Sources and Acquisition of Streptomycin Resistance in Erwinia amylovora, the Fire Blight Pathogen

Sources and Acquisition of Streptomycin Resistance in Erwinia amylovora, the Fire Blight Pathogen

Patricia McManus, Assistant Professor, Dept. Plant Pathology, 1630 Linden Drive, UW-Madison, 608-265-2047, psm@plantpath.wisc.edu

Vi Best, Senior Research Specialist, Dept. Plant Pathology, 1630 Linden Drive, UW-Madison, 608-262-2093, vmb@plantpath.wisc.edu

CALS Institute for Pest and Pathogen Management

June 1-December 31, 1999

To determine if plant-grade antibiotics contain bacterial DNA, specifically genes for antibiotic resistance.

Streptomycin and oxytetracycline, the only two antibiotics registered for use on plants, are in Priority Group 3 for tolerance reassessment. The use of antibiotics in agriculture is being scrutinized closely as the antibiotic resistance problems in hospitals and communities worsen. One concern is that antibiotic formulations for plant use might be impure, containing antibiotic resistance genes from the Streptomyces bacteria used to produce the antibiotics. Then by spraying the antibiotic onto apple trees, growers might be providing antibiotic resistance genes that can be taken up by Erwinia amylovora, the fire blight pathogen. If E. amylovora becomes resistant to streptomycin and oxytetracycline, control becomes even more difficult.

Wisconsin has about 325 apple orchards covering about 6500 acres. The most common varieties (McIntosh, Red Delicious, Cortland) are not highly susceptible to fire blight, and antibiotics are rarely used on these trees. However, there is significant acreage of the highly susceptible Golden Delicious, Gala, Paulared, and Idared. A new favorite, Honeycrisp, is also highly susceptible. Nearly all the dwarfing rootstocks, which have become popular in the past decade, are susceptible to fire blight. Thus, many growers rely on streptomycin for control of fire blight. There are no hard figures for the economic impact of fire blight in Wisconsin. However, if left unchecked the disease can completely destroy a young, high-density planting, causing $5000-10,000 loss per acre in a single season.

We did not detect DNA in several samples of plant-grade streptomycin products. The two methods used (DNA staining with a fluorescent marker followed by microscopy and the polymerase chain reaction [PCR]) are highly sensitive techniques that were used by other researchers to detect DNA in veterinary and reagent grade antibiotics. In order to verify that our techniques were adequate, we "spiked" samples with known amounts of bacteria. The lower limit of detection was 300 cells of Streptomyces bacteria per milligram of antibiotic formulation. There seemed to be some inhibition of PCR by streptomycin products, possibly because the antibiotic bound DNA, or because product additives inhibited enzyme activity in PCR.

 

 

We conclude that DNA, including antibiotic resistance genes, is either absent or present at low, undetectable levels in plant-grade antibiotic formulations. It is unlikely that growers are enriching the environment with antibiotic resistance genes when they spray streptomycin to control fire blight.

This short-term project was not intended to produce a lot of information for outreach directly to growers. Rather, the data is more directly relevant to defending the use of antibiotics on plants, a practice that has come under attack in recent years. This research was presented briefly by McManus at the annual meeting of the American Association for the Advancement of Science in February 2000. In addition to scientists, several policy makers and reporters were present. Currently McManus is co-authoring an article on plant-use of antibiotics to appear online at http://www.scisoc.org in June 2000.

This project complemented a survey of Wisconsin apple orchards for streptomycin-resistant strains of Erwinia amylovora conducted in 1998. We have not been successful at getting federal funds at the level needed to do further work on this project. Thus, for the time being, we are not pursing this work further.