We once had a contamination of an atypical mycobacterium (Zn positive) from
the laboratory tap water supply we used to wash slides after dewaxing and
rehydrating.  I don't remember the exact name but it was a common soil borne
organism. The water also had iron as a contaminant - Perl's Prussian Blue
positive!  The source was a slime mould inhabiting the plastic pipe attached
to the tap.

Phil Hall
St Michael's Hospital Bristol, UK

	----------
	From:  Gayle Callis[SMTP:uvsgc@msu.oscs.montana.edu]
	Sent:  24 March 2000 15:47
	To:  histonet@pathology.swmed.edu
	Subject:  contamination of a section, control on slide, where does
it come from

	I would think a likely source of cross contamination is not from a
control
	section to a patients section on the same slide, but rather as a
floater in
	the water bath, the bits and pieces that tend to break away from
other
	sections.

	Do people still skim the waterbath with Kimwipes BETWEEN each case
that
	they cut?  

	An interesting question and discussion.  I can see why microbiology
would
	be concerned, they use smears, and then flame the smears to fix them
to the
	slide before doing an AFB.  An aerosol droplet could land somewhere
else,
	where an unknown is being dealt with, best to have smears separated.


	I would assume that once a tissue is fixed with NBF (yes, we use it)
and
	the microorganisms are within the confines of a fixed tissue, they
are not
	going to exfoliate from the section, UNLESS this happens with a
fragment of
	positive tissue getting loose on the waterbath, and if they do break
away
	during the staining process, the various rinses, etc will let them
be
	rinsed away or you have very friable tissues at cutting, bits and
pieces
	crunched off by the knife, and onto the next section. 

	 In fact, the waterbath could be a source of false positive AFB
organisms,
	from poorly cleaned plastic carboys filled with distilled water.
This has
	been published since these saprophytic (did I spell that one
correctly?)
	AFB exist out there and contaminate your thought to be pristine
water
	containers.  

	I always did separate controls, but then I also did not have large
volumes
	of slides in a day.  Has there ever been a study to determine if
control
	sections contaminate the unknown on the same slide?

	  

	  
	Gayle Callis
	Veterinary Molecular Biology
	Montana State University
	Bozeman MT 59717-3610
	406 994-4705
	406 994-4303