Re: Why specifically is agar used as a bacterial growth medium?

Hi David,

Thanks for the great question. The history of Agar is pretty interesting in that it is one of those things that makes so much sense in hindsight that we tend not to think about its origins. As with so many things in the sciences, it was basically a series of lucky events that led to the use of Agar as a culture medium.

Agar was first used as a culture medium by Robert Koch (the Robert Koch, who developed Koch's Postulates for determining if a microbial agent was responsible for causing a disease) in 1881. Prior to then, gelatin was used as a culture medium. However, there were several problems with gelatin; it melted at high temperatures, and there were some bacteria that liked to eat it (causing it to "melt" at low temperatures).

At the time, Angelina and Walther Hesse were working in Koch's lab (Walther as the modern equivalent of a post-doc, and Angelina as her husband's technician and as a medical illustrator). Walther noticed that the jellies and puddinges made by his wife would remain solid in temperatures that caused gelatin to melt, and asked her what she used in them. Angelina described Agar-Agar (a Malay word that means "jelly"), an additive that had been used for centuries to "jell" foods in the hot regions of in south-east Asia. Angelina had learned of Agar-Agar from a Dutch neighbor who had lived in Java. The rest is history.

Agar has several qualities that make it superior to gelatin as a culture medium. It is a polysaccharide and is resistant to proteases. It melts at 100 degrees celcius, so it can be sterilized by boiling, but remains solid in the range of normal laboratory temperatures. It can be stored for long periods of time, permitting the maintenance of long-term cultures.

A very nice article written by Angelina and Walther's grandson appeared in a 1992 edition of ASM (American Society for Microbiology) News (Volume 58, #8, pages 425-428), describing his grandparents and their contribution to the modern science of microbiology. It might give your students an interesting perspective on the history of modern lab techniques.