Traditional European cheese makers let milk naturally acidify all the time, but for the average hobbyist, that is a difficult and somewhat dangerous thing to do. You should never try this with pasteurized milk because all of the naturally occuring bacteria have been killed off. By letting the milk just sit overnight, you are allowing both naturally occurring and contamination bacteria to multiply. So even very tiny quantities of coliforms or disease causing bacteria can become a real problem. If you don't want to use "store bought" cultures you can use buttermilk (mesophilic) or yogurt (thermophilic). With Moz you would use yogurt. One way or another, you can't skip the bacteria.