Refrigeration and thymol are harmless and will slow down microbial
  growth, but isn't the preservation of a microwave retrieval
  solution built in with its use, especially in a laboratory that
  does immunohistochemistry every day?  If slides are heated in the
  liquid to near boiling for several minutes every day, and the 
  container has a cover over it while it's in use, cooling down, 
  doing nothing or warming up, few bacteria and fungal spores should 
  fall into it, and most of those that do get in will be executed 
  en masse, every day, when the solution is used again.

  This is a circumstance in which it is not only more economical but
  also more microbiologically hygienic to use a solution repeatedly
  than to throw it out every day. Obviously you can't go on for
  ever, because there will be evaporative losses (easily corrected),
  bits of sections in the retrieval solution (easily removed by
  filtering) and eventually some sort of grunge derived from any 
  accumulated boiled bacterial corpses and bits of hyphae that are 
  small enough to pass through filter paper. There's no obvious way 
  to define an end-point for the last item. If the solution is 
  transparent to the eye, it's unlikely to deposit crud on the 
  sections, but it is conceivable that residual dead bacteria might
  cause false-positive results in immunohistochemical searches for
  bacterial antigens. Is this really a potential problem, or is it
  just a theoretical one?    
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On Mon, 16 Oct 2000, Amos Brooks wrote:
> I'd try to free up some refrigerator space to keep the buffer in at
> least. You could also try some thymol to prevent fungi, I don't think
> that would interfere with the buffer, would it??
> free space haha what's that .....  > Amos Brooks

> Luis Chiriboga wrote:
> > I usually make large quantities of microwave solution on the order of
> > 20L (I use a lot and I'm lazy). Has anyone ever added a preservative to
> > their buffer?  I occasionally get some growth (nothing bigger than an
> > occasional tadpole) in the carboy.
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 John A. Kiernan,
 Department of Anatomy & Cell Biology,
 The University of Western Ontario,
 LONDON,  Canada  N6A 5C1