Agar: making liquids solid.

A fundamental piece of my day-to-day routine as a microbiologist involves culturing the microbes I want to work with. As explained in my recent post, using the correct combination of nutrients is necessary for growing certain types of microbes but is that the only important aspect?

Isolation is the key.

Let’s say I want to identify the microbe responsible for a new disease or outbreak. Robert Koch defined some famous postulates or requirements to make such a claim at the end of the 19th century. Three out of the four postulates require that we isolate the accused and demonstrate that this microbe alone can cause disease in a healthy organism.

Sounds easy enough but is the process of isolating microbes that easy?

As I have said before, microbes are everywhere – even in space – and they even cover our entire bodies. Hence, isolating microbes and keeping them isolated is a hard task microbiologists battle with on a daily basis.

The original growth media was meat infusion extract produced by the Liebig Extract of Meat Company (Lemco). While effective at growing microbes, this broth also grew a lot of different microbes simultaneously. Combined with an isolation method called streaking, physically separating microbes on a solid surface could give isolation of the microbes in a sample.  However, Koch had difficulty finding a good gelling agent.

On the left, agar comes in a powder that can be added to liquid media, heated to high temperature and cooled into a gel that can take any shape. Advantageous over liquid broth alone (middle), solid media in a circular shape provided by Petri dishes allows microbiologists to isolate microbes using the streaking method (right). Photo credit: Kenny Flynn. 

On the left, agar comes in a powder that can be added to liquid media, heated to high temperature and cooled into a gel that can take any shape. Advantageous over liquid broth alone (middle), solid media in a circular shape provided by Petri dishes allows microbiologists to isolate microbes using the streaking method (right). Photo credit: Kenny Flynn. 

Gelatin seemed like a good option but there were two problems: 1) gelatin melts below body temperature and 2) microbes commonly eat gelatin as a tasty snack. These issues made working with disease-causing microbes difficult.

Luckily, the wife of Walther Hesse, who was working for Koch, suggested the use of agar, a polysaccharide derived from red seaweeds used in making fruit jellies. With a higher melting temperature and decreased tastiness, agar is still used in microbiology labs around the world today.

You can find numerous agar plates on my bench on a daily basis. Photo credit: Kenny Flynn

You can find numerous agar plates on my bench on a daily basis. Photo credit: Kenny Flynn