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Open any school or college microbiology practical and you will meet the agar plate: a shallow dish of firm jelly on which bacteria grow as separate dots. That jelly came into science from a kitchen. This post covers what is known about Fanny Hesse, why the obvious alternative failed, a worked example of the kind of counting her husband did, and what students can take from the story.
How much do we actually know about her?
Less than you might expect. Almost everything comes from one article, The introduction of agar-agar into bacteriology, published in the Journal of Bacteriology in 1939 by Arthur Hitchens and Morris Leikind. They wrote that bacteriologists had long known agar was introduced "by a certain Frau Hesse", but not who she was. In 1937 they obtained the facts from her son, Dr Friedrich Hesse, a surgeon in Dresden. So the story is a family account, published by two careful historians, and we should read it that way.
Who was Fanny Hesse?
According to Hitchens and Leikind, she was born Fannie Eilshemius in 1850 in a place then called Laurel Hill, now part of Jersey City in New Jersey. Her father had emigrated from Germany. In 1874 she travelled in Europe and in Germany met Walther Hesse, a district physician in Schwarzenberg, Saxony, whom she married.
In the winter of 1881–82 Walther Hesse studied the new science of bacteriology under Robert Koch at the Imperial Health Office in Berlin. He began a study of the bacteria in air and continued it at home, where his house became his laboratory. Hitchens and Leikind describe Fanny as his technician and illustrator as well: she cooked broth for his bacteria, and she painted the originals for the colour plates in his 1884 paper.
Why was gelatin not good enough?
Koch had shown that bacteria could be grown as pure cultures on a solid, transparent, sterile surface, and gelatin was the most promising jelly. The 1939 article lists its two defects. Some bacteria liquefied it. And gelatin is fluid at 37 °C, the incubator temperature needed for bacteria that grow at body heat.
Walther Hesse measured air bacteria by drawing measured volumes of air through tubes lined with gelatin, then counting the colonies that grew. The liquefying gelatin ruined many of his experiments.
What did she suggest?
Hitchens and Leikind write that Fanny Hesse suggested agar-agar, which she had used for years in her kitchen to make fruit and vegetable jellies. She had the recipe from her mother, who had it from Dutch friends who had lived in Java, where agar from seaweed had long been used for jellies and soups. Agar stayed firm at every temperature at which bacteria grow, and the organisms Walther met did not liquefy it.
The discovery was sent to Koch by letter, probably late in 1881. In his 1882 paper on the tubercle bacillus, Koch made what the authors call the first printed reference to agar, a single sentence. No formal paper on it was ever published. Agar spread quickly all the same: by February 1887 Julius Petri, in Koch's laboratory, was recommending his new flat dishes especially for agar plates, which do not stick well to plain glass (Centralblatt für Bakteriologie, volume 1, page 280). The Nobel Prize biography of Koch describes his pure cultures "on agar kept in the special kind of flat dish invented by his colleague Petri", and Koch's wider work is covered in our post on Koch's postulates.
Worked example: counting bacteria in air
This is the kind of calculation Walther Hesse's method needed. The numbers are sample numbers.
Problem. You draw 20 litres of air through a tube lined with agar. After incubation you count 36 colonies. Estimate the colony-forming units per cubic metre of air.
Colonies per litre = 36 ÷ 20 = 1.8.
One cubic metre is 1,000 litres.
Colonies per cubic metre = 1.8 × 1,000 = 1,800.
What the number means. Each colony grew from at least one cell, but a dust particle can carry a clump of several cells and still give one colony. That is why the result is quoted as colony-forming units, not as cells.
Common mistakes:
Treating colonies as cells. A colony is the visible offspring of whatever landed there, one cell or many.
Forgetting the unit change. Litres to cubic metres is a factor of 1,000, not 100.
Ignoring temperature. On gelatin at 37 °C the medium would melt and the colonies would run together, the very problem agar solved.
What can a student take from her story?
Good ideas can come from outside a field. Fanny Hesse knew a material that experts did not, and she connected it to their problem. The story also shows how history is built: from a short sentence in Koch's paper, a son's memory and two historians' questions. When you read a famous story, ask where it comes from.
If the steps of growing and isolating a pure culture are hazy, the Fermi Concept Library covers every concept in physics, chemistry, biology, mathematics and computing, so you can look up each idea as you need it. The dish that agar is poured into has its own history, told in our post on Julius Petri and the dish.
Try it now: redo the worked example for 50 litres of air and 120 colonies.
