A couple of weeks ago Dr. Jeff Ullman, formerly of WSU, gave a provocative seminar on the fate of various constituents of pharmaceuticals and personal care products in the environment. He and his co-researchers have discovered that a wide range of chemicals from these products do not degrade when going through our bodies, animal bodies, or wastewater treatment facilities, and can sometimes be detected at very low levels in drinking water. He focused on recent work1 trying to test the hypothesis that antibiotics fed to livestock (often in continual sub-therapeutic doses) can be excreted by the animal, remain biologically active, exert selection pressure on human pathogens that might be present in the environment outside the animal, lead to the development of antibiotic resistance by these pathogens, and then be ingested by another animal. Ultimately, their careful step-by-step study did show it was possible for this to occur. However, they found that not all antibiotics act the same. Cefoxitin and florfenicol, for example, retained their bactericidal activity and thus could select for resistance, while tetracycline and ciprofloxacin were almost completed deactivated within 24 hours of contact with the soil. They conclude that efforts to control antibiotic contamination might best be focused on those compounds that retain their biological activity in soil since these are the ones that could exert a selective pressure for resistance in the environment.
At the recent National Organic Standards Board meeting in Portland, the topic of oxytetracycline use, in general and specifically in organic apples and pears for control of fire blight, dominated the conversation and testimony. The primary reason put forward to eliminate this use was the risk of horizontal gene transfer of resistance from a non-pathogen to a human pathogen, thus endangering the effectiveness of a medically important drug. Tetracycline is widely used in animal agriculture; however, the research described above suggests that the resistance risk outside the animal is low due to its deactivation by soil. Since oxytetracycline (OTC) is directly applied to trees, it may leave residues on the tree or fall to the soil where it is presumably deactivated. Dr. Virginia Stockwell presented results from her research at the meeting showing rapid degradation of OTC by sunlight and rapid washing from leaves by rain. She did see an increase in the presence of OTC resistance genes in bacteria on the leaf surface immediately after treatment, but this reverted to the pre-treatment level within about a week. A recent study in Wisconsin (Yashiro and McManus, 2012) found no difference in resistance genes in orchards treated annually with streptomycin (another material used for fire blight control) for 10 years versus orchards never treated. And human pathogens are rarely found on apple flowers or leaves, limiting the opportunity for horizontal flow of resistance genes.
The Board faced a difficult decision about extending the use of OTC for two more years while on-going research is completed. Evidence presented by Dr. Glenn Morris of University of Florida, based on antibiotic use in animal agriculture, suggested imminent risk, while evidence from studies in orchards and on the environmental fate of tetracycline did not. The Board ended up not approving an extension.
Based on surveys I have done of organic growers, over 70% indicated that they would reduce their acres of organic apples and pears without a proven alternative to OTC, or exit organic production entirely. In that case, the number of acres being treated with OTC would not change much, as growers would use the material (when fire blight risk is high) and lose their organic status rather than their trees. With a two-year extension, again OTC use would probably not change much in that time, but with the additional time for the research to be completed, for new products to be registered and be made commercially available, and for growers to try out the new control methods, the odds that growers would be successful with non-antibiotic control would arguably increase. With success, more growers would stay in organic production (price premiums are still very attractive), which would decrease future OTC use on those acres. And since many growers also farm “conventional” orchards, they may adopt parts of the non-antibiotic control on those acres (as has occurred with other pest controls in organic orchards) and reduce OTC use even more. However, if growers experience a “train wreck” with their first attempts at non-antibiotic control, without the full benefit of the emerging research and new controls, that will set adoption back for years and reinforce the reliance on antibiotic control.
Concerns were expressed about antibiotic residues on the harvested fruit as another risk factor. I conducted some initial residue testing and found no detectable residues on fruit from seven orchards. More testing is needed to validate these findings. And antibiotics are found commonly in the natural world. Several of the biocontrol organisms used against fire blight produce antibiotics as their mode of action. There are other types of antibiotics, such as kasugamycin, that are not used for either human or livestock health. Could these play a role in controlling bacterial diseases on plants, including in organic systems? Early organic certification programs allowed antibiotics because they were considered naturally occurring compounds (they are) and did not scrutinize how products are made.
I agree with the goal to move all agriculture away from use of antibiotics important in human medicine. But I wonder if the NOSB decision is a case where a decision made to achieve this goal may end up causing the opposite result. See my earlier comments on antibiotics for fire blight management here.
1 Subbiah et al. 2011. β-Lactams and Florfenicol Antibiotics Remain Bioactive in Soils while Ciprofloxacin, Neomycin, and Tetracycline Are Neutralized. Applied Environ. Microbiol. 77:7255-7260.