Some comments on this really fascinating thread from an old pathologist, 
trained in Baltimore where there's a LOT of tuberculosis.

The acid fast bacilli (AFB) that come out of the water faucet have names like 
Mycobacterium aquae and Mycobacterium gordoneae. They occasionally cause 
infections in extremely immunosuppressed patients, but they aren't hazardous 
in the laboratory. As far as I know, they have the same staining properties 
as Mycobacterium tuberculosis.

If an AFB isn't in the plane of the tissue section, it's probably a 
contaminant, and the pathologist should usually disregard it.

AFB are curved and distinctly beaded, and these features distinguish them 
from acid-fast debris and from acid-fast lipofuscins. Your pathologist ought 
to know this, but a lot of the young folks don't. Somebody needs to sit you 
down at a double-headed microscope and show you this feature until you get a 
feel for it. Dr. Carson, I really think this information should be in the 
next edition (and there IS going to be a third edition!) of Histotechnology: 
a Self-Instructional Text - with photomicrographs to support it.

The fluorescent technique is much more sensitive (perhaps fivefold) than 
light microscopy, and light microscopic acid fast staining ought to be 
abandoned for the diagnosis of mycobacterial disease. Pathologists tend to 
order this stain as a ritual, or because they're afraid the lawyers will get 
them if they don't. This approach needs to be discouraged.

Bob Richmond
Samurai Pathologist
Knoxville TN