EM (Effective Microorganisms) is a liquid product cultured in a tank with more than 80 species of microorganisms of 10 genera of 5 families. It contains both aerobic and anaerobic microorganisms. In other words, microorganisms that cannot live without oxygen and microorganisms that hate oxygen coexist. That is one of the most distinctive features of EM.
Until now, there was a stereotyped concept among microbiology researchers, or those who deal with microorganisms, that they can treat microbes only with specific types. Attempts to "combine different types of microbes with others" were never put in consideration. They thought that "if you do that, the different microbes are bound to fight each other."
Thus, in conventional microbiology, such high-level co-cultivation has been considered impossible. So skeptical people about the efficacy of EM always point out this problem. In reality, however, coexistence of aerobic and anaerobic microorganisms is possible and realizable.
For instance, Photosynthetic bacteria and Azotobacter in soil are types of microorganisms that play important roles under the same purpose in nitrogen fixation; however, the conditions for these two to exist are completely opposite. Photosynthetic bacteria are anaerobic and do not like oxygen, and Azotobacter is aerobic and likes oxygen. It's just like water and oil. So until now, coexistence of these two was thought to be impossible. In EM, however, they coexist beautifully. This was an absolutely astonishing discovery.