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Source Attribution of Salmonella in Ground Beef in Non-fed Processing Plants


Salmonella in ground beef continues to represent a major challenge for the non-fed sector
of the U.S. Beef processing sector. Because of the USDA regulations with respect to the
presence of Salmonella in ground beef destined for the school lunch program and in general
USDA focus on Salmonella, it behooves the industry to bring the same level of focus in
controlling Salmonella as that of controlling E. coli O157. A number of Check Off funded
projects have determined the prevalence of Salmonella as various steps in processing. These
projects have also determined that lymph nodes can be a significant source of Salmonella.
Because the lymph nodes from shank and brisket are not removed in non-fed plants and are
subsequently ground, lymph nodes can be a major source of Salmonella in ground beef. The
other primary source of Salmonella in ground beef is carcass surface tissue (originating from
hide). In this project we set out to determine the relative contributions of the lymph nodes and
carcass in the prevalence of Salmonella in ground beef. We will sample carcasses, flank lymph
nodes from the same carcasses, brisket lymph nodes from the same carcasses and ground beef
made from the same carcasses. We will obtain finger print of Salmonella isolates from each of
the above samples. Comparing the finger prints observed in ground beef with that of lymph
nodes and carcasses will enable us to determine most probable source of Salmonella in ground
beef. This information should enable the industry to better control the presence of Salmonella

1. Determine the prevalence of Salmonella on cow carcasses after hide removal and before any
intervention. <BR/>
2. To conduct PFGE analysis on Salmonella isolates from carcass samples. <BR/>
3. Determine the prevalence of Salmonella in lymph nodes from chuck and flank of the same
carcasses sampled above.<BR/>
4. To conduct PFGE analysis on Salmonella isolates from lymph nodes. <BR/>
5. To determine the prevalence of Salmonella in ground beef produced from carcasses sampled
above. <BR/>
6. To conduct PFGE analysis of the Salmonella isolated from lymph nodes<BR/>
7. Using the above information determine the most likely source of Salmonella in ground beef
and recommend appropriate measures to reduce or eliminate Salmonella from ground beef.

More information

Findings: Results indicate that while none of the air samples were positive for Salmonella, all
animal/carcass samples had at least one positive sample for Salmonella. Of 457 samples collected 163 were confirmed to be positive for Salmonella (35.7%). As expected the highest number of positive Salmonella samples (96%) was hide samples followed
by carcasses right after hide removal (47%). Eighteen percent of lymph nodes, 7.2% of trim and
1.7% of ground beef were positive for Salmonella. The PFGE results indicate that ground beef
Salmonella had similar pattern as that of carcass and hide isolates but the trim isolate had the
same pattern as that of lymph nodes. If we account for sampling errors due to no random
distribution of Salmonella, the conclusion is that the predominant (if not the sole) sources of
Salmonella (including multi-drug resistant Salmonella) are hide and lymph nodes.

Koohmaraie, Mohammad
IEH Laboratories & Consulting Group
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