Picture Quiz Number 5: Answers

Have a look at this stain on a sputum sample and answer the questions below:


i) What type of stain is demonstrated?

Ziehl-Neelson stain. Originally invented by  Franz Ziehl and then Friedrich Neelson.


ii) What is the initial dye used in such a stain called?, and what is used to “decolourise” this dye?

Carbol-fuschin giving the bacilli their reddish colour. All other material in the slide is then decolourised by a mixture of acid and alcohol, just leaving the alcohol acid fast bacilli stained (AAFBs), which don’t decolorise due to their thick waxy cell wall. The slide is usually counterstained with malachite green or methylene blue or similar. Other minor variations of the ZN stain have sprung up over the years.


iii) What is the possible diagnosis?

Has to be smear positive tuberculosis until proven otherwise. The beading and slight curvature of the bacilli also suport this. These days such a sample would often undergo a PCR test to quickly confirm the presence of Mycobacterium tuberculosis without having to wait for the organism to grow on culture.


iv) What other type of stain can be used to make the potential diagnosis?

An auramine stain is another alternative for looking for acid alcohol fast bacilli. It is generally more sensitive but less specific than Ziehl-Neelson at picking up acid alcohol fast bacilli.


Click here for a nice diagramatic description of ZN staining.


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