Medical Fun Facts Podcast

MFF0021: Ghon

Tonight I’m thinking of Anton Ghon. Ghon was an Austrian pathologist and he lived from 1866 to 1936. He specialised in bacteriology and became an expert in meningitis and tuberculosis. It’s his work in tuberculosis that earned him medical immortality. Both the Ghon focus and Ghon’s complex are named after him.

The Ghon focus is a subpleural primary tuberculous lesion. It’s usually found near a fissure either at the lower part of an upper lung lobe or at the upper edge of a lower lobe. It often forms in childhood after airborne exposure to the tubercle bacillus in a nonimmune human.

The Ghon focus is a small area of granulomatous inflammation (a form of chronic inflammation) and is only really seen antemortem by chest x-ray when the lesion calcifies or suddenly grows larger. Of course, you can get a better look post-mortem when you open the chest cavity, remove the lungs a have a good feel for hard lumps.

When a Ghon focus involves infection of nearby lymphatics and hilar lymph nodes, it transforms into a Ghon’s complex.

Tuberculosis is caused by the bacterium Mycobacterium tuberculosis. Mycobacteria are special in that their cell wall contains large amounts of mycolic acid which confers a waxy property. This means traditional bacterial stains, like Gram’s stain, fail to penetrate the cell wall and in clinical specimens, the clue is to look for ghosting bacterial cells. The usual stains used for mycobacteria are acid-fast stains like the Ziehl-Neelsen stain which is commonly used in mycobacteriology or the Wade-Fite technique used in histopathology to stain acid-fast bacilli (AFB).

Other genera that contain an appreciable amount of mycolic acid includes the corynebacteria and rhodococci.

Acid-fast stains like the Ziehl-Neelsen stain can be modified to also stain other bacteria like Nocardia species.

Mycobacteria are fairly common in the environment and we often see them as contaminants or commensals in clinical specimens. The clinical significance of such so-called atypical mycobacteria is low unless the patient is immunocompromised. Then these atypical mycobacteria can cause significant pulmonary infection. It’s important to note that infection with atypical mycobacteria should NOT be called tuberculosis. Unfortunately, I’ve heard some nonmedically qualified people make an assumption that atypical mycobacterial infection can be called tuberculosis. That creates a lot of confusion especially because treatment and overall patient management including infection control measures can be very different.

One of the most interesting mycobacterial infections I’ve come across is the Buruli ulcer which is caused by Mycobacterium ulcerans. While this tropical infection is well known in sub-Saharan Africa, it also goes by the name Bairnsdale ulcer in coastal Victoria here in Australia. Another pocket of infection is found in far north Queensland where it is known as the Daintree ulcer. The interesting thing about these ulcers is the size they can get to and the edges are often undermined so that you can run your finger around and under the edges. While treatment with antimicrobials is important for a cure, surgical intervention is often required too.

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