Medical Fun Facts Podcast

MFF0008: Blood cultures


I was at work today and a friend asked how long does it take after his blood is taken to get a result. After a bit of a chat, I worked out that he meant blood for culture.

It reminded me that when I was a boy we used Castaneda bottles for the culture of blood. These are biphasic bottles containing both liquid (broth) phase and solid (agar) phase growth media. I remember when I was putting myself through medical school working in a medical testing laboratory, I’d have to open the blood culture incubator in the morning and shake the rack of bottles to agitate the broth and get it to cover the solid agar. The bottles would get read twice a day to see if any bacteria were growing. It was a long slow process and I remember in the tea room before work in the morning we’d regularly check the death notices in the paper and we’d often find that person had a positive blood culture that day.

Compare that with today. We have amazing automated blood culture instruments which use changes in carbon dioxide in the media as a result of bacterial metabolism to detect growth. The change in carbon dioxide concentration causes subtle pH changes which result in a change in indicator colour in a disc in the base of the bottle. A beam of light is shone on the permanently rocking bottles every eight minutes. Any change is recorded and using a growth algorithm when sufficient change is measured the instrument signals that the bottle is positive. This leads to the bottle being removed and sampled. A Gram’s stain is performed along with a wet mount preparation. This information is communicated with the referring medical team along with specialist microbiologist advice about patient treatment.

These days with MALDI-TOF a bacterial identification is often available hours later as soon as solid phase growth appears after subculture from the positive bottle. Antimicrobial susceptibility and resistance results are usually available the following morning.

I love the technology we use in a modern medical testing laboratory to help get our patients better.

If you’re in school or university and thinking about studying medicine, I can highly recommend pathology training. It is one of the most fulfilling careers around.

This has sparked a memory of an article written about me while I was working in Darwin. You can check it out here (in the show notes)

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