Getting some bacteria with your food is the risk you take if you eat birthday cake after the candles have been blown out, dip a chip in salsa after someone else double-dips or eat toast that has fallen on the floor -- yes, even just for five seconds.
Every year, Dawson and a group of his undergraduate and graduate students come up with a research project related to our food habits and then quantify how dirty it actually is. He and another colleague are also working on a book about his fascinating food findings.
At one point in an iconic "Seinfeld" episode
, George Costanza takes a bite of a chip filled with dip. When he goes for more dip with the same chip, he is quickly confronted about "double-dipping."
"That's like putting your whole mouth right in the dip!" another character says.
Those who oppose double-dipping say it spreads germs from your mouth into shared food. After watching a rerun of that episode, Dawson was inspired to test the theory
. He and his team put chips in three types of dip with different pH levels and consistencies: salsa, chocolate syrup and queso.
No mouth bacteria were found in the dips when chips were dipped only once. But when double-dipping occurred, there were much higher bacterial populations in the dip. And the type mattered: Salsa that had been double-dipped had five times more bacteria than the chocolate and cheese.
Dawson attributed this to some basic food science principles.
"Common sense tells you that if you bite it and dip it in the salsa and more of it falls back into the bowl and doesn't stick to the chip, then there's going to be more bacteria going back in the bowl with it," he said.
Thick chocolate syrup and cheese dip are more likely to stay on a chip, resulting in less bacterial transfer.
Five seconds on the floor
Urban legend says that if you pick up dropped food from the floor within five seconds, it's safe to eat, because it is not enough time for contamination to take place.
Dawson and his students set out to understand the truth behind this food trope.
salmonella bacteria on tile, carpet and wood. After five minutes, they put down bologna or bread and left it there for five, 30 or 60 seconds. They did the same thing after bacteria had been on the surface for two, four, eight and 24 hours.
"There was enough bacterial transfer in five seconds that from a practical standpoint, it's not a really good idea to eat food from the floor," Dawson said.
He said he doesn't think it's time on the floor that matters so much, but rather the type and amount of bacteria.
"Really, it depends on what's been on the surface. The odds are, there's probably nothing there. But there's always the chance something could be," he said.
, a food microbiologist at Rutgers University, conducted the only other peer-reviewed five-second rule study
. His findings, published last year, confirmed Dawson's and showed that longer contact times with a contaminated floor resulted in the transfer of more bacteria.
"In one sense, the five-second rule is not true, because there was no safe amount of time (for food to be on the floor), but on the other hand, time really did seem to make a difference, especially with certain foods," he said.
Schaffner found that the type of food, as well as the surface on which the food fell, also influenced the amount of bacteria transferred. Wet foods were more likely to pick up bacteria, and surfaces such as carpets were less likely to transfer bacteria. He said these factors are important to consider when weighing whether to apply the five-second rule.
"If it's my floor in my house and I know how clean my house is and it's a relatively dry piece of food like a piece of cereal, yeah, I'll eat that up off the floor," Schaffner said.
"But if it's a big sloppy wet piece of watermelon that drops in my kitchen, where I walk and my dogs walk, no, I'm definitely not going to eat that, because I know the risk is much higher."
The worst party favor: germs
Blowing out candles on a birthday cake is a tradition that some scholars say dates to ancient Greece. But because your mouth contains bacteria that can spread through blowing, the practice is pretty dirty.
that when candles are blown out on a cake, there was 1400% -- or 15 times -- more bacteria on the frosting than on frosting with candles that were not blown out.
To simulate a birthday party and activate the salivary glands, Dawson's research team first ate some pizza. But instead of a cake, they spread frosting on foil and placed it on top of a Styrofoam disc, along with 17 candles. After all the candles were blown out, the number and type of bacteria in the frosting were measured.
"The amount of bacteria varies a lot from person to person based on how sloppy someone is when blowing their candles out, but it does occur," Dawson said. "I don't know the chance of this occurring, but in fact if someone is sick, carrying a disease, and blows on the birthday cake, there is going to be bacterial transfer."
Beware the beer pong ball
It's a simple concept practiced by college students across the country: Throw a pingpong ball across a table and hope it lands inside a plastic cup filled with beer on the other side. If the ball goes in, the other person has to drink the whole beer.
Except a pingpong ball may be covered in more bacteria than you might expect. The ball could have been bounced on the table, fallen on the floor, been in someone's hands or landed in a cup of beer -- surfaces that all contain bacteria.
For this experiment
, Dawson sent his students to collect pingpong balls from indoor and outdoor tailgates during a football game at Clemson.
The highest levels of bacteria were found on balls from outdoor beer pong games. They also discovered that nearly all of the bacteria on the pingpong balls transferred directly into the beer.
Mind the popcorn hands
Sharing a bowl of popcorn at movies, sporting events, fairs and concerts is common practice. And it seems to carry only a very low risk
of bacterial transmission.
Dawson and his team spread a noninfectious E. coli strain on people's hands and then measured how much was transferred to the popcorn they picked up and the kernels left in the bowl. Though they observed that bacteria transferred to both the popcorn in the hand and the popcorn in the bowl, the rate of transfer was only 0.2% and 0.0009%, respectively.
Of 136 tests, 24 resulted in no bacterial transfer to the popcorn at all.
You might want to think again after telling a waiter that you would like a lemon in your ice water. In restaurants, lemons are often left in open containers at room temperature, where anyone can access them, and are sometimes refrigerated overnight and used the next day to reduce waste. Food workers also may not always have clean hands when handling lemons. And depending on where ice is stored and how it's picked up, it can contain bacteria, too.
Dawson and his researchers set out to test the rate
of bacterial transfer to both lemons and ice by inoculating their hands or ice scoops with E. coli and then handling the two items. One hundred percent of the bacteria were transferred to wet lemons, but only 30% was transferred to dry lemons.
For ice, an average of 19% of the bacteria on hands were transferred, while an average of 66% of bacteria on the ice scoop were transferred.
They also found that lemons contaminated with E. coli and left at room temperature for 24 hours had an increased bacterial population. Inoculated lemons refrigerated for 24 hours had less E. coli the next day than those left at room temperature overnight. But the bacteria in refrigerated lemons were still able to survive.
Checking out the menu
As soon as you sit down at a restaurant, the waiter hands over a menu. You flip through all the pages and check out the back to make sure you don't miss anything delicious. But those restaurant menus are often not washed and can be another culprit in the spread of bacteria.
After collecting random samples of local restaurant menus and testing them for bacteria, Dawson and his team discovered
that for the most part, there were low numbers of bacteria living on them. At busier restaurant times, higher numbers of bacteria were on menus than in less busy periods.
When the researchers contaminated the menus with E. coli, 11% of the bacteria were transferred to people's hands. Low amounts of the bacteria were also able to survive on the menus over a period of one or two days.
But is eating bacteria actually that bad?
Dawson's studies illustrate that there is a risk of bacterial transfer from many everyday food habits.
But besides feeling grossed out, should we be worried?
As Dawson and Schaffner pointed out in their five-second rule studies, risk is often dependent on the type of bacteria. And that mostly depends on where the bacteria came from.
, a spokesman for the American Dental Association
and an oral infection control expert, said that for some of these scenarios, such as double-dipping or blowing out candles on a birthday cake, it depends on the health of the people you're sharing with.
"When someone is sick, they don't have normal oral flora in the mouth. If they have a strep throat, that's a type of bacteria that can cause an infection in people if it spreads," Molinari said. "For the most part, everyone just has common nonpathogenic bacteria in their mouths. But when a person gets sick, that dynamic changes."
, a food microbiologist at Cornell University, agreed.
"I would say that a good rule is, if a person is sick, they really shouldn't be sharing food or preparing food to share with other people," he said. "You don't want to end up like Typhoid Mary."
Mary Mallon, who became known as Typhoid Mary
, worked as cook in the early 1900s and spread typhoid to many people who ate her food despite never developing symptoms herself.
Worobo added that in situations involving bacteria on the floor, like the five-second rule and beer pong, most bacteria are probably normal organisms found in soil.
"But if it's in the kitchen and you've dropped meat or poultry or meat juice has dripped and not been cleaned properly, that has the potential for campylobacter and salmonella to be present on the floor," he said. "So safety really depends on what has been on the floor prior."
Schaffner applauds Dawson's efforts to teach undergraduate and graduate students how to conduct research. But he thinks these studies may need more outside replication and statistical analysis to confirm the results. He also says the overall risk of contracting a disease through any of these situations is low.
"Relatively speaking, these are going to be rare events. We obviously don't see a lot of disease outbreaks from this," Schaffner said. "But people do need to aware of the problem and evaluate their level of risk based on the situation."
And that's the message Dawson intends to convey: not that people should be afraid to eat but that they should be knowledgeable about the right ways to do it.
"These studies are not really big food safety issues, but they're interesting and fun. I hope they do make people aware of good hygiene," he said. "But, I don't want anyone to be a germophobe about it."