it is possible that some bacteria could have remained alive in the middle of the sponge. However, I pressed the sponges down hard on the agar plate so water from the inside came out onto the plates. So, germs from the middle of the sponge should have shown up on the plates, but I can't guarantee that there was no bacteria hiding in the middle of the sponge that I didn't detect. 

I also swabbed the dishes before and after the dishwasher. They got very clean in my dishwasher, too. There was 1 colony. 

I repeated the sponge experiments in my neighbor's dishwashers. I always used a hot cycle with heated dry. I tested their sponges before and after. I also tested their dishes before and after.

Bethany's dishwasher

Bethany's dishes and sponge produced no colonies after the dishwasher. 

Jennifer's Dishwasher

Jennifer's sponge and dishes had a few colonies after the dishwasher.

Kelly's dishwasher

Kelly's sponge and dishes had a few colonies. So, the sponge wasn't always perfectly clean in every dishwasher tested, but there was a big improvement. My dishwasher is new and the 4 hour cycle was much longer than anyone else's so maybe that is it.

Is heated dry necessary?

I wanted to determine if the heated dry in my dishwasher was necessary. So, I put a dirty sponge in the dishwasher on just the normal cycle without the heated dry. It was a 2.5 hour cycle.

It was not quite as clean as it was on the longer cycle in my dishwasher, but it was still quite good. 

I also tested Jennifer's dishwasher on the normal cycle without heated dry. Her sponge was still pretty germy. 

I have not tested the other dishwashers with sponges WITHOUT the heated dry. The bottom line is that the more heat you can use in your dishwashing cycle, the more germs you are going to kill. Overall, I think putting your sponge in the dishwasher when you run it is a great idea. You can also check out my page of dishwasher testing here. 

Washing Machine

I already have a long page of laundry/washing machine experiments. You can see that laundry page here. So, I already know that the I haven't found anything that really kills most bacteria in the laundry besides the addition of a half cup of chlorine bleach. (Obviously you can only use chlorine bleach on laundry that you don't mind fading.)  The hot sanitize cycle on a new front loader machine comes in second place to chlorine bleach. But you can't do that to your nice clothes either. HE top loaders are the worst in my experience. So, I've accepted that most of our clean laundry has germs. However, there is no reason that a sponge or dishcloth can't get nearly sterile. Here is an experiment where I tested dirty dish cloths and a sponge before and after washing on the hot sanitize cycle with chlorine bleach in my Whirlpool duet HE front loader. 

First, I blotted the dirty cloths and the sponge onto their own agar plates. Then they were washed on the hot sanitize cycle, heavy soil, with an extra rinse (almost a 2 hour cycle). I blotted them again on another agar plate after they were done in the washer. 

As you can see, the dish cloths and the sponge got super clean. I didn't see any colonies. 

If you are not using chlorine bleach, then the washing machine would be a BAD place to clean your sponges. They will still be disgusting and spread their germs to other laundry if you just wash them in a normal load. 

Boiling

I've already done experiments to show that boiling for 10 minutes (and it must be the full 10 minutes) is a great way to disinfect cleaning cloths and dish cloths (like Norwex). You can see those results here.  So, I expected the boiling to work great for the sponge unless the fact that the sponge floats caused a problem. To do these experiments, I pressed the dirty sponge onto an agar plate, boiled it for 10 minutes, let it cool a few minutes, and then pressed it onto another agar plate. To make sure that the sponge got hot enough even though it floated, I put a lid on the pan while it was boiling and kept the lid on for the whole 10 minutes. 

As you can see, the boiled sponges were very clean. There was not a single colony. Boiling sponges is a pain because it is an extra chore you have to do. However, it is the most effective disinfecting method. Dishwashers and washing machines can vary greatly in performance and ability to disinfect but boiling is always the same. If you want to be sure your sponge or dishcloth is perfectly sterile, boil it for 10 minutes with the lid on. In the Little House on the Prairie books, they used to boil their laundry. 

Soaking in 3% hydrogen peroxide

Soaking a sponge is a huge pain because sponges float. So, I already didn't recommend this disinfecting method before I even got the results. For this experiment, I soaked a dirty sponge in a container of 3% hydrogen peroxide for 5 minutes.  I had to hold it down with a beer can. I pressed the sponge onto an agar plate before and after the soak. The hydrogen peroxide was bubbling wildly as it worked to kill the bacteria in the sponge. After 5 minutes, I rinsed the sponge in tap water, squeezed it out, and pressed it onto the agar plate. 

After the 5 minute soak, the hydrogen peroxide was still bubbling like crazy. That means that there was still bacteria in there. The results also show that there was still a ton of bacteria in that sponge after the 5 minute soak. So, it is possible that a longer soak in hydrogen peroxide would have worked. However, soaking is a huge pain, so I just don't recommend it. Also, I did test a brand new sponge with hydrogen peroxide and it did not bubble at all. 

Soaking in 10% chlorine bleach

Chlorine bleach is the gold standard in germ killing, so I expected chlorine bleach to do very well. For this experiment, I soaked a dirty sponge in a 10% solution of chlorine bleach in water for 10 minutes.  

As you can see, the chlorine bleach did a great job disinfecting the sponge. It was annoying because the sponge floated, and I didn't like how the sponge smelled strongly of bleach afterward. However, bleach certainly works. 

What do I use at the kitchen sink?

I have never used a sponge at the kitchen sink. I use cotton or microfiber dishcloths and thin Scotch-Brite Scour Pads

. I put out a clean dish cloth at least once day. At the end of the day, it goes into a laundry basket reserved for only kitchen stuff. Every few days, I wash that laundry (cloths, towels, and scour pads) on the hot sanitize cycle with chlorine bleach. The scour pads are easier to clean because they are thinner. However, they still scratch all the food off of a pan. Yes, my dish towels fade, but I don't care. I prefer them to be clean. 

The scouring pads also get really clean in my washing machine when I use chlorine bleach. There was not a single colony, just some marks left on the agar caused by the scratchy surface of the pad. 

Results Summary

My official recommendation is don't use a sponge at all. They harbor too much bacteria. But if you insist on using a sponge this is what I learned from my experiments. Microwaving is not a good way to disinfect a sponge or cloth. Washing machines can vary greatly in performance, but washing dirty dishcloths and sponges on hot with a half cup of chlorine bleach usually does a great job disinfecting. However, if you don't use bleach in your laundry, don't bother putting your sponge in there. Soaking sponges is a pain because they float. However, soaking in 10% chlorine bleach for 10 minutes with a weight on the sponge to keep it down is an effective way to kill germs. Boiling for 10 minutes with the lid on is the best way to disinfect a sponge or cloth. However, boiling is an extra chore that you have to remember to do. I rarely find the time to make my bed. All dishwashers aren't perfect at killing germs. However, getting into the habit of putting your sponge, bottle brush, or scrubber into the dishwasher every time you run it would keep it very clean AND does not require any extra work from you. I have a neighbor who keeps her scrub brush in the dishwasher all the time when it is not in use. It doesn't sit by the sink. She actually keeps it in the dishwasher. I think that is brilliant! If you have a bottle brush for baby bottles, you should get in the habit of putting it in the dishwasher every day (unless you boil it).