The seaweed-derived substance introduced for solid bacterial culture was agar. Its use solved a major problem in early microbiology because the gelatin then used for culture media could melt at incubation temperatures or be broken down by microorganisms.
The suggestion is credited to Fanny Hesse, whose husband Walther Hesse worked in Robert Koch’s laboratory. Fanny had learned about agar’s heat-resistant gelling properties and used it in cooking. When gelatin proved unreliable for growing bacteria, she recommended agar as a substitute. Walther Hesse and Koch then used agar-based media to produce stable cultures.
Agar remains useful because it stays solid at temperatures suitable for bacterial growth and is comparatively resistant to bacterial enzymes. This allowed researchers to isolate individual colonies and study organisms in pure culture, a crucial step in identifying disease-causing microbes.
A common mix-up is crediting Koch alone or describing agar as gelatin. Agar is a polysaccharide extracted from certain seaweeds; gelatin is a protein derived from animal collagen. Agar-based media later became closely associated with culture plates and the Petri dish.