medium is achieved. Basically, the same principle of alternating exposure of thecultures to the nutrient medium and the air was applied may years later to developthe RITA system, and similar setups described in Chapter 7.To prevent microbial contamination, the culture vessels can be closed by cottonwool wrapped in cheesecloth, as a simple method. However, many other materials,such as aluminum foil, or more costly products on the market, can be used instead.