Re: AAFB vs AFB
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?
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