Bacteria are usually cultured in Petri dishes that contain a culture medium, usually nutrient agar. Petri dishes were developed in 1887 by Julius Richard Petri (1852–1921), a member of Robert Koch’s laboratory. The top of the dish is larger than the bottom so that when the dish is closed a strong seal is created, preventing contamination of the culture. Agar was developed as a culture media for bacteria by Robert Koch. Koch was interested in the isolation of bacteria in pure culture. Because isolation was difficult in liquid media, he began to study ways in which bacteria could be grown on solid media. After sterile, boiled potatoes proved unsatisfactory, a better alternative was suggested by Fannie E. Hesse (1850–1934), the wife of Walther Hesse (1846–1911), who was one of Koch’s assistants. She suggested that agar, which she had used to thicken sauces, jams, and jellies, be used to solidify liquid nutrient broth. Agar is generally inexpensive and, once jelled, does not melt until reaching a temperature of 212°F (100°C). If 1–2 grams of agar are added to 100 milliliter of nutrient broth, it produces a solid medium that is not degraded by most bacteria. Stacks of Petri dishes culturing bacteria are one of the most common items in a microbiology laboratory.