WASHINGTON — Incoming raw poultry is the primary source of Listeria monocytogenes contamination in commercial chicken cooking plants, according to a 21-month study conducted by Agricultural Research Service (A.R.S.) scientists and their collaborators at the University of Georgia, Athens, and published in the Journal of Food Protection.

The researchers conducted their study in a new commercial cooking facility and began taking samples before the facility was operational. As a result, the research team was able to track the sources of contamination.

Potential sources of L.m. were tested by taking samples of soil and water around and near the facility exterior, and by testing heavily traveled floor surfaces following personnel shift changes. Samples also were collected and tested from incoming air from air vent filters and from monthly swabs of incoming raw meat.

The plant was free of L.m. when first constructed; floor drains in the facility were sampled approximately monthly to determine at what point the plant would become colonized with the bacteria.

Within four months of operation, L.m. was detected in floor drains, indicating that the organism had been introduced from some outside source. No L.m. was recovered from any floor samples in the plant entryways, locker room or cafeteria. Likewise, the organism was not detected on air vent filters during the survey. The only tested source found to be consistently positive for L.m. was incoming raw poultry meat.

The researchers believe the results of the study will help cooking operations more sharply focus their sanitation processes to reduce cross-contamination. Listeria monocytogenes is a bacterial human pathogen that is sometimes found in fully-cooked, ready-to-eat processed meat and poultry products.

Co-authors of the study included A.R.S. microbiologist Richard Meinersmann in Athens, University of Georgia scientist Joseph Frank, and former A.R.S. researcher Scott Ladely.