A Bacillus Megaterium and mycobacterium smegmatis, how can I differentiate them from one another?

Question

My instructor gave me a broth culture with two different organisms in it. I need to identify both. One is proteus vulgaris. I am still trying to determine what the other organism is. A Gram stain showed Gram negative rods (proteus vulgaris) and Gram positive rods. A spore stain revealed mostly red vegetative cells with small bits of blue green that may or may not be endospores. The acid fast stain showed purple cells with a few tiny red cells that might be acid fast. The stains aren't giving me a definite answer. Are there any differences between Bacillus Megaterium/ subtilis and Mycobacterium smegmatis that can help me decide between the two?

Answer 1

B. subtilus is non-acid fast, Mycobacterium is acid-fast. Do the stain again, and try to get better results, stains are not inconclusive, they are either positive or negative, the only reason they would not be is if the stain was done improperly. It can be tricky to get the stain right, so just keep trying.