Let’s get this out of the way quickly. Your earlier post was "ignored" because I read too fast and missed it.
As someone with a background in biology, though, I do have to keep pointing out that bacteria don’t magically "separate curds from whey". The biochemical processes of bacterial fermentation in milk produce lactic acid which has the same effect on milk protein as the addition of acid such as lemon juice.
To quote Wikipedia: "Lactic acid is primarily found in sour milk products, such as: koumiss, leban, yogurt, kefir and some cottage cheeses. The casein in fermented milk is coagulated (curdled) by lactic acid."
It is therefore quite reasonable in modern America to take a product, yogurt, which is both a source of milk protein AND (the critical part) pure, active lactobacillus and use it to produce the desired result (which can admittedly also be achieved in other ways): curds (i.e casein).