In 1881 Robert Koch demonstrated a new technique at the International Medical Congress in London. Koch had recognized the difficulties of using broth media for isolation of pure cultures and had looked for solid media alternatives. He evaluated media such as coagulated egg albumen, starch paste and an aseptically cut slice of a potato (as used by the German biologist Schroeter), but then moved to a meat extract with added gelatin. The resulting ‘nutrient gelatin’ was poured onto flat glass plates which were inoculated and placed under a bell jar. This new plate technique could be used both to isolate pure cultures of bacteria and to sub-culture them either onto fresh plates or nutrient gelatin slopes in cotton-wool plugged tubes.
Although nutrient gelatin was a major advance, gelatin had two major disadvantages as a gelling agent:
• It turned from a gel to a liquid at 25 °C – preventing plates from being incubated at higher temperatures.
• It was hydrolyzed by gelatinize – an enzyme produced by most photolytic organisms.