Food Protection Trends - May 2010 - 276

FIGURE 1. Dendrogram generated from the Xbal patterns of the 58 Salmonella isolates using UPGMA clustering analysis with the BioNumerics software. A positive tolerance of 1.5% was chosen.

RESULTS
Salmonella serotypes and antimicrobial susceptibility testing
A total of 54 of 58 (93.1%) of the Salmonella species belonged to the serotype Typhimurium vars Copenhagen. The rest (4/58, 6.9%) were Salmonella Typhimurium. AMR testing showed that all isolates were resistant to more than one of the antibiotics (Table 1). All but two of the isolates (ID 26 and 30) were resistant to more than two of the antibiotics tested, with 96.6% (56 of 58) of the isolates being MDR. All isolates tested were susceptible to amikacin, cefoxitin, cetriaxone, ciprofloxacin, gentamycin, nalidixic acid, and trimethoprim-sulfamethoxazole. Almost all of the isolates recovered from this study had a similar antimicrobial resistance pattern.

isolates showed a 233-bp PCR product by use of primers intI2, suggesting the presence of class 2 integron. Both of the two isolates also carried the class 1 integron.

respectively. The relationship between molecular types, integron carriage and resistance phenotypes of the Salmonella Typhimurium serovars Copenhagen is shown in Table 2.

PFGE analysis
The PFGE analysis identified 9 distinguishable Salmonella genotypes. Of the 9 PFGE profiles, STC and ST accounted for 94.8% (55 of 58) and 5.2 % (3 of 58) of the isolates, respectively (Fig. 1). Type IV, V, VII, VIII, and IX derived from cattle at two sampling periods showed 100% similarity. Type X (1 isolate from sampling 2), which is the most distant strain, showed only 73% similarity in PFGE banding patterns. Genotype V was the most prevalent (28/58, 48.3%) of the isolates, followed by types VI (15/58, 25.9%), IV and IX (both 3/58, 5.2%). Additionally, genotypes I, II, II, and X each represented 1.7% (1 of 58) of the genotypes (Fig. 1). Isolates 49 and 65, which were positive for both Int 1 and 2, belonged to genotypes I and IV,

DISCUSSION
In this study, all but two of the Salmonella isolates were resistant to more than two of the antimicrobials tested, with 96.6% of the isolates showing multidrug resistance. The widespread AMR of Salmonella isolated from cattle in North Dakota has been reported before (23), with most animal strains showing more multidrug resistance than human Salmonella isolates show, possibly due to a difference in antimicrobial selection pressure exerted on the microorganisms in the two populations. The emergence and dissemination of MDR among Salmonella isolates from healthy cattle have potential adverse implications to public health. Since the first description of class 1 integron by

Presence of integrons
Regardless of sampling points (1, 2, or 3), 29 were positive for class I integron (280 bp product) while only two of the

276 FOOD PROTECTION TRENDS | MAY 2010
Food Protection Trends - May 2010

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