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