From personal experience I do know that acid fast organisms are present in 
some tap water.  We cultured them from water after some bad experiences with 
false positives.  Wang reported that somewhere over 30% of water fountains showed 
the presence of acid fast organisms (non-pathogens, but who can tell the 
difference on a stained slide).  This was done using the fluorescence technique.  
As a consequence, we filtered our water bath water through a Millipore filter 
after it was already passed through a charcoal filter and a deionizing column. 
 We also used no tap water prior to the carbol-fuchsin and we ran a negative 
control (uterus - extra cuts from blocks as the routine H&Es were cut)  from 
the same days workload with each batch of AFB slides.   So take care with your 
AFB stains.

Freida Carson

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