If you have access to a compound microscope, it’s fun to look at yogurt bacteria at high magnification.
Make a wet mount with a tiny toothpick amount of yogurt and a drop of water. Under 400x, you can see individual bacteria swimming around. Store-bought yogurt often contains these species: Streptococcus thermophilus, Lactobacillus delbrueckii subsp. bulgaricus, Lactobacillus acidophilus, Bifidobacterium, and Lactobacillus casei.
The first two are the main yogurt-makers and work in synergy to digest lactose and produce lactic acid. They work so well together because they each make a waste product that the other uses for metabolism and are both thermophilic (survive at relatively high temperatures) bacteria that grow best at 105–113°F (40–45°C). The other three strains work better at temperatures closer to the human body temperature of 98.6°F(37°C). They are often added as probiotics since they are already present in different parts of your body. As the temperature and pH of the yogurt changes, different species are able to metabolize at better or worse rates, and the end result is a unique flavor profile of everyone’s waste.
In general, these bacteria come in two main shapes, with the Lactobacillus having an oblong rod shape and the Streptococcus, which is Greek for “twisted berry,” like little spheres. After they divide they can still be attached to each other; they sometimes look like beads in a chain.