Project Summary

Does Antimicrobial-Free Production of Beef Cattle Reduce Foodborne Transmission of Resistant Bacteria to Human Consumers? 

Principle Investigator(s):
Molly McClurg1, Enrique Doster1, Lee J. Pinnell1, Emily Rice2, Cory Wolfe1, Harrison M. Cottingham3, Mahesh N. Nair2, Keith E. Belk2, Wayne W. Campbell3, Paul S. Morley1
1VERO Program, School of Veterinary Medicine and Biomedical Sciences, Texas A&M University
2Department of Animal Science, College of Agriculture, Colorado State University
3Department of Nutrition Science, College of Health and Human Sciences, Purdue University
Completion Date:
June 2021

  • Significant differences in antimicrobial resistance genes or in the bacterial populations found on “raised without antibiotics” (RWA) vs. conventional beef products were not found.
  • Eating RWA beef products did not change antimicrobial resistance found in feces of study subjects at any point in the dietary trial.


Consumers in the U.S. are increasingly prioritizing “healthfulness” when making food purchasing decisions. Production of animals and poultry without exposures to antimicrobial drugs (i.e., “raised without antibiotics” or RWA production) may be the claim with the greatest impact on the meat and poultry industries in the past decade. A 2012 survey of U.S. consumers reported that a majority of respondents would pay more for meat if animals were raised without antibiotic exposures, and 37% said they would pay at least a dollar more per pound for “raised without antibiotic” products. Nearly 75% of respondents were very concerned that using antibiotics during production of food animals was responsible for creating “superbugs” that cause human illness that cannot be cured. Consumer interest in meat products with this claim has increased to the point where many of the largest restaurant chains in the U.S. have issued a variety of pledges that they will source poultry and other meats from certified RWA production systems. These concerns and a willingness to pay more for meat carrying a label certifying that meat was produced in RWA production systems suggests that consumers believe they will have a lower likelihood of being exposed or infected with AMR bacteria if they eat these products.   

Antimicrobial resistance is one of the most important global health issues, as evidenced by the White House, CDC, WHO, and other groups that are calling for increased scrutiny and minimization of antimicrobial drugs (AMD) use in food animals. Groups such as the CDC and WHO have explicitly stated that there is an increased risk of transmitting resistant bacteria to people through food when food animals are exposed to AMDs. However, no published scientific studies that directly examined whether eating beef or other meat and poultry products was associated with differences in risks for AMR in human consumers have been identified. Specifically, no published study has previously directly evaluated the risks for transmission of resistant bacteria through consumption of label certified RWA beef products with risks related to consumption of beef with conventional labels (i.e., there is no claim about AMD exposures, and some or all animals on farms where animals are raised may have been exposed to AMDs for prevention or treatment of diseases).

A randomized, blinded, cross-over dietary trial of people that consumed a basal diet emphasizing beef products as a protein source was conducted with a single exposure difference for the two Trial Periods: people consumed beef produced from cattle using conventional production practices to produce products with standard labeling (i.e., no claims regarding AMD exposures) in one Trial Period, and consumed beef produced in verified RWA production systems in the other Trial Period. Study subjects were aware that the intent was to study AMR related to diet, but they were not informed that the specific comparison of interest was between the two types of beef products. Laboratory personnel and data analysts were blinded as to which samples were obtained from the different Trial Periods or product types (i.e., conventional vs. RWA) until after analyses were complete. Subjects participated asynchronously in the 16-week dietary trial (i.e., not all subjects started on the same date) from March 1, 2022, through November 15, 2022. After screening and enrollment, the order of dietary interventions (RWA- vs. conventionally-labeled products) were randomly assigned to participants. During study weeks 1 - 2, 6 - 9, and 13 - 16 (“washout periods”), all subjects consumed normal, self-chosen diets. During both 3-week Trial Periods (weeks 3 - 5 and 10 - 12), subjects consumed prescribed diets that only differed in the two types of beef products. Detailed daily menus were distributed to each subject, and all food items consumed were provided without cost to the participant. Researchers provided all food from a commercial grocery retailer, and food was prepared in individual households. Prescribed daily diets included 6 oz. portions of ground beef and/or lean beef steak; dairy and eggs labeled as USDA Certified Organic were also included but no other meat or poultry products.  

Participants were provided with commercially prepared fecal collection kits, which they used to aseptically collect and store fecal samples at 7 paired time points (n = 14 samples per subject) during the 16-week study period. Paired fecal samples were collected from subjects with the specific intent of pooling DNA at key time points to minimize the potential impacts of daily variability in microbiome composition. Additionally, additional samples of food items provided to participants were obtained weekly and used to characterize bacteria found on fresh beef products and other dietary components. Raw ground beef and rinsates of steaks and other foods were collected for analysis. All samples were frozen until further analyses were conducted. After all sample collection was complete, samples were thawed, DNA was extracted and purified, and high-throughput genetic sequencing was conducted to assess the microbiome and resistome using specialized methods developed in our laboratory for detection of all known AMR genes and analysis of resistome sequencing data. Data analysis regarding the resistome and microbial communities found in human fecal samples was completed in two steps, relative to the crossover study design. First, fecal microbial communities were assessed for differences when subjects ate beef produced in conventional or “antimicrobial-free” rearing systems. The microbial community structure between timepoints in the study and among meat and food products were then compared.

Sequencing results showed that antimicrobial resistance genes (ARGs) most commonly identified in feces were determinants for resistance to tetracycline, macrolide-lincosamide-streptogramin (MLS), betalactam, aminoglycoside, and sulfonamide classes of AMDs, as well as to genes conferring multicompound resistance (e.g., drugs and biocides, multi-metal resistance mechanisms). In contrast, ARG communities in meat samples had markedly higher relative abundance of ARGs conferring resistance to tetracylines, more variable relative abundance for betalactam and MLS ARGs, and lower relative abundance for aminoglycosides. While the most abundant classes of ARGs were found in both feces of subjects as well as beef products they consumed, the specific types of ARGs (classification groups and mechanisms) differed by sample type (feces vs. beef) supporting the conclusion that the antimicrobial resistance in bacteria found on beef products did not significantly impact the antimicrobial resistance of gut microbial communities in consumers. Of greatest relevance to the primary study objective, there were no significant differences in the fecal resistome composition of study subjects when consuming RWA or conventionally labeled beef products, at any of the study timepoints, regardless of the order of dietary interventions. These findings indicate that neither of the dietary interventions (RWA or conventionally-labeled beef products) had a significant impact on AMR in the feces of consumers.

Surveys of consumers have found that label choices (“organic”, “natural”, “RWA”, etc.) are driven by a perception that these types of products are more “healthful”, in part because of concerns that antimicrobial drug exposures in food producing animals is creating “superbugs” that cause incurable human illness. Despite the significant pricing premium that is associated with beef products that carry these label claims, this research finding did not identify any differences in microbial community composition or populations of AMR genes that are related to this labeling difference. Most importantly, neither type of beef product (RWA or conventional) was associated with identifiable differences in antimicrobial resistance genes found in the feces in consumers. This study represents a first-of-its-kind examination of the impacts of the microbiome of beef products on antimicrobial resistance in the gut microbiome of consumers.