Listeria monocytogenes is an environmental pathogen. It can become persistent if it finds niches that are not adequately cleaned and sanitized. These niches can be on equipment, or can be on floors, drains, etc. It moves throughout the facility via a number of vectors including water, personnel, and movable equipment.
Finding it in a product means the facility either had an ingredient with the organism (where that ingredient was added with no further kill step such as heating) or the organism made its way to a product contact area (located downstream from a kill step) such as piping, filling equipment, etc.. Recently, we saw where an ingredient supplier had an issue in their facility (cookie dough). But in many of these cases where product is positive, it is within the facility's own environment. In these cases, and especially when the facility does not have an active Listeria control program, it is hard to know the extent of the risk, and with that, are forced to expand the recall.
FDA Recall Notice
http://www.fda.gov/Safety/Recalls/ucm540897.htm
Ruth’s Salad Charlotte NC Expands Recall of Ruth’s Salads Pimento Spreads due to Possible Health Risk. May Contain Listeria monocytogenes
For Immediate Release
February 9, 2017