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.
Fifteen animals were sampled initially and then weekly for a period of three weeks to establish a baseline of resistance in their natural flora. On the third week, 10 of the animals were given the standard neomycin dose of 10 milligrams per pound of body weight for three days. Samples were collected on days -14, -7, 0, 1, 2, 3, 7 and 14 to determine the initial resistance patterns and patterns that emerged over time. For reporting purposes, the sampling days were grouped into three periods. Period 1 = days -14, -7 and 0; Period 2 = days 1, 2 and 3; Period 3 = days 7 and 14. Neomycin was fed on days 0, 1 and 2. Enterococci, commensal E. coli, Salmonella and E. coli O157:H7 were isolated and tested for antimicrobial susceptibility.
No E. coli O157:H7 were recovered during the duration of the study. Of the isolates recovered, 226 were generic E. coli, eight were Salmonella and 193 were Enterococcus.
The antimicrobial drug susceptibility of the commensal organisms in the animals was tested prior to administration of neomycin. Some resistance was naturally present in the generic E. coli. The percentage of generic E. coli isolates recovered from cattle that were either fed neomycin sulfate versus those that were not is outlined in Table 1.
Only eight Salmonella isolates were recovered. Three of those were recovered from samples collected during Period 1, four during Period 2 and one during Period 3. The four Salmonella isolates recovered during Period 2 were obtained from both the exposed (n = 2) and unexposed groups (n =2). One isolate was resistant to seven antimicrobial drugs (tetracycline, streptomycin, sulfamethoxazole, kanamycin, gentamicin, trimethoprim/sulfa and ampicillin). This Salmonella isolate was only detected once and did not propagate in response to treatment.
The percentage of Enterococcus isolates recovered is summarized in Table 2. Of the Enterococcus isolates recovered, 90.2 percent were E. durans, 9.3 percent were E. faecium, and 0.5 were E. gallinarum.
Overall, resistant development was observed most commonly and most pronounced to streptomycin, kanamycin, tetracycline and erythromycin.
Feeding neomycin sulfate appears to result in cross-resistance to other antibiotics in both gram- negative and gram-positive bacterial isolates. The majority of the resistance was not observed seven to 14 days after exposure to neomycin, however some levels did persist. Neomycin has a one-day withdrawal period and results of this study indicate that resistance peaks during that time. However, since some levels of resistance did persist even longer, an extended withdrawal period may be warranted to reduce the likelihood that resistant organisms enter the human food supply.