What happens to the bacteria in yogurt as yogurt expires?

I like to answer in two sections: First: I see some statements in your narration that need clarification as they are basically not correct. Commercial yogurt are processed by heating grade A milk to about 90 C for several reasons,including killing of pathogens ( harmful bacteria). read more

Spoilage of yogurt: The bacteria that are added in yogurt, will not cause spoilage ( except the few I mentioned above). However during handling after opening the container, mold, yeast, some spoilage Gram positive and Gram negative bacteria can get in from the environment. read more

Yogurt does not expire, almost for long time, if it kept in refrigerated, may be bit sour, due slow progression of lactic acid production. Make sure it is sealed once seal is broken then mold will grow eventually. There may some whey on the top, and less live bacteria but nutritionally all the values will stay almost intact. read more

Bacteria also grow and accumulate on aged or improperly preserved foods, such as yogurt. Diarrhea is a common symptom that occurs after one has consumed expired yogurt, as the body is attempting to rid itself of harmful toxins the yogurt provided. read more