Do Good Apple and Pear Management Practices Promote the Development of Streptomycin-Resistant Erwinia amylovora?

The use of antibiotics in crop protection, and the subsequent emergence of antibiotic-resistant plant pathogens, has in many respects paralleled antibiotic use and resistance development in human and veterinary medicine. Streptomycin-resistant strains of Erwinia amylovora, the fire blight pathogen of apple and pear, have been found in widely separated geographic regions.

The stars indicate regions with streptomycin-resistant Erwinia amylovora.

Molecular analyses of streptomycin-resistant E. amylovora have revealed
two genetically distinct mechanisms of resistance to streptomycin.

One mechanism of resistance stems from a point mutation in a single codon of the chromosomal gene rpsL. The resulting amino acid substitution renders the bacterial ribosome insensitive to streptomycin.

A second way in which bacteria become resistant to streptomycin is through the conjugative transfer of the genes strA and strB. These genes encode the enzyme streptomycin phosphotransferase, which inactivates streptomycin. The streptomycin-resistance genes strA and strB have been found in at least 16 different genera of clinical and environmental bacteria, often associated with transposons, conjugative plasmids, or both.

While the medical, veterinary, and crop protection fields are stymied by genetically similar mechanisms of resistance, antibiotic use in the agroecosystem presents unique circumstances that could strongly impact the build up and persistence of resistance genes in the environment. Examples of how conventional apple and pear management might actually promote streptomycin resistance in E. amylovora are presented to provoke discussion and potentially spark new research enterprises.

In regions of dense apple and pear production, streptomycin is applied by air-blast spray equipment to hundreds of hectares of nearly contiguous orchards. While growers strive to minimize drift by spraying during calm weather, non-target organisms on plants, in the soil, and in water are exposed to low doses of streptomycin. Low doses of antibiotics applied to large areas over long periods of time contribute to the build-up of resistance in clinical bacteria. Might a similar scenario be played out in apple and pear orchards?

The past decade has seen a dramatic increase in high-density plantings of fire blight susceptible apple cultivars and rootstocks, creating a situation analogous to clinical settings in which immune-compromised patients are believed to be reservoirs for resistant pathogens.

Nonpathogenic bacteria are being exploited for biological control of fire blight. Pantoea agglomerans (formerly Erwinia herbicola) occupies some of the same niches on apple as E. amylovora. This may provide an opportunity for transfer of streptomycin-resistance genes between the two species. The presence of an identical plasmid carrying strA and strB in E. amylovora and P. agglomerans, and conjugal transfer of this plasmid between these species in vitro, suggests that P. agglomerans might be a reservoir for, or an intermediary in the transfer of, resistance genes to E. amylovora.

Fertilizers and fungicides applied to apple and pear are rich sources of divalent cations. The concentration of Mn²⁺ or Ca²⁺ in mancozeb fungicide or CaCl₂ fertilizer, respectively, when applied at recommended rates, is similar to the non-physiological concentrations recommended for transformation of bacteria in the laboratory.

The purity of antibiotics used in crop protection is unknown. Reagent and veterinary formulations of antibiotics have been found to contain antibiotic resistance genes from the producing Streptomyces spp. Plant-grade antibiotics are unlikely to be purer than those used for treating humans; could they themselves be an origin of antibiotic-resistance genes in agroecosystems?

Stability of DNA is enhanced by attachment to particulate matter, and transformation is generally more efficient when DNA and recipient cells are immobilized on a support matrix such as particulate matter. Moreover, DNA is tightly bound by aminoglycoside antibiotics, such as streptomycin, which could protect the DNA from nucleases and even enhance its uptake into bacterial cells. Could E. amylovora acquire resistance genes from antibiotic preparations or lysed bacteria by means of artificial transformation?

Questions or comments? Please contact Dr. Patty McManus at the University of Madison-Wisconsin Fruit Pathology Laboratory.

Literature Cited:

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