Date of Award

January 2022

Document Type

Open Access Thesis

Degree Name

Master of Public Health (MPH)


School of Public Health

First Advisor

Melinda Pettigrew


Background: Previous research has shown a potential link between antibiotic use among food-producing animals and antimicrobial resistant enteric pathogens. In late 2016, the Food and Drug Administration completed the implementation of its Guidance for Industry #213, through which growth promotion was removed as an indication for use of medically-important antimicrobials among food-producing animals. This study aimed to describe trends in antimicrobial resistance among human Salmonella isolates from 2013-2020 in the United States before and after the implementation of this regulation.Methods: All non-Typhoidal Salmonella isolates sent to the National Antimicrobial Resistance Monitoring System for Enteric Bacteria (NARMS) from 2013-2020 were included in this study (n=16,183). Thirteen antibiotics were tested by NARMS for the years included in this study. The Pearson’s chi-squared test and the Fisher’s exact test were used to evaluate differences in proportion of resistant isolates for thirteen antibiotics. Median-unbiased odds ratios were calculated for the change in proportion of isolates resistant to different antibiotic classes. Trends in resistance were assessed using the Cochran-Armitage trend test. Results: A significant decrease was observed in the proportion of isolates resistant to ampicillin (p<0.001) and streptomycin (p<0.001), while an increase was observed for chloramphenicol (p=0.002), nalidixic acid (p=0.001) and SMX-TMP (p<0.001). Isolates from 2017-2020 had a lower likelihood of being resistant to at least one aminoglycoside (OR=0.69, 95% CI 0.62-0.76), penicillin (OR=0.80, 95% CI 0.72-0.88), or sulfonamide (OR=0.87, 95% CI 0.79-0.96) antibiotic, though a higher likelihood of being resistant to chloramphenicol (OR=1.28, 95% CI 1.09-1.50) or at least one fluoroquinolone (OR=1.97, 95% CI 1.72-2.27). Significant (p<0.05) decreasing trends over 2013-2020 were observed for aminoglycosides, penicilins, and sulfonamides, while there was an increase in resistance over time to amphenicols, fluoroquinolones, and macrolides. Conclusion: Though likelihood of resistance to some antibiotics has decreased since the implementation of GFI #213, there are still concerning trends in antibiotic resistance among Salmonella. As more data become available from isolates after implementation, further research is needed to better understand the impact of these regulatory changes.


This is an Open Access Thesis.

Open Access

This Article is Open Access