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Impact of Feeding Neomycin on the Emergence of Antibiotic Resistance in E. coli O157:H7 and Commensal Organisms

Loneragan, Guy; Brashears, Mindy
Texas Tech University
West Texas A&M University
Start date
End date
Neomycin sulfate is an aminoglycoside, an antimicrobial that interferes with protein synthesis. Aminoglycosides are commonly used to treat infections in humans and animals. Feeding neomycin sulfate to cattle prior to slaughter has been proposed as a potential antimicrobial intervention to inhibit E. coli O157:H7. Currently, such an application of neomycin sulfate is not approved by the Food and Drug Administration (FDA).

It is necessary to validate neomycin’s potential to serve as a preharvest intervention in the prevention of E. coli O157:H7 contamination of beef products. However, before that can be done, concerns that the widespread feeding of neomycin sulfate might lead to antibiotic resistance must be addressed. While there is no data available to support that assumption, the question must be dealt with prior to using neomycin sulfate as a preharvest intervention.

In this project, researchers not only sought to understand the potential for direct resistance of E. coli O157:H7 to antimicrobials, but to also examine the impact of neomycin sulfate on commensal organisms in the animal. Commensalism is a relationship between two organisms in which one partner benefits from the association and the other is unaffected. Commensal E. coli represents a large population of gram negative organisms in the animal, while Enterococci are gram positive organisms. Antibiotic resistant enterococci are intrinsically less susceptible to many common antimicrobials. Some strains are even resistant to vancomycin, an antibiotic that is usually reserved as a last resort for treating life-threatening infections caused by gram positive bacteria. Since the resistance in vancomycin resistant enterococcus (VRE) is encoded on a plasmid, the resistance is potentially transferable to other organisms. Resistance in commensal organisms can become a part of the natural flora of the animal and be transferred through the food supply to humans, potentially having an impact on human medicine.

The objective of this study was to determine the impact of feeding neomycin sulfate to cattle and its effect on the antimicrobial drug resistance of commensal E. coli organisms in the animals.

More information
Findings: Neomycin appears to result in cross-resistance to other antibiotics in both Gram-negative and Gram-positive isolates. Resistance development was observed most commonly and most pronounced to streptomycin, kanamycin, tetracycline, and erythromycin. While most resistance was not observed 7-14 days after exposure to neomycin, some resistance did persist. Neomycin has a 1-day withdrawal period. Results of this study indicate that resistance peaks during this time and a longer withdrawal period may be warranted to reduce the likelihood of resistant organisms entering the food supply. It may be important to evaluate the duration of efficacy of neomycin in reducing E. coli O157 after administration. More research is also required to quantify resistant and susceptible bacterial loads to determine if administration of neomycin markedly increases the number of resistant organisms or simply increases the likelihood of recovery of them without affecting their numbers. A more extensive study is needed to examine these issues.

For complete projects details, view the Project Summary.

Funding Source
Nat'l. Cattlemen's Beef Assoc.
Project number
Escherichia coli
Food Defense and Integrity