Acid
Fastness -
To
stain
tubercle
bacillus,
in
smears
or
in
tissues,
the
commonly
used
method
is
the
Ziehl
Neelsen
method.
The
smeared
specimen
is
heated
to
steaming
for
2-3
min.
in
carbolfuchsin
(
a
mixture
of
triphenyl
methane
dyes
rosaniline
and
oararosaniline
in
aqueous
5%
phenol)
Subsequent
washing
in
95%
ethanol
-
3%
HCI
(acid
alcohol)
decolorises
most
bacteria
in
a
few
seconds,
whereas
acid
fast
organisms
retail)
the
stain
much
longer.
This
property
is
attributed
to
high
lipid
content
in
their
cell
wall.
Mycobacteria
appear
to
be
Gram
positive,
but
they
take
up
the
stain
weakly
and
irregularly
and
without
requiring
iodine
treatment
to
retain
it
