Molecular Epidemiology of Multi-drug resistant Salmonella in swine production systems
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2001-09-28
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The swine industry is often implicated as one of the most significant contributors to foodborne Salmonella infections in humans. As the trend in swine production in the United States is toward larger and more intensive farms, but fewer farm units, it is likely that antimicrobial usage is also increasing. The purpose of this research was to study antimicrobial resistance and determine phenotypic and genotypic diversity of multi-drug resistant Salmonella in swine production systems. We found resistance to specific classes of antimicrobials including tetracycline, b-lactams and chloramphenicol to be common and stable over the study period. Over the time period of the study, we also noticed a significant shift in the patterns of multi-drug resistance, despite the fact that no change in antimicrobial usage had taken place, suggesting that other, currently unidentified selective pressures could be of significance. Bacteriophage typing also revealed that two phage types of importance to public health, DT104 and DT193, often had two predominant pentaresistance patterns. Susceptibility testing on a number of human S. Typhimurium isolates, however, revealed that multi-drug resistance was infrequent among isolates originating from humans. The findings in studying the genetic diversity of these Multi-drug resistant strains imply that vertical transmission, most likely originating from the sow farm, is an important source at the nursery stage, but that horizontal transmission of clonal groups is significant at the finishing phase. Resistance genes detected on multi-drug resistant Salmonella Typhimurium DT193 revealed that the resistance genes and locations are specifically different from that of DT104.Overall, the study findings show that antimicrobial resistance in the swine production environment is common and may be linked to the level of antimicrobial usage. Despite the common occurrence of public health important strains among isolates from swine, strains from humans were distinct and were less likely to be resistant to multiple antimicrobials. This area needs to be of high priority for further research to assess the risk of human Salmonella infection from swine or pork products. Further research on the genetic diversity of Salmonella also must be considered in order to identify significant factors contributing to the dissemination of multi-drug resistant Salmonella in modern swine production systems.
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PhD
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Comparative Biomedical Sciences
