I think it is possible to relay on some natural bacteria but the outcome can go both ways, either be really good or really bad. In the past people didn't use any specially selected strains of bacteria they simply used raw milk (together with its natural bacteria) and the rennet from the cow and they made successful cheeses but the problem is they also made some very bad cheeses if some unwanted bacteria strain got into the milk. So I think the best substitute would be to use yogurt with the live probiotic bacterial cultures. This in theory should provide bacteria that shouldn't spoil the cheese. But I have not tried to make it in this way so I can not say for sure that it will work. If you try to make cheese with some alternative method let me know. I would love to learn it as well :)