Project Summary

Source Attribution of Antibiotic Resistant Salmonella in Cattle Processing Plants

Principle Investigator(s):
J. M. Bosilevac, T. A. Arthur, S. D. Shackelford, T. L. Wheeler and M. Koohmaraie
U.S. Department of Agriculture, Agricultural Research Service
Completion Date:
May 2008



There is considerable data on the prevalence of Salmonella and MDR-Salmonella in processing plant environments; however, it is impossible to attribute the source of the organism to any specific group of slaughtered cattle due to the significant impact of the lairage environment.  Also, Salmonella may enter the beef chain through alternate methods such as in lymph node tissue that may be included in ground beef materials. Salmonella that have acquired resistance to multiple antibiotics (MDR-Salmonella) are of increasing concern.  Many studies have implicated cull cattle from particular production systems as a reservoir of MDR-Salmonella that may be entering the beef chain, but conclusive data is lacking.  

The shedding of Salmonella has been described to increase in cattle due to the stress involved in transport and lairage, and the lairage environment itself may be responsible for the increased prevalence.  The best sample to collect would be one that is free of the effects of the lairage environment, such as feces from cattle at slaughter, but this too has been brought into question by studies from pork production where the acquisition of Salmonella was observed during very short times in lairage.  The studies of objective 1 were initiated to determine if feces collected from the distal portion of the colon of cattle at the processing plant are an accurate indicator of Salmonella rates in the production environment.  

Several items that are not strictly carcass surface trim tissue, such as lymph nodes, are included in ground beef.  There are several reports of bacteria isolated from the lymph nodes of cattle at slaughter; however most of the studies have dealt with mesenteric lymph nodes that are not normally incorporated into ground beef.  There are only a few lymph nodes that are located in products that go into ground beef.  As such, these tissues may be a source of Salmonella that could potentially contaminate the ground product.  The studies of objective 2 focused on determining the presence, levels, and resistance status of Salmonella in beef lymph nodes likely to be incorporated into ground beef.  

As mentioned above, cull cattle from particular production systems such as dairies have been implicated as a reservoir of MDR-Salmonella that may be entering the beef chain, but conclusive data is lacking.  There are processing plants that routinely process cull dairy and beef cows and bulls as well as fed cattle.  Samples collected at processing plants are desirable because multiple lots of cattle from wide ranging geographic areas can be sampled at a central location.  Using the fecal sampling method validated in Objective 1, feces was collected at processing plants across the U.S. to build a comprehensive set of data to identify whether a specific segment of the production system is more likely to introduce MDR-Salmonella into the beef chain. The stated objectives for this work were to: 

  • Demonstrate that the collection of feces from distal colon provides a sample that represents the levels of Salmonella in the production environment and that this particular sample is not affected by factors such as transport and lairage.
  • Determine if lymph nodes are a source of Salmonella that could potentially contaminate ground beef products.
  • Determine if MDR Salmonella can be attributed to cattle from a particular production environment such as cull beef cows, dairies, or feed lots.

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