Submitted to: Journal of Food Protection
Publication Type: Peer reviewed journal
Publication Acceptance Date: 8/28/2012
Publication Date: 1/7/2013
Citation: Van Kessel, J.S., Sonnier, J.L., Zhao, S., Karns, J.S. 2013. Antimicrobial resistance of Salmonella enterica isolates from bulk tank milk and milk filters in the United States. Journal of Food Protection. DOI: 10.4315/0362-28X.JFP-12-263. Interpretive Summary: Resistance of pathogenic bacteria to antibiotics or other antimicrobials has become a growing concern. Zoonotic bacteria such as Salmonella have frequently been associated with dairy cows and their environment and therefore milk and milk products can become contaminated. Salmonella isolates were recovered from the bulk tank milk (raw milk before it leaves the farm) from US dairy farms as part of two surveys conducted by USDA-APHIS and other participants in 2002 and 2007. Filters are used to trap large particles from the milk before it is put into the bulk tank. These filters were also tested in the 2007 survey. The objective of this study was to characterize antimicrobial susceptibilities and genetic relatedness of the Salmonella isolates recovered from the raw milk and milk filters in these two surveys. Each of 176 Salmonella isolates was tested for their ability to grow in the presence of 15 different antibiotics. Thirty isolates (17.0%) exhibited resistance to at least one antimicrobial agent. Twenty isolates (11.4%), displayed resistance to a wide range of antibiotics including ampicillin, chloramphenicol, streptomycin, sulfonamide, and tetracycline (ACSSuT), plus resistance to amoxicillin-clavulanicacid, cefoxitin, ceftiofur and ceftriaxone. The isolates that were resistant to at least one antimicrobial were tested for the presence of particular elements in their DNA that are known to correspond with antimicrobial resistance. These elements were detected in primarily the MDR isolates. The results of this study suggest that there is a low but appreciable risk of infection by multi-drug resistant Salmonella from consuming non-pasteurized milk and dairy products. This information will be useful to other scientists, veterinarians, and other government regulatory agencies.
Technical Abstract: Non-typhoid Salmonella is frequently associated with dairy cattle and their environment. Despite well-developed milking hygiene protocols, fecal contamination can occur and Salmonella has often been isolated from bulk milk. Salmonella isolates were recovered from US bulk tank milk as part of the NAHMS Dairy Surveys in 2002 and 2007. In-line milk filters were also tested in the 2007 survey. The objective of this study was to determine antimicrobial susceptibilities and genetic relatedness of Salmonella from bulk milk and milk filters in the NAHMS surveys. Susceptibilities to 15 antibiotics were determined for 176 Salmonella isolates representing 26 serotypes using a Sensititre automated antimicrobial susceptibility system. Resistant isolates were screened by PCR for the presence of the blaCMY gene and class I integrons, and further characterized by Pulsed-Field Gel Electrophoresis (PFGE). Thirty isolates (17.0%) representing six serotypes [(Newport (14/14), Dublin (7/7), Typhimurium (3/5), Kentucky (4/22), Anatum (1/13), and Infantis (1/2)] exhibited resistance to at least one antimicrobial agent. Twenty isolates (11.4%), including all 14 S. Newport, three S. Dublin, two S. Typhimurium and one S. Infantis, displayed the typical MDR-AmpC phenotype which showed resistance to ampicillin, chloramphenicol, streptomycin, sulfonamide, tetracycline (ACSSuT), plus resistance to amoxicillin-clavulanic acid, cefoxitin, ceftiofur and ceftriaxone. All MDR-AmpC isolates carried the blaCMY gene and five contained class I integrons (2.8%). Two-enzyme (Xba I and Bln I) PFGE discerned clades within serotypes, though it did not discriminate isolates based on year, antibiotic resistance profile, or geographic location. These results suggest that there is a low but appreciable risk of infection with multi-drug resistant Salmonella infection from consuming non-pasteurized milk and dairy products.