Expert Blog

Untangling the antibiotic resistance puzzle: Expanding global use and a new herbicide risk

Carmen Cordova (Alum)

It's been an interesting week with two important studies moving the needle on our understanding of the impact of antibiotic use in agriculture on a global scale. Both give me a chance to put on my scientist hat, dive deep into the details, and untangle another piece of the antibiotics resistance puzzle for colleagues, family members , and faithful readers. Unfortunately, these new findings don't paint a great picture as there is a huge potential global increase in antibiotic use looming and an additional resistance threat has been linked to three common herbicides.

A PNAS study was published earlier this week that estimated the increase of antibiotics (described in the paper as antimicrobials) use on a global scale. Despite having incomplete data, the authors use the best methods available to estimate antibiotic use for an impressive 228 countries by using antibiotic usage data available from a few countries and maps of areas of concentrated livestock across the world.

There are several key points made:

  • There needs to better data collection. The authors found that only 32 countries collected data on antibiotics use for livestock operations, which represents only a small fraction of the countries in the study. Even here in the US, the data collected could be better because we only know gross amounts sold, and remain the dark about how they are being used and by who, in the livestock industry.
  • Antibiotics use in animal agriculture is estimated to go up globally by 67% by 2030. While this number also includes ionophores (a group of animal-only antibiotics) and doesn't measure increases of individual antibiotics, this is a huge increase of antibiotics across the board in less than two decades.
  • The increase in antibiotics use in livestock will drive a global increase in antibiotic resistant bacteria because of how connected we all are through transportation and trade.

The authors caution against the apparent rapid growth of antibiotic use in animal agriculture and they again emphasize, as others have done, that local use of antibiotics on the farm can pose a risk to others at farther distances, even at a global scale. Antibiotic resistant bacteria are quite adept at moving around the environment and escaping from farms into communities.

A second study looks at the antibiotic resistance issue from a new vantage point, but one that is all too common in modern agriculture: what happens to the efficacy of antibiotics when herbicides are in the mix? The authors exposed both E. coli and Salmonella to combinations of one of five antibiotics and one of the three herbicides: Dicamba (Kamba), Glyphosate (Roundup), and 2, 4-dichlorophenoxyacetic acid (2,4-D), herbicides that pose health risks even when they aren't mixed with antibiotics.

Here are the findings:

  • Exposing the bacteria to one of the herbicides and antibiotics generally made them temporarily resistant to the antibiotic. (There were a few instances when the opposite happened and the herbicide increased the potency of the antibiotic).
  • The authors suggest that bacteria may be reacting to the presence of the herbicide and going into overdrive to pump out or "kick out" the herbicide and anything else, including the antibiotic, out of their bodies.
  • The study reveals a temporary resistance effect occurs when certain combinations of herbicides and antibiotics are mixed. There isn't any evidence in this study that, after removal of the herbicide, the bacteria or their next generation, would continue to be resistant to the antibiotic.
  • This temporary resistance could be a problem for people or animals who are exposed to higher levels of these herbicides, while being treated with an antibiotic at the same time. This is especially a concern for agricultural workers and also for honeybees.

This emerging science points to a potential additional resistance threat linked to the ubiquitous practice of herbicide use. Together with the global assessment of antibiotic use in agriculture, these studies reinforce the public health threat of resistance caused by widespread use of antibiotics in animal agriculture. Antibiotics need to be used responsibly in animal agriculture, as well as in human medicine. A global report released last year by the World Health Organization shows that there is scientific agreement on the public health risk related to antibiotic use in animal agriculture. This emerging science only reinforces that antimicrobial stewardship is necessary if we want to keep antibiotics working in modern medicine.