Staining an unknown bacterium?

Question

What type of differential stain would likely be the first used when identifying an unknown bacterium? Explain.

Answer 1

Gram stain is always the first stain you should do when you get an unknown. The reason is because the classes of Gram-positive species and Gram-negative species are both large so you are dividing the pool of possible species approximately in half.

If you stained for, say, acid-fastness or spores first and you end up with a negative result then you still have a ton of classes and genera of bacteria that your unknown could be. If the stain is positive however you get lucky because the number of bacteria that are acid-fast or spore-formers is relatively small.

Answer 2

Gram stain because it would identify if the bacteria had a cell wall or not.  This knowledge would allow us to choose what stain would be best to further identify the cell.