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There is currently limited scientific data to support the common perception that reduced antimicrobial use in food animal populations will directly benefit the public health. <P>
The overall objective of this application is to fill critical information gaps regarding the effect of antimicrobial use on qualitative and quantitative measures of pathogens and antimicrobial resistance both pre- and post-harvest in order to fill both exposure and release assessment components of antimicrobial resistance risk analysis. In addition, data will be utilized to understand the role of commensal organisms in dissemination of specific antimicrobial resistance genes. <P>Our central hypothesis which is based on our own preliminary data is that differences in pre-harvest and post-harvest pathogen prevalence, antimicrobial resistance in pathogens and commensal organisms, and the fecal concentrations of pathogens and antimicrobial resistant commensal bacteria are associated with on-farm antimicrobial use protocols. <P>We plan to test our central hypothesis and accomplish the overall objective of this research by pursuing the following objectives: <OL> <LI> Quantify the impact of reduced antimicrobial use on food-borne pathogens pre-harvest. The working hypothesis is that herd antimicrobial use policy is associated with food safety risk as measured by the prevalence, antimicrobial resistance, genotypic diversity, and fecal concentration of Salmonella, Campylobacter, and STECs both pre- and post-harvest in beef and swine production systems. <LI> 2. Assess the response of targeted fecal commensal bacterial populations to reduced antimicrobial use in food animal production systems. The working hypothesis is that antimicrobial use is associated with the prevalence and quantity of commensal bacteria resistant to antibiotics in the fecal flora as well as genotypic diversity among resistance genes and mobile genetic elements. 3<LI> . Characterize the effect of antimicrobial use on the risk of zoonotic food-borne transmission of resistant pathogens. The working hypothesis is that beef and pork products labeled as produced under antibiotic-free production systems will differ in the prevalence, antimicrobial resistance, genetic diversity and quantity of Salmonella, STEC, Campylobacter and commensal bacteria as compared to meat products not labeled as antibiotic free. </ol> We believe that this work will fully evaluate the impact of antimicrobial use in animal agriculture both pre- and post-harvest using not only multiple concurrent measures of antimicrobial resistance risk, but also the quantitative exposure risk on a broad number of bacterial species associated with antimicrobial use on US farms. At the completion of these studies, we expect to have generated key data regarding the association of antimicrobial use in cattle and swine with antimicrobial resistance exposure risk pre- and post-harvest. This will have a positive impact on pre-harvest food safety and the sustainability and continued success of US agriculture, because it will provide data necessary for science-based public policies for antimicrobial use.
NON-TECHNICAL SUMMARY: The US food supply is among the safest in the world. However, human illness caused by food-borne pathogens from livestock remains of great public health significance. In addition, increasing development of bacterial resistance to antimicrobial drugs may be the most important food safety issue worldwide. Antimicrobial drugs are commonly used in food animal production for the treatment and prevention of disease, and for the enhancement of animal growth. This use of antimicrobial drugs in agriculture has led to real concerns that the resulting selection pressure has lead to the emergence of resistant bacteria of public health concern. Complete removal of antimicrobials from agriculture may have severe financial ramifications for producers, animal welfare implications, and an unknown impact on human health. Our central hypothesis is that differences in pre-harvest and post-harvest pathogen prevalence, antimicrobial resistance in pathogens and commensal organisms, and the fecal concentrations of pathogens and antimicrobial resistant commensal bacteria are associated with on-farm antimicrobial use protocols. We plan to test this hypothesis by pursuing the following objectives: 1. Quantify the impact of reduced antimicrobial use on food-borne pathogens pre-harvest, 2. Assess the response of targeted fecal commensal bacterial populations to reduced antimicrobial use, 3. Characterize the effect of antimicrobial use on the risk of zoonotic food-borne transmission of resistant pathogens. We expect our results have a positive impact on food safety and the sustainability of US agriculture, because it will provide data necessary for science-based public policies for antimicrobial use.
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APPROACH: The study populations will include pens of beef calves in Nebraska feedlots and groups of finishing swine in Michigan and Ohio. Entry criteria for both cattle and swine populations will include accurate keeping of antimicrobial treatment records. For each of these populations, the sampling unit will be groups of animals, although individual animal observations will be made in order to classify the group as to pathogen prevalence, antimicrobial resistance phenotypes and genotypes, as well as fecal concentration of pathogens. In addition, fresh retail meat products will be obtained by the investigators based in Ohio, Michigan, and Nebraska. The investigators will identify a total of nine metropolitan areas or geographic regions with a population base to ensure a large number of retail grocery stores. Within each of the metropolitan areas, 5 grocery stores will be identified for sampling based on the criteria of selling both antibiotic free and conventional beef products, for a total of 45 grocery stores. Within each metropolitan area, the five selected stores will be from different corporate chains in order to reduce the likelihood of unknown repetitive sampling of similar products from the same source. At each store, 10 fresh beef products and 10 fresh pork products that are routinely available will be purchased. Of the 10 beef and 10 pork products purchased from each store, 5 of each will be labeled as ABF and 5 will have no label indicating antimicrobial use procedures. Of each of the 5 products purchased within antimicrobial use categories, 3 will be fresh ground meat products and 2 will be individual cuts of meat. All animal and meat samples will be cultured for the presence of specific pathogens including Salmonella spp., Campylobacter spp., and Shiga-toxigenic E coli. Minimum inhibitory concentrations (MICs) to a panel of antimicrobial drugs will be determined for each isolate. The MICs will be determined using a Sensititer semi-automated broth micro-dilution system using the current NARMS panel of antimicrobial drugs for Gram negative bacteria. We will use Pulsed Field Gel Electrophoresis (PFGE) for genetic fingerprinting. Fecal concentrations of pathogens will be determined using quantitative real-time PCR assays. In addition to potential pathogens, commensal E coli and enterococcus will be cultured by direct inoculation onto selective agar. Innoculation onto agar both with and without predetermined concentrations of antimicrobials will allow for the detection of specific resistance genes present in the flora of the samples. The primary comparison of interest is between conventional and antibiotic free classification groups both across and within species and retail meat product types. Specific prevalence estimates and their 95% confidence intervals will be generated for the presence of each pathogen and antimicrobial resistance genotype.