Acid fast stain and Gram Stain question?

Question

In microbiology class today, we had to figure out if an unknown is Gram positive or negative, their morphology, shape, acid fast or non acid fast, and spore former or not.

My unknown was Streptobacillus. Gram +, Rod, and it is a spore former. I had trouble with figuring out whether it is Acid fast or not because the slide under microscope showed pink chain of rods. The correct answer is that they are non acid fast. Is it true that if a bacteria is Gram + it is always non acid fast and vice versa?

thanks!

Answer 1

Usually with a simple unknown exercise, you're better off only doing a Gram stain to start.  If it shows up, you already know it's not acid fast, as acid fast cells wouldn't show up with a gram stain.  You would get purple rods for Strep, and if it had spores that were colorless inside of them, you would know that they were Bacillus.  Mycobacterium is acid fast and is also Gram positive, so it is NOT true that Gram positives are non acid fast.  It's just that you need an acid fast stain to see them.