Researchers Bust Antimicrobial-Resistance Myth

John Maday, Editor, Bovine Veterinarian ( Lori Hays )

The following commentary does not necessarily reflect the views of AgWeb or Farm Journal. The opinions expressed below are the author's own.

We know that when a population of bacteria are exposed to an antibiotic, some level of resistance develops over time. So it would seem logical to assume that beef from cattle treated with antibiotics would carry more resistant bacteria than beef from “antibiotic-free” production systems.

Recent research from scientists with the USDA’s Agricultural Research Service (ARS), however, shows similar levels of antimicrobial resistance (AMR) in ground beef raised with and without antibiotics. The report, titled “Similar Levels of Antimicrobial Resistance in U.S. Food Service Ground Beef Products with and without a Raised without Antibiotics Claim,” appeared in the Journal of Food Protection.

Over three months, the researchers collected 370 samples of ground beef from three food-service suppliers. The samples included 191 from conventional production systems and 179 with “raised-without-antibiotics” claims. From those samples, they cultured several strains of bacteria:

  • Escherichia coli
  • Tetracycline-resistant (TET) E. coli
  • Third-generation cephalosporin-resistant (3GC) E. coli
  • Salmonella enterica
  • TET S. enterica
  • 3GC S. enterica
  • Nalidixic acid–resistant S. enterica
  • Enterococcus spp.
  • Erythromycin-resistant Enterococcus spp.
  • TET Enterococcus spp.
  • Staphylococcus aureus
  • Methicillin-resistant S. aureus (MRSA)

The team also isolated metagenomic DNA from each sample to measure the abundance of antibiotic resistance genes using quantitative polymerase chain reaction (PCR) tests.

They found that overall, antibiotic-resistant bacteria in all the ground beef samples made up a small portion of the total bacterial flora. The most common drug-resistant organisms were tetracycline-resistant E coli, which was detected in 54% of conventional samples compared with 35% of RWA samples, and tetracycline-resistant enterococcus, which was found in 94.8% of conventional samples and 91.1% of RWA samples.

The researchers note though, that detection of the resistant bacteria differed significantly by supplier.

As for the other pathogens tested, detection of tetracycline-resistant Salmonella, third-generation cephalosporin-resistant E coli, erythromycin-resistant enterococcus, and MRSA was lower, and similar, in both sample groups, and there was more variation between suppliers than between the production systems.

When the researchers assessed AMR gene abundance, they found that two of the most common tetracycline-resistance genes—tetA and tetB—were significantly more abundant in the RWA ground beef.

Overall, the researchers found about levels of resistance in meat from both production systems. They concluded that their results were “consistent with prior research suggesting antimicrobial use in U.S. beef cattle has minimal impact on the AMR of bacteria found in these products. These results should spur a reevaluation of assumptions regarding the impact of antimicrobial use during U.S. beef production on the AMR of bacteria in ground beef.”

While these results raise questions about the value of label claims for RWA beef, they do not absolve antibiotic use in cattle as a potential contributor to resistance. The pathways through which AMR bacteria find their way into beef from “antibiotic-free” production systems are not fully understood. Cross-contamination in processing plants could contribute, as could live-animal exposure to AMR pathogens through manure, water, dust or other environmental sources associated with antibiotic-treated cattle.

Judicious use of antibiotics remains critical for minimizing risk of resistance in pathogen populations. However, as this study shows, the pathogen population (antibiotic-resistant and susceptible) in ground beef varies more between processors than between production systems. This suggests that overall pathogen control, including pre-harvest management and sanitation practices in processing plants could have the greatest overall effect on reducing risk of food-borne illness associated with ground beef.