Animals are free from a defined set of pathogenic organisms that may cause clinical or subclinical disease. The health status will depend on individual lists of exclusion organisms. It is important to realize that SPF is not a standardized definition and that mice from different facilities may have different pathogens.
http://www.uff.br/animaislab/a12.pdf

In terms of Listeria monocytogenes control, the University of Georgia’s research into using competitive exclusion organisms in the drain to out-compete Listeria in this critical environmental niche looks very promising, and a major commercial sanitation supplier has expressed interest in taking this technology from the laboratory into a real-world setting.
http://www.foodsafetymagazine.com/magazine-archive1/december...

Control of Lm in Food Processing Facilities by Competitive Exclusion Microorganisms
xxx xxx, University of Georgia
This research determined whether the introduction of harmless bacteria in
the food processing facilities would reduce and/or eliminate Lm
via competitive exclusion.
http://www.meatami.com/sites/amif.org/ht/d/sp/a/GetDocumentA...