MFF0029: Nocardia


Tonight it’s Monday 9 January 2017 and I want to share with you the joy that is Nocardia. The genus Nocardia is fascinating, to the layperson it sits somewhere between normal bacteria and the mycobacteria. Not being a bacterial taxonomist nor a gene jockey I can’t say with any authority that that is where the nocardiæ should sit but it’s a reasonable assumption in my mind.

I remember the first time I saw one. It was beautiful and wholly unexpected. As I’ve mentioned before I do love to sit behind a microscope and read Gram’s stains. This would have been in the mid-1980s at Queensland Medical Laboratory. I was in university and I put myself through by working part-time in a medical testing laboratory. I started in 1983 straight out of high school, as a technical assistant and then became a technician and then in 1987, after completing my bachelor of medical science midway through my bachelor of medicine and bachelor of surgery degrees, I was made a laboratory scientist with a pay increase to match. Completing all those years of medical school was made so much easier being able to pay for books, union fees, the Higher Education Contribution Scheme, board, petrol and car maintenance (my 1978, CL Chrysler Regal Le Baron with a Hemi 265 cubic inch straight six was very thirsty, especially with the Holley twin barrel and extractors).

Anyway, back to the Gram’s stain. It was a sputum specimen. Sputum Gram’s stains are usually underwhelming and unrewarding, and I’d estimate roughly 5 per cent have any really decent findings. On this day, as I peered down the microscope and moved the stage slowly I came across an odd and peculiar sight. In amongst the pus cells, yes we call them pus cells, they’re really polymorphonuclear leucocytes, but it’s easier to say pus cells. Do you like how I say pus? Anyway, as I was looking at the pus cells I saw some whorls of blue but they were very odd. Blue means Gram-positive, that is the material has had the crystal violet fixed by the iodine mordant and has resisted decolourisation with the acetone. These whorls of blue were not continuous but had a beaded appearance, almost like a blue ‘pearl necklace’. No, not that sort of pearl necklace, a necklace made of pearls. Having never seen this before I asked another scientist, I think it may have been Steve the kiwi. He explained that I was seeing a species in the genus Nocardia. The cell wall contains waxy mycolic acid and so penetration of Gram’s stain is not complete. He also explained that by modifying the Ziehl-Neelsen stain we could effectively confirm the diagnosis of pulmonary nocardiosis. A standard ZN stain uses hydrochloric acid in isopropyl alcohol (or methanol) as a decolourising agent while a modified ZN for nocardiæ uses sulphuric acid. Sure enough, these whorls of bacteria stained pink in the modified ZN stain.

Nocardiæ grows a little slower than normal bacteria and they’re not always that easy to identify in simple biochemical tests. Because of their relatively slow growth, they can be outgrown on routine clinical agar if there is a lot of other flora in the specimen. One of the ingenious ways to isolate nocardiæ is to use tap water agar. There are just enough trace elements and nutrients in standard tap water to enable the nocardiæ to grow while virtually nothing else does. These days we have more advanced technology and nocardiæ can be identified more readily.

My final comment and this, unfortunately, has to be an archival addendum because it would be improper of me to encourage this sort of behaviour. Nocardia on agar after incubation smells like freshly mown grass. It’s a real earthy odour. Sniffing or whiffing agar plates is frowned on these days for biosafety reasons, especially with one of my all-time favourite bacteria, viz., Burkholderia pseudomallei because laboratory infections have occurred as a result of overzealous plate sniffing. I could go on about how many plates I’ve wafted under my nose, but that wouldn’t remove the potential danger, so please do not whiff culture plates. It could be very dangerous.

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