Medical Fun Facts Podcast

MFF0045: Agar

If I’ve timed this correctly, tonight’s show is dropping on Monday 6 March 2017.

So why am I talking about agar tonight? Many listeners know I write a food blog called Yummy Lummy and many listeners will also know that agar is used as a gelatin substitute to make jellies, puddings and custards. People who do not eat meat for ethical reasons prefer agar over gelatin because gelatin is derived from animals.

Apart from its culinary uses, agar is used in microbiology to culture microorganisms, mainly bacteria.

Agar is derived from algae and was first described in the 1600s in Japan. Agar is the mixture of two components, viz., a linear polysaccharide agarose and a heterogeneous mixture of smaller molecules known as agaropectin.

The history of agar in bacteriology is fascinating. The German microbiologist, Walter Hesse who was an assistant in Robert Koch’s laboratory began using agar at the suggestion of his wife, Fannie Hesse.

Because it has a higher melting temperature, agar is preferred to gelatin. This allows bacteria to grow at a higher temperature before the media melts. For example, sometimes we like our incubators to be at 42 °C, the same body temperature of birds when we’re trying to grow pathogens like Campylobacter.

Typically, agar is mixed with nutrients like brain heart infusion, or cysteine lactose electrolyte deficient broth or blood. In clinical microbiology, blood is popular. In the USA sheep blood seems to be preferred while in Australia horse blood is popular. You may not realise it, but there are subtle differences in how some bacteria look on each species of blood agar. The hæmolysis can look differently so when you’re reading a textbook written in the USA, you need to be aware of that.

There are also differences in blood agar depending on how you prepare it. If you’re careful blood agar looks red. If you heat it to a higher temperature the erythrocytes break apart and you end up with a brown agar which we call chocolate agar. The broken erythrocytes release more nutrients, making chocolate agar useful for many fastidious bacteria.

Another ploy is to freeze a bag of blood before melting it and adding it to the agar before heating. The resulting red blood agar is translucent and this is called laked blood agar. The nutrients in laked blood agar include those that would normally be destroyed by overheating.

Going back to horse blood, I remember once when I was training, a new batch of our horse blood agar failed in its quality control test to grow Escherichia coli. We went back to the media making vessels and lines, checked if we had previously made agar with antimicrobials, we replaced the tubes with fresh ones and washed out everything, still the blood agar failed to grow E. coli, however, it did grow an Extended Spectrum b-lactamase (ESBL) producing Klebsiella pneumoniæ. This seemed odd. My boss went to the supplier of the horse blood and asked if the horses were okay. It turns out one was ill and had been given a shot of gentamicin. We got blood from a healthy horse and everything grew as it should. It was a lesson to me in good laboratory quality management.

Even though I’m excited about the future of clinical microbiology as we embark on a change to whole genome sequencing and metagenomics. I know as an old-fashioned bacteriologist, I will have an enduring fondness for agar. As a senior pathologist once said on asking me what I wanted to do when I grew up and explained microbiology, “So Lum, with your ethnicity, you can be a market gardener on a very small scale”.

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