N.A. Kenneison in the UK asks about staining of AFB (acid fast bacilli) and 
AAFB (presumably Atypical AFB), in particular Mycobacterium leprae.

Pardon me for starting at an elementary level: but an acid fast organism has 
a waxy coating that's difficult to transport dyes either into or out of. 
You're allowed to get the dye into the bug in any way you please - xylene, 
phenol, peanut oil, DMSO, or gnomes with itty bitty hypodermic needles.

The specificity of the stain results from the method you use to get the dye 
out. Mycobacterium tuberculosis resists the strongest of the customary 
reagents, acid alcohol, but acid alcohol may remove the dye from 
Mycobacterium leprae and from other variably acid fast organisms such as 
Nocardia, and weaker differentiating reagents are used in AFB stains modified 
for these organisms.

Mycobacterium leprae is of course the etiologic agent of human leprosy 
(Hansen's disease), which also infects the north American nine banded 
armadillo (Dasypus novemcinctus). Acid fast variants such as the Fite-Faraco 
technique require a M. leprae control - does anyone on this list know where 
to get these? - I would suppose that one leprous armadillo would supply the 
world until kingdom come. M. leprae has never been cultured.

Clinically, other human diseases are caused by what are called "atypical 
acid-fast organisms" (the term "atypical" refers to clinical presentation, 
not to staining properties) such as Mycobacterium kansasii and M. fortuitum. 
Since the advent of AIDS, the most important of these is Mycobacterium 
avium-intracellulare (MAI), which causes a disseminated acid-fast disease 
which is one of the commoner causes of death in AIDS. 

As far as I know, these atypical acid-fast organisms have the same staining 
properties as Mycobacterium tuberculosis, but I have never seen an entirely 
unambiguous statement on this point - does anyone know? Is MAI a suitable 
staining control for M. tuberculosis, and vice versa?

Bob Richmond
Samurai Pathologist
Knoxville TN

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