Wednesday, June 15, 2011

Study: Sub-Therapeutic Antibiotic Exposure Drives Resistance



# 5630



For years battle lines have been drawn between agricultural interests and concerned scientists over the extensive and routine use of sub-therapeutic doses of antibiotics in farm animals.


Low levels of antibiotics – delivered to healthy herds via their water or feed – have been found to promote growth, lower production costs, and reduce animal mortality and morbidity on crowded factory farms. 


Which makes them very popular among farmers.


But worried scientists point out that most of the antibiotics being used on the farm are functionally identical to those used in humans, and that their overuse (or misuse) can lead to the development of resistant organisms.


Late last year it was disclosed that roughly 80% of all antibiotics sold in the United States are used in agriculture  (see Maryn McKenna’s Update: Farm Animals Get 80 Percent of Antibiotics Sold in U.S.)


The economic and public health stakes here are enormous, and so over the past few years we’ve seen a steady stream of conflicting statements and studies on the matter designed to influence the public, and regulators.


Despite a rising tide of resistant organisms being detected on the farm, the agricultural lobby maintains that scientific proof that the use of sub-therapeutic doses of antibiotics drives bacterial resistance is lacking.


Countering that position, today we’ve a new study appearing in the peer reviewed journal Microbial Drug Resistance that finds that low level exposures to three common antibiotics: amoxicillin, tetracycline, and enrofloxacin drives the acquisition of resistance in Escherichia coli.


First some excerpts from the press release, then a link to the study.


Mary Ann Liebert, Inc./Genetic Engineering News

E. coli bacteria more likely to develop resistance after exposure to low levels of antibiotics, reports a study in Microbial Drug Resistance

New Rochelle, NY, June 14, 2011—E. coli bacteria exposed to three common antibiotics were more likely to develop antibiotic resistance following low-level antibiotic exposure than after exposure to high concentrations that would kill the bacteria or inhibit their growth, according to a timely article in Microbial Drug Resistance, a peer-reviewed journal published by Mary Ann Liebert, Inc.


Bacterial resistance to commonly prescribed antibiotics is an enormous and growing problem, largely due to misuse of antibiotics to treat non-bacterial infections and environmental exposure of the bacteria to low levels of antibiotics used, for example, in agriculture. In the article "De Novo Acquisition of Resistance to Three Antibiotics by Escherichia coli," the authors studied the mechanisms by which E. coli acquire resistance to three common antibiotics: amoxicillin, tetracycline, and enrofloxacin.


Depending on the antibiotic and the level of exposure, different mechanisms may come into play. The authors report that exposure to antibiotics at relatively low levels--below those needed to inhibit growth of the bacteria--are more likely to result in the development of antibiotic resistance. "Exposure to low levels of antibiotics therefore clearly poses most risk," a finding that "contradicts one of the main assumptions made questioning the threat of usage of antibiotics in food animals," conclude the authors.



The study (available as a PDF) is called:


De Novo Acquisition of Resistance to Three Antibiotics by Escherichia coli

Michael A. van der Horst,Jasper M. Schuurmans,
Marja C. Smid, Belinda B. Koenders, and Benno H. ter Kuile



For more on the legal wrangling surrounding the use of growth-promoting antibiotics in agriculture, I would direct you to Maryn McKenna’s Superbug Blog and her recent article:


Growth Promoters: If You Can’t Convince Them, Sue Them



Meanwhile - as scientists study, lobbyists lobby, and regulators contemplate their next move – nearly 80,000 pounds of antibiotics are used in this country every day on farm animals (28.8 million pounds/year).


Making regulation of the sane and sensible use of antibiotics on the farm desperately needed sooner rather than later.