The Known Unknowns – Pulling Back the Cover on Macrolide Resistance on Feedlots

Project Title

Known Unknowns: Macrolide Resistance at Beef Cattle Feedlots


Antonio Ruzzini (WCVM)

Murray Jelinski (WCVM) and Tim McAllister (AAFC Lethbridge)

Status Project Code
In progress. Results expected in March, 2026 ANH.01.22


The beef industry uses several macrolide antibiotics (e.g., Tylosin, Micotil, Zuprevo, Draxxin and Zactran) to prevent and treat a number of important diseases. Over time, bacteria develop resistance to antibiotics. Determining which antibiotics bacteria are resistant to has traditionally used culture-based tests. These tests essentially expose the bacteria to the drug. If the bacteria die, they are antibiotic susceptible. If they survive, they are resistant.

Macrolides are a class of antibiotics used to treat bacterial infections in large groups of animals. These antibiotics include Tylan, Mycotil and Draxxin among others.

New genomic tests are a lot faster and may become more economical than traditional culture-based tests. But the problem with genomic tests is that you have to know what genes to look for– if there’s a whole new gene in the mix that you aren’t aware of, the test won’t find it and will produce misleading results. We are pretty sure there are macrolide resistance genes that we simply haven’t identified yet. This team has discovered a bacterial esterase enzyme that can break down 16-membered macrolides (e.g., Tylan, Micotil, Zuprevo). Other enzymes are known to break down 14-membered macrolides like erythromycin (which is used in human medicine but not cattle). They suspect there might be other, as-yet-undiscovered enzymes that break down the 15-membered macrolides like Draxxin and Zuprevo. That’s what they want to look for in this project. While these enzymes could lead to antibiotic resistance in disease-causing microbes, they could also be a tool to help break down macrolide residues in the environment.


  • Characterize a new family of macrolide esterases, and
  • Screen for new enzymes capable of hydrolyzing macrolides with 15-membered rings.

What they will do

Esterases are enzymes that break down and inactivate specific molecules (like antibiotics)

This team has previously identified some suspect DNA sequences in macrolide-resistant bacteria found in cattle and the environment. They will use these isolates to find genes coding for enzymes that break down the 16-membered macrolides. They will look for related gene sequences; some of these may code for enzymes that might break down the 15-membered macrolides which are as-yet-undiscovered.

These genes will be cloned into E. coli and then tested to see whether they can resist 8 different macrolides used in livestock. Enzyme activities will also be investigated to determine which components are important to allow the enzyme to bind to the macrolide. This could provide a clue on how to develop modified antibiotics that can escape these enzymes.

Then they will look for esterases that may explain resistance to the 15-membered macrolides. They’ll do this by screening for Draxxin resistant bacteria from feedlot samples (animals, waterers, feed, soil, etc.) to determine if the Draxxin is being degraded or modified. The bacteria that degrade Draxxin will be studied further using long-read sequencing of DNA or RNA to identify new resistance genes and further characterize their abilities to inactivate macrolides.


Identifying potential antibiotic resistance will help producers choose which antibiotics will be most effective to treat their herd without contributing to further antibiotic resistance. Using new genetic tests could become a more efficient method of identifying these resistances, though, identifying the genes contributing to resistance is critical for this to replace culture-based testing. Further, understanding the enzymes that break down macrolides in the feedlot environment may also be a strategy to improve antibiotic development, and mitigate antibiotic resistance on-farm.