The microbiology of double-dipping

You're probably familiar with the Seinfeld episode where George commits yet another social faux pas, getting caught "double-dipping" a tortilla chip. Just in time for your Superbowl festivities, turns out a soon-to-be-published manuscript (described in the New York Times) examined just how many bacteria are actually transferred by "double-dipping." I have more at Correlations, and the Seinfeld clip is after the jump.

[Edited to add: Steve uploaded the poster describing the research here.]

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I'm honestly curious here: is 50 to 100 bacteria per bite really significant when the mouth contains billions of bacteria per square centimeter? How large of an inoculum would be required to set up a Neisseria infection in a healthy mouth? Chances are the bacteria transferred are just normal flora anyway, right?

That's a great question FutureMD. I'd really like to know too.

By Jon Strong (not verified) on 31 Jan 2008 #permalink

One thing I hate about stories like this is that they've written the story about it long before the paper is out, so I can't say 100% about their methods, but it sounds like yes, this would mostly be normal flora that was transferred (though it would be very interesting to speciate some of them and see which ones transferred most commonly). 50-100 bacteria per bite isn't a lot and most likely would be harmless, but for some bacteria (and viruses, which they didn't examine) it takes just a small inoculum to cause infection. Think if someone was a carrier of Strep pyogenes, for instance--those only take a small amount of bacteria to cause an infection, and can be pretty nasty.

I don't see this as any kind of huge public health risk (like they say, it's akin to kissing), but I think it does show that it's pretty rude to double-dip and share your germs with everyone else. I've never understood why people can't just break the chip in half prior to dipping anyway--problem solved.

Cross-posted at Correlations, plus I should add that human ID50 for meningococcus can't really be established directly, but the relatively low secondary case rate in households suggests the ID50's probably higher than for gonococcus:

Depends on the bacteria in question. The urethral ID50 for gonococcus is 1000 bacteria, but I don't think oral infectivity has been examined that closely. Infection with Campylobacter jejuni, a major player in grody foodborne infections, has been induced with 500 organisms, but unless your fellow partier had *just* yakked, you wouldn't be at particular risk, given the organism mostly hangs out in the gut.

I think you're right, that most of the organisms transferred would be benign flora, but the implications for viral transmission are a little more ominous. Herpesviruses, rhinoviruses and noroviruses come to mind as potentially icky and quite transmissible with double-dipping.

Mind you, I'd also like to know how many bacteria are transferred from hand to chip bowl during the grubbing-and-grabbing phase of the transfer.


The methodology described in the Times article is questionable, IMO. The bitten crackers were dipped for three seconds -- hardly a real-world simulation.

Eh, I think it's possible, especially if you're moving a chip around trying to scoop up the dip. Again though, hard to tell without the whole manuscript to evaluate.

I teach micro at a community college. Several years ago I had a student who did a similar study for her final project. She got her fiance to help: he filled his mouth w/ bakers' yeast, double dipped (black bean dip) then she dipped. She recovered the bakers' yeast from her mouth to everyone's delight. I bet it tasted awful. I wish I could find her now to tell her about this study.

Thanks for sharing that, aeh. Ain't science great!

Turns out a soon-to-be-published manuscript examined just how many bacteria are actually transferred by "double-dipping."

Ha ha ha ha ha! So now the double-dip hysteria shit is about to hit the fan. And pray tell me, how about the french kiss? Any studies done? Must be a million times worse than that sauce mistreated by some anti-social double-dipper!
Well, what the heck. Be happy cultivating that little hysteria of yours, I don't care much about micro-organisms, although I couldn't possibly live without them of course.

BTW, a French commercial recently told me to wear a mask in case of a flu infection. To protect other people, it said. HAHAHAHAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA!!!!!!!!

Ahh, the Germaphobes are in full bloom. Here a germ, there a germ, everywhere a germ, germ.

Ever have intercourse, Dr. Smith? Pretty massive mutual transfer of germs from one to another. Seldom get sick, rather enjoyable activity, I might add.

On the germ scale, puts double-dipping to shame, I would think.

The idea that what goes on between your ears can be described as thought, David, puts all your other fantasies to shame.

As a tried and true yeast geneticist, I have to cry foul with the supposition that baker's yeast tastes awful.... excuse me while I go for a beer.

On average, the students found that three to six double dips transferred about 10,000 bacteria from the eater's mouth to the remaining dip.

That's more than enough for me! Double dipping goes on the no-no list! Bacteria don't always seem to want to play nice! LOL!
Dave Briggs :~)

Why should the issue ever arise? There is never any need to have a common dip bowl, much less a common chip bowl. Give everyone their own bowls of chip and dip. If they need more, they can pour or scoop with a utensil that never touches their bowls.

I admit, I'm not big on communal eating in general. There are always the mouth breathers who spray chewed-up food out their gaping pieholes because they insist on yammering on while eating, people rooting their grubby hands in your bowl or plate, people picking their teeth with their fingers and then reaching into a shared food source etc. And of course those who think it the height of wit to theatrically let rip a huge belch or fart right in front of everyone.

PS: typo: "faux pas", not "faux paus"

By not a comment (not verified) on 02 Feb 2008 #permalink

Everybody's so concerned about mouth bacteria! What I worry about with shared food is butt bacteria. Too many people just don't wash their hands! I say, beware the shared bag and shared bowl more than the shared dip.

On several of the blogs where this topic has come up, people have minimized the impact by saying that the number of bacteria transferred by double-dipping is so much smaller than what one would get when kissing, French kissing, or having sexual intercourse. But surely the vast majority of people are much more particular about who they kiss and/or have sex with than who they share snacks with?

As for breaking the chip in half: Since the dipped part of the 2nd half is likely to have been handled by the dipper, wouldn't that decrease mouth bacteria contamination at the expense of increasing hand bacteria contamination of the dip?

I wouldn't really be worried about eating dip that other people had double dipped in. But, if I was going to save the dip for later, or it sat out a while, I think a 10'000 bacteria inoculation would make it go bad a lot faster, no?

There were six test dips: sterile water with three different degrees of acidity, a commercial salsa, a cheese dip and chocolate syrup.

It would probably be bad before long anyway- the salsa, dip and syrup sound like some nice growth medium. A couple of hours of log phase growth in that kind of stuff (especially with staph) and people are going to be throwing up left and right..

By Dark Matter (not verified) on 04 Feb 2008 #permalink

oops...I mean "media"....

By Dark Matter (not verified) on 04 Feb 2008 #permalink

There are a billion and one places we share infectious organisms. It's a trade off of risk and benefit.

One isn't necessarily a germaphobe for simply collecting the data and making a choice based on the evidence rather than a choice based on social images and political correctness.

I only share dip with a few intimate friends. I gave up drinking fountains years ago and salad bars more recently. My son brought enough infections home from school to fill all the openings on my dance card. Something had to go.

It's all a trade off of risk and benefit. If you prefer an occasional extra viral infection to giving up any of the above (though I cannot imagine anyone who couldn't forgo the free grocery samples that include a dipping bowl) then do nothing except perhaps cut down during influenza season, because flu kills people, and avoid the sources with high traffic or obvious signs of likely unmonitored contamination.

Think I'm being a tad too obsessive compulsive about the community salad trough? Try a Google search of "e coli salad bar". Again, when the infectious disease risk includes life threatening infection, my avoidance behavior increases.

Free samples at the grocery store occasionally include dip. Doubles or not, customer fingers clearly contribute to the mix. Gross.

But the worst place of all the germ exchange venues involving food and beverage are public drinking fountains. Yes folks, public drinking fountains. It's been over a century since the good Dr Snow mapped out the concentric cholera case density that was inversely proportional to the distance from one infamous town water pump. And to this day, we still drink from the community tap.

Lest you think chlorine in the water takes care of things, I remind you it takes ~30 minutes for chemical disinfection of clean water. So whatever is on that fountain that washes into your mouth has not had 30 minutes of exposure to the chlorine in the water before reaching your lovely pharynx or stomach.

Since cholera is not too common in these parts and e-coli on a drinking fountain has been documented but more rarely than e-coli in the salad bars, I would drink from that public drinking fountain if I was desperate. I simply prefer an evidence based risk assessment to base the decision on.


Skeptigirl (ARNP, infectious disease field)

By Skeptigirl (not verified) on 05 Feb 2008 #permalink

Do any of these people shake hands or touch doorknobs? Maybe they cover their sneezes with their hands -- maybe even the same hand as they use for dipping. As for the butt bacteria, maybe we could redo the bakers yeast experiment.

People have lived in groups for a long time. We live in a bacterial world. There might actually be more harm done by lack of exposure to other people's normal microbes. Refer to the "hygiene hypothesis".