Eating part of a food item and then re-dipping it into a communal sauce, known as double-dipping, does transfer bacteria from the mouth into the dip, according to Professor John Tregoning, author of Live Forever? A Curious Scientist’s Guide to Wellness, Ageing and Death.
Professor Tregoning referenced a study that examined salsa, chocolate sauce, and cheese dip, which found that bacteria were transferred to all three, with salsa exhibiting the highest level of contamination.
Despite this transfer, Professor Tregoning suggests that double-dipping is unlikely to make people sick. He notes that most individuals' mouth bacteria are relatively stable and can combat bacteria introduced from others. While the risk could be higher for those who are severely immunocompromised or have recently taken antibiotics, such infections are expected to be rare.
He also highlighted that bacteria transferred via hands pose a greater concern than mouth bacteria. Studies on shared foods like popcorn have indicated some cross-contamination from fingers to food and then to mouths. Professor Tregoning emphasised that faecal bacteria, such as Salmonella and norovirus, which can cause diarrhoea and vomiting, are more likely to transmit, particularly when people do not wash their hands during food preparation.