Holiday season means parties, and parties usually mean lots of dips and chips/crackers to dip in them. But besides the problem of lots of people packed in party spaces, there's another way to pass the germs around double dipping!
Although the study isn't new, its message needs to be reiterated dipping a tidbit into a sauce after one has already taken a bite is by no means a sanitary practice.
A group of undergraduate students at Clemson University in South Carolina investigated whether dipping a tidbit into several different sauces would transfer bacteria into the dip, and what effect the type of dip had on the extent of bacterial contamination. They examined three different dips: a chocolate sauce, a salsa and a cheese dip. The salsa was the most fluid dip, and also the most acidic.
To start, the investigators (eight students) dipped crackers into sterile water before and after biting them, and found that the bitten crackers indeed transferred more bacteria than the whole crackers (the crackers themselves were not sterilized). They further tested the effect of acidity (pH) on bacterial growth, and found that the water with the lowest pH (the most acidic) both initially and after two hours post-dipping had the fewest bacteria.
Finally, they tested the real sauces. And they found that although more bacteria initially were transferred to the salsa probably because it is more liquid than the others two hours later there were fewer bacteria in this more acidic dip.
Thus, you can, indeed, transfer bacteria from bitten crackers to dips. And, although it wasn't tested in this study, it's unlikely that simply turning the chip around so that the unbitten portion enters the dip is not a good fix. Although mouth bacteria wouldn't transfer, hand bacteria assuredly would. So one scenario might be the following: a party-goer doesn't wash his or her hands after using the restroom, then uses the dip, turn and dip again technique. Not so great.
Perhaps the best advice is to put your favorite dip on a plate and eat from that sparing both yourself and others from the dangers of unsanitary double-dipping!