Phenol and acid fast bacilli

<< Previous Message | Next Message >>
From:Roy Ellis <>
To:Histonet <>
Content-Type:text/plain; charset=iso-8859-1

Mycobacterial cell walls contain complex lipids composed of beta-hydroxy
carboxylic acid (mycolic acids) with chain lengths of up to 90 carbon atoms
and long chain length phthiocerols and polymethyl branched fatty acids. Both
alpha and hydroxy mycolic acids have been isolated from mycobacteria. Other
complex lipids and glycolipids have also been isolated.

The property of acid fastness is related to the carbon chain length of the
mycolic acid.

The mycolic acid (and other cell wall lipids) present an initial barrier to
dye entry as well as elution. In the originl ZN method this is partly
overcome by addind phenol, a lipophilic agent, to a concentarted aqueous
solution of basic fuchsin, and partly by heating and extending the staining
time. Phenol is said to increases lipophilia and thus aid pssage od the dye
through the protective lipid wall enabling the dye to bind with negatively
charged ions present in the bacterial structure. After staining, the lipid
coating is recostituted  by removing the lipophilic agent. Subsequent
difrentiation in acid/alcohol solutions removes the dye from all tissue
structures but leaves the dye in the organism, the lipid wall now acting s a
brrier to the removal of the dye.

Phenol is a particularly hazardous substance especially in concentrated form
and can be replaced in the carbol fuchsin solution with many alternative
lipophilic agents with great success. The best alternative is LOC (liquid
organic cleaner) sold by many of the large chemical companies.

REFERENCE: Safer staining method for acid fast bacilli. Ellis R and
Zabrowarny L. Journal of Clinical Pathology 1993;46: 559-560.

<< Previous Message | Next Message >>