The scientists took samples from the participants' tongues and their saliva.
Your mouth is home to around 700 different types of bacteria. Kort and his team discovered that on average the bacteria in your mouth is more similar to your partner's than a random, unrelated person, which makes sense.
But they also found differences between the extent to which the bacteria on your tongue and in your saliva match up with your partner's. It seems that your salivary bugs are more changeable, but those on your tongue like to stay put.
The researchers found that at least nine kisses a day led to couples sharing more salivary bacteria than not. In general, more kissing and a shorter time since the last kiss was related to the couple sharing more bacteria in their saliva.
But when it came to tongues, though couples were more likely to have similar bacteria, it didn't seem to have anything to do with kissing. So while kissing does transfer bacteria, that bacteria doesn't necessarily end up on your tongue.