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Cleaning a Dirty Sponge Only Helps Its Worst Bacteria (nytimes.com)
50 points by aaronbrethorst 11 months ago | hide | past | web | favorite | 45 comments

If you want an effective means to disinfect a sponge, it's not unreasonable to turn to the standard for sterilizing intravenous infusion fluids (pharmaceutical grade sterilization) back in the 1980s. That was designed to reduce a bacterial titre of 1 million organisms per ml down to one organism surviving per million litres, in other words by 15 orders of magnitude. And you can achieve this in your own kitchen quite easily.

The trick is to use saturated steam at 1 bar overpressure — a pressure cooker at 15 psi will do this — for 15-20 minutes. (Pharmaceutical grade autoclaves are just pressure cookers that operate on a large scale.) 1 bar overpressure corresponds to saturated steam at a temperature of 121 degrees celsius and it makes a real mess out of anything with a cell wall or proteins. This is a whole lot more effective than merely boiling your sponges at standard atmospheric pressure, let alone the microwave-and-pray method.

(Note: I'm going from third-of-a-century-old memory of pharmaceutical manufacturing standards here, rather than an actual citable source. Confirming it is an exercise for the reader and actual specifics may vary. A sterilized sponge, no matter how thoroughly boiled, may still contain residual toxic compounds left from bacterial decomposition. And as I allowed my professional registration as a pharmacist to lapse a long time ago this is not professional advice.)

You need to be able to hold the sponge above the water and air levels so it's in the steam. Real autoclaves either vacuum out the air or have some other method to keep the air from insulating the item being sterilized. One could construct a small wire stand to hold the sponge near the top of the pressure cooker, that would probably help.

Why does it need to be in the steam? Both the steam and the water are at 121C and water is a lot more effective at heat transfer.

News at 11:

When you kill bacteria via technique X, some bacteria are affected by it more than others. Since not all bacteria are killed, this leads to the enrichment of some types of bacteria.

When a scientist took the most populous bacteria and scoured the encyclopedia for any way in which this bacteria could possibly be dangerous, he finds that "It is widespread in nature and lives on the human skin. It can cause infections in people with weak immune systems.", which is about par for the course.

Scaremongering ensues.

It's where I keep my pet tardigrades. :-)

It always amazes me the traction this sort of story gets. We've been cleaning hands and dishes and counter tops with towels and sponges for hundreds of years, and the more we worry about the bacteria, the worse we make things.

Trust that we've evolved to live in bacteria filled environments and not die horribly.

You can think of it this way as well - breathing pure oxygen sounds good in theory, but the reality is, it would kill you.

How about a new product, "probiotics for your sponge", i.e., a dose of "healthy" bacteria to keep the bad ones at bay?

Living sponges, possibly bioengineered, with an immune system that keeps itself healthy/clean?

Isn't it obvious to most people that sponges (damp, room temperature environment) are bacteria ridden?

I use mine to get food off of the dishes, and then rinse the dishes in running hot water. There's no way I'm expecting the sponge to be bacteria free.

The entire article is based on people trying to disinfect sponges using pretty ineffective means (like microwaving???), we dump boiling hot excess water from our tea kettle onto our dish washing sponge / scouring pad. I'm skeptical than anything survives that.

A better headline for the article would be "most people don't know how to properly disinfect their stuff".

I did enjoy the comparison of used sponges with human feces, the article is definitely worth a read.

Boiling was specifically mentioned in the study. Previous studies only measured the bacterial load, but not the bacterial composition. Apparently the worst types of bacteria continue to survive and repopulate the sponge in greater numbers:

"Sanitation by boiling or microwave treatment has been shown to significantly reduce the bacterial load of kitchen sponges19, 21 and can therefore be regarded as a reasonable hygiene measure.

However, our data showed that regularly sanitized sponges (as indicated by their users) did not contain less bacteria than uncleaned ones. Moreover, “special cleaning” even increased the relative abundance of both the Moraxella– and Chryseobacterium–affiliated OTUs (Fig. 3B). Presumably, resistant bacteria survive the sanitation process and rapidly re–colonize the released niches until reaching a similar abundance as before the treatment (Fig. 6A). This effect resembles the effect of an antibiotic therapy on the gut microbiota40, 41, and might promote the establishment of higher shares of RG2-related species in the kitchen sponges.

Although further analyses, including controlled sanitation experiments, are needed to substantiate these findings, our data allow careful speculation that a prolonged application of sanitation measures of kitchen sponges is not advisable."

- "Microbiome analysis and confocal microscopy of used kitchen sponges reveal massive colonization by Acinetobacter, Moraxella and Chryseobacterium species" https://www.nature.com/articles/s41598-017-06055-9

I wonder how freezing the sponge for 24+ hours would compare.

Does freezing kill bacteria? In school we were warned to always let a chicken fully defrost before cooking it, otherwise the center doesn't cook properly and salmonella (which presumably survived being frozen) will survive the cooking.

Not all of them - which likely means it'd have the same result or worse as the boiling mentioned above.

I believe freezing is, however, extremely effective again parasites (but there won't be in the sponge though).

Given the extremes required for sterilization in terms of either temperature or time, I'd really prefer an actual study to your intuition; pouring boiling water over something only exposes it very briefly, which may not actually be sufficient.

>Given the extremes required for sterilization in terms of either temperature or time

It can be as little as 4 minutes for steam at 270*F/~32 psi for medical instruments. That is usually for immediate-needs and 45 min cycles were more typical for what I had to do for a job. Boiling water isn't certainly going to do it.

It's probably safer and cheaper to just buy them in bulk and throw them away after a few days of usage than try to sterilize them.

132C @ 30 psi is pretty extreme, even for just a few minutes!

(Our "flash" protocol is 10 minutes instead of four, but it probably depends on how heavily wrapped the item is).

Couldn't you just soak the sponge in bleach? Bleach kills everything and it's a liquid, which can get to the depths of your sponge.

And now you are cleaning your dishes with a sponge giving off residual amounts of bleach. Which is poisonous.

The amount of bleach left on your dishes after washing and rinsing isn't going to poison you. Also, bleach breaks down to mostly salt and water.

The real problem with using bleach to clean dishes or eating utensils is that some of us can taste it very strongly, even hours or days later. For example, the first job I had was at a fast food place and we had one idiot who used bleach to soak the plastic soft drink dispenser nozzles in overnight, instead of the provided cleaning solution (which was tasteless). I was the only one working there who noticed but I knew every single time he did it without fail.

The sanitizer used in most restaurants I worked in was chlorine based. Obviously in certain quantities, chlorine would be dangerous, but in less quantities it is still commonly used to sanitize stuff you eat off of.

bleach rapidly breaks down into salt and water. I'd be more worried about the salt being too abrasive on your non-stick pans ;)

Some amount of bacteria is good for our immune systems. Letting children play in the dirt, eating their boogers, and all sorts of other gross things that children do has been proven to result in them having stronger immune systems.


On the flip side, people who use hand sanitizer all the time? Yep. You guessed it. They end up having crappy immune systems as a result, and in some cases, toxicity from over-absorption of the hand cleaning chemicals such as Triclosan.


The soapy sponge is just to remove any adhered matter and oils from the dish so that a smooth surface is left. The real cleaning occurs when the dish dries. I'm surprised this isn't common knowledge.

I do microwave the sponge after use, for about 2 minutes so that it's bone dry. The reasoning is simple: most of the bacteria are going to hang out in the water. It's still going to be infected, but it doesn't have to be disinfected. It just has to remain close enough to the ambient load of the kitchen that it doesn't obviously inflame my hands when I pick it up. After all, my hands are germy too, most of the time. The soap, water, and pressure applied during cleaning are supposed to do most of the work.

What a frustrating article. Did the research show that a boiling water wash to kill bacteria doesn't work, or that some peoples techniques don't work.

That no cleaning method they tried works.

We store our sponge vertically in a glass with slanted sides. The sponge stops about 1-1/2" before the bottom. The sponge drips dry after every use, depriving bacteria of the "wet" they need to thrive. It surely doesn't kill all bacteria but it takes about 2 months before the sponge gets into the ewww zone, which from my standards, greatly extends sponge life over storing them flat.

In sterile processing we toss brushes, albeit not exactly a sponge, daily- if not more often- because of the bio-burden/micro-organism build up and efficacy. At home I'm a bit more relaxed as most utensils are only touching semi/non-critical tissue but i try to stay clear of anything that holds moisture for cleaning; unless I'm using it with a disinfectant ala bleach.

“Now I’m an expert in how to clean sponges,” said Dr. Egert, who wants to compare disinfection methods in a follow-up study. “I’m waiting for the sponge industry to call me.”

How about sponge with soap?

I use soap when washing dishes, and after I'm done, I just leave it there (I don't clean the soap from the sponge).

(Non-antibacterial) soap just sticks to bacteria, making them more slippery, so they can be washed away. They're still alive in the drain and septic/sewer.

But soap doesn't just magically do this - it has to be foaming, as scrubbing allows the soap molecules to magnetically organize. As the hydrophobic soap molecules stick to the non-polar bacteria, they form a bubble shell that's not unlike a cell wall. To produce this foam with normal hand soap takes ~20 seconds of rotational scrubbing - a soap-Rasengan, if you will.

Bacteria are in everything, including every breath of air and gulp of water. Our immune system kills the ones which get through our small intestine's mesh system.

Is it possible to use electricity to kill bacteria? A soaked sponge would be conductive enough to pass a current through. Essentially a sponge Chidori.

> scrubbing allows the soap molecules to magnetically organize

Do you have any links about this?

Soap doesn't kill germs, it simply makes it easier to wash germs off by making water fit into smaller areas.

So, a damp sponge is a perfect environment for growing bacteria.

Now if you are using antibacterial soap your sponge may be fine like that, but then those soaps have concerns of their own (eg. They kill good bacteria that we apparently want to keep around)

it's difficult to google but i read somewhere that normal soap makes bacteria pop due to dissolving their membranes

Soap is mostly for helping to remove dirt by helping both scrubbing and the mixture of oils in water. I think it has some side-effect as an antibacterial due to pH or something, but it's not inherently a bug killer (of course, you can get anti-bacterial soap these days. Arguably a bad thing globally).

Wasn't antimicrobial soap just banned in the US?

Not quite banned, but a marketing ban:

> The U.S. Food and Drug Administration today issued a final rule establishing that over-the-counter (OTC) consumer antiseptic wash products containing certain active ingredients can no longer be marketed. Companies will no longer be able to market antibacterial washes with these ingredients because manufacturers did not demonstrate that the ingredients are both safe for long-term daily use and more effective than plain soap and water in preventing illness and the spread of certain infections. Some manufacturers have already started removing these ingredients from their products.


Why would anybody clean a spounge god damn it, its not like they cost a fortune.

OK, so how do I wash the dishes by hand? Use e thinner cloth?

Just wait until they study underwear.

I read somewhere else that the best way to kill bacteria in sponges is to microwave them for about 30 seconds.

I thought the same thing, but the first line of the article is "Stop. Drop the sponge and step away from the microwave."

I've switched my household over to using these instead:


The reason why sponges are great for bacteria is their ability to hold water in little pockets. These dry out completely and much faster. The bacterial load won't be zero, but I suspect it will be much better than with sponges, especially with some judicious disinfecting. (Which I will be using the pressure cooker for!)

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