Question

You INCORRECTLY perform an acid-fast stain on a smear from a mixed culture of Mycobacterium smegmatis (acid-fast) cells and Staphylococcus aureus (non-acid-fast) cells. Your mistake is that you forget to use methylene blue during the procedure. If you viewed this slide with a microscope, what would each cell type look like?

Answer 1

Answer:

Acid fast staining procedure involves staining slide with Corbol Fuchsin (ZNCF), followed by decolourization and finally staining with couter stain methylene blue.

Once the slide has been decolourized and washed, the background becomes colourless or picks up a dull pale colour or grey colour due to acid in the decolourizer.
In absence of counter stain Methylene blue the back ground will remain so and Staphylococcus aureus will also be partially visible as dull grey mass in the background. Mycobacterium smegmatis, which is acid fast would have retained the pink colour of Corbol Fuchsin and will appear as pink rods in chinese letter formation on the dull background. However, due to absence of background colour the contrast will be missing and light from the condenser will need adjustment in order to at least make things visible on the slide.