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Food Safety: The Five Second Rule

February 14, 2012

Every once in a while a scientist/SF writer needs to get down to Earth. The other day a couple of Kenyon biology students stopped by my office to propose a term project in food safety. They plan to test an apparently well-known maxim known as the five second rule. This rule (well known, apparently, to everyone but me) says that if a piece of food is dropped on the floor, it’s safe to eat so long as you pick it up within five seconds.

Although I could not find much published in Science or Nature, several researchers do claim to have put this pressing question to the test. Paul Dawson at Clemson University said his students tested it and found that the rule absolutely doesn’t work–the fallen food is full of picked-up microbes. However, I smelled a rat (no pun) since I saw no raw data.

Then I found Dawson’s article from Journal of Applied Microbiology, “Residence time and food contact time effects on transfer of Salmonella Typhimurium from tile, wood and carpet: testing the five-second rule.” There’s a mouthful all right. The data there are really more about survival time of pathogens on floors (measured in hours or days–a sobering thought in itself) but it does show that, be it five seconds or thirty, your slice of white bread is going to pick up plenty of hitchhikers to the microbial galaxy of your human body.

A high school student, Jillian Clarke, doing summer research at the University of Illinois at Urbana-Champaign, found different results. She found that the university floors at Illinois were too clean to pick up any microbes. (Take that, Clemson.) She also found interesting social observations, such as that “Cookies and candy are much more likely to be picked up and eaten than cauliflower or broccoli.”

Would you eat food picked up off the floor? In your home? Camping in the woods? If there were no more left?

  1. February 15, 2012 12:28 am

    I may be neurotic on this subject–I always thought it was a joke one makes of oneself when the urge to pick up one’s food and eat it, bacteria be damned, puts one in an awkward situation–when there are witnesses…

    But it is a sliding scale, both through each individual’s discernment, and through the details–what food, what flooring, what condition or apparent cleanliness of flooring.

    The worst damage is usually being done by a restaurant kitchen worker more concerned for keeping the job than the safety of the patrons–but even without dropping your food on the floor, they can still neglect to wash their hands in the restroom.

    And, ultimately, we have the option of simply accepting the fact of bacteria (or other micro-baddies) infusing every surface, every clod of dirt, every drop of water–our own skins, our digestive tracts–everywhere–in the very air we breath, with alien life (but there’s the human perspective, not the objective one).

    And our continued battle, or perhaps ‘dance’ suits better, with bacteria, et. al. that we begin as soon as our own bloodstreams take over the job from the mother’s immune system, is a vital part of living–too much protection makes us extra-vulnerable, and too many counter-measures, like antibiotics, only breed stronger, more virile invaders. We really have no choice but to bend our medicine and technology to the inevitable balances of nature and physics that rule our existence–perfectionism is a dangerous game when considering the human immune system, or the environment it thrives in.

  2. February 15, 2012 1:03 am

    Just did it today. As a field biologist, I’m a firm believer in the hygiene hypothesis. A little dirt’s fine, especially in wild areas where antibiotic resistance might not have spread.

    Personally, I’m more worried about the DNA-binding chemicals on the floors of biology labs than I am about the bacteria, and I’d never eat anything that hit the floor (or any other common surface) in a health care facility.

  3. paws4thot permalink
    February 15, 2012 5:57 am

    Which makes me wonder – is a plate which was washed at bare hand friendly temperatures hours (sometimes days; who actually washes plates they don’t use daily before using them?) earlier actually micro-biologically cleaner than a linoleum or hardwood floor?

    • February 15, 2012 3:35 pm

      Um, I do. Jokes aside, when you’ve had mycology and a bit of sterile procedure, cleanliness takes a different level of importance.

      The basic point with the water isn’t sterilizing it, it’s getting the stuff off. Surfactants and elbow grease do a lot of that work.

      • paws4thot permalink
        February 16, 2012 5:58 am

        That was an “I’ll bet most people don’t do this” comment. Certainly my microbiologist sister will happily use the “good china” that’s lain in the cupboard for months since it was last washed.

  4. February 15, 2012 7:03 am

    We not only make jokes about the five-second rule and whether our floors are “clean enough to eat off of” (some days we think so, other days not), we’ve been known to pluck an ambient cat hair out of the air and put it into the family stewpot when it’s just us. Because we figure we’re ingesting it anyway, and it’s an old joke.

  5. February 15, 2012 11:30 am

    These undergrads, on the other hand, found that you have up to thirty seconds.

  6. February 15, 2012 8:50 pm

    Well, I don’t know about those Connecticut students. I’d like to see their data, too. We’ll see how the Kenyon students make out–which campus is the “cleanest” of them all.

    As for cat-hair stew, that’s certainly a new one.

  7. JamesPadraicR permalink
    February 15, 2012 9:12 pm

    Just spent too long looking for a link to the MythBusters segment on this, from a couple years ago, but couldn’t find it. The Wikipedia article on the five-second rule mentions that they did it, but their list of episodes doesn’t. From what I remember, their result was that transference is instantaneous.

    Personally I wouldn’t eat any kind of wet/sticky food off the floor. Maybe dry stuff, if it can be wiped off or rinsed. I agree with Heteromeles above, and will add that in your home you’ve probably already been exposed to anything you’re likely to ingest. I definitely wouldn’t do it elsewhere.

    • February 15, 2012 9:35 pm

      I agree: Once food touches the floor, it’s got microbes.

      But a lot of other surfaces turn out to have just as many microbes as the floor, if not more. For instance, the kitchen table or counter. My students found antibiotic-resistant bacteria in a crevasse of a bar table.

      And then there’s the dollar bill or pound note in your pocket–let’s not go there.

      • JamesPadraicR permalink
        February 16, 2012 1:25 pm

        I’ve always heard that anything that’s frequently handled is a good source for bugs, particularly doorknobs and phones. I’ve also always been a (not compulsive) hand washer, reinforced a couple years ago by my mother’s post surgical MRSA infection (stubborn, but not life threatening).

        I’ll add that the MythBusters result should have seemed pretty obvious.

  8. February 16, 2012 12:33 am

    Diet is impotant to us,personally I wouldn’t eat any kind of wet/sticky food off the floor. Maybe dry stuff, if it can be wiped off or rinsed. I agree with Heteromeles above, and will add that in your home you’ve probably already been exposed to anything you’re likely to ingest. I definitely wouldn’t do it elsewhere.

    • JamesPadraicR permalink
      February 16, 2012 1:16 pm

      Apologies if I’m wrong, Abercrombie, but you look like Spam.
      Mostly copied and pasted from my previous comment.

  9. Alex Tolley permalink
    February 16, 2012 1:53 am

    I think people have different rules – 2 seconds, 5 seconds ….

    Climate probably is important too – I am much less sanguine about eating food off a tropical floor than a temperate one.

    If the food is dry, like a cookie, I will eat food off a “clean” floor, but not sloppy or wet food. If it falls on grass, fine, but not on soil (disgust factor?).

    I suspect that microbes on the floor are less a problem to ingest than those on a imperfectly cleaned counter top, which may include Salmonella from chickens.

    If you take your shoes off when entering your house, the bugs on your floors seem less likely to contain pathogens you might have stepped in with your shoes from outside. I do think that “pooper scooper” laws in many municipalities has reduced the problem of pathogens from animal feces in the streets.

    I also think that if we are comfortable with kids eating dirt, perhaps we are being over cautious about bugs, unless we believe that cleaning is evolving highly resistant bugs in our homes.

    Has anyone actually tested the effects of eating “dirty” food to see how serious the potential effects are?

  10. paws4thot permalink
    February 16, 2012 6:03 am

    Actually, given how a self-limiting infection can travel around the islands here, I suspect that airborne infections are more of an issue in terms of “getting ill” than domestic floor/worktop bacteria are.

    Most outbreaks of the nastier food poisoning strains tend to trace to food retailers, wholesalers or caterers AFAIK.

  11. February 16, 2012 8:46 am

    An interesting distinction to make:
    Microbes can be “environmental” dustborne bacteria and mold spores, such as Bacillus subtilis or Micrococcus luteus, that do not grow in humans. We breathe them all the time. The “microbiologist sister’s” china probable collects mainly these.

    On the other hand, what’s on someone’s kitchen counter, cash, or hand, is much more likely to be something growing on a human–and therefore capable of growing on the next human. Most of those are harmless, but a few can be pathogens; or opportunists, that don’t make me sick but do sicken you (if you’re immunocompromised).

    Then there are occasionally cases of “emerging pathogens” that were around in the environment but suddenly start causing problems. For example, Bacillus anthracis (anthrax) from buried cattle that died of the disease; a generation later, the spores come up from the soil. More modern examples: SARS and West Nile virus, prevalent in animal populations, then introduced into humans.

  12. Alex Tolley permalink
    February 16, 2012 10:24 am

    So what are the pathogens you are going to pick up from eating food dropped on your kitchen floor?

    I would have thought E.Coli is the most likely bug. But modern cleanliness is so much higher that 2 generations ago that for homes without pets (cats and dogs) this is going to be a very small risk. Salmonella on a counter top that was incompletely cleaned seems to me a greater potential risk, especially when prepping food on butcher block that is never put in a dishwater.

    SARS, West Nile and Anthrax don’t strike me as probable pathogens on a kitchen floor.

    As paws4thot notes, food establishments may be a bigger risk than home. Certainly every case of suspected food poising in my family I have encountered has been due to restaurant food.

    In my younger days, I would eat meat/chicken that was clearly way past its best by making a spicy curry of it. I never had a food poisoning event from my cooking, although now I shudder slightly at the risks I took and my wife removes food from the fridge and disposes of it much earlier.

    • February 16, 2012 11:04 am

      From the kitchen floor it could be anything from dropped food; or tracked in from outside. For instance, from animal droppings outdoors.

      You’re right that outside food establishments are more of a problem, mainly because they circulate a wider variety of pathogens that your family is not yet immune to. However, our groceries are starting to look the same. Produce comes from all over. In the USA, our produce comes mainly from South America, and if we don’t cook it we’re exposed to those bugs. Listeria is an emerging problem because it grows at refrigeration temperature.

      I try to eat “local foods” when available, although they too can have local contaminants, especially if grown downstream of a cattle farm. I draw the line at food from China–not just for the pathogens, also the chemical pollution, which is the worst anywhere.

      • Alex Tolley permalink
        February 16, 2012 11:32 am

        China – I was told that the US has drastically reduced its imports of food, toiletries and clothes from China due to fears of contamination. As one person put it – “if they will poison their own people, imagine what they will do to us”. China is really going to have to correct that problem, both in real terms (regulations and enforcement) and image perception.

        Given our own poor imported food testing (thank you Congress), I concur that it is a good idea to steer clear of Chinese food wherever possible. But I suspect that global transport of ingredients and limited food labeling will make this impossible to do for processed foods. I suppose another reason to go with Michael Pollan’s – only buy from teh edges of the grocery store, where the fresh food is stocked.

  13. paws4thot permalink
    February 16, 2012 11:31 am

    Qualifier on food establishments, wrt E Coli and Salmonella – These are “notifiable diseases” here in the UK, so every case is likely to be investigated and traced from patients back to source (certainly all those where a physician is involved will be). Of the cases I can remember the press coverage of, some have been due to poor food hygene in restaurants, but the majority have traced back to wholesale butchers or poulterers, or to UK factory farms.

  14. February 16, 2012 11:42 am

    Another problem–Noroviruses and hepatitis (A and B?) from carriers working in the restaurant or grocery. Honestly, why do we bother to eat at all. 🙂

    I’m off to Boskone, folks, but I’ll post this evening if I can.

  15. February 16, 2012 5:24 pm

    Have fun at Boskone! Hope you get to eat lots of good food, shake lots of happy hands, and don’t get sick at all.

  16. Daren Thomas permalink
    February 21, 2012 8:52 am

    It’s just dirt. You’re just a bacteria bag anyway, so don’t worry too much about picking something up. Your body can take quite a lot of heat, and you want to train it, right? Except, in a hospital, I wouldn’t – those places collect nasties…


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