Milk curdles in the presence of bacteria as they produce byproducts that cause it to coagulate. Not even necessarily lactic bacteria, but that's quite likely.
It forms a clabber, which is a culture of bacteria with milk destabilized enough to curdle. There is no way of knowing what type of bacteria they are without testing.
That said, people have eaten clabber for many centuries without getting sick. And you can do that same if you want. You can also use this bacteria as a starter for cheese or to make more of the clabber. Yogurt is made with a very specific process, by scalding milk, and adding specific bacteria that like high heat, and then culturing at that high heat, which is about 100F.
